Marina V. FRONTASYEVA, Joint Institute for Nuclear Research, Michel WAUTELET, Mons-Hainaut University, Belgium;

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2 Journal r l of f Science i and Arrtts EDITOR IN CHIEF Ion V. POPESCU Valahia University of Targoviste, director of Multidisciplinary Research Institute for Science and Technologies VICE-EDITOR-IN-CHIEF Valerica Gheorghe Cimpoca Valahia University of Targoviste, technical director of Multidisciplinary Research Institute for Science and Technologies DEPUTY DIRECTOR OF PUBLICATION Cristiana RĂDULESCU Valahia University of Targoviste Editorial board: Bakki AKKUŞ, University of Istambul, Turkey; Delia DIMITRIU, Manchester Metropolitan University, England; Dimiter L. BALABANSKI, University of Sofia, Iacob GUŢU, University de Stat din Moldova, Chisinau, Bulgaria, IKS, Katholieke Universiteit Leuven, Belgium; Republica Moldova; Alexander Vladislavovich BELUSHKIN Joint Institute for Florin IORAŞ, Buckinghamshire Chilterns University, England; Nuclear Research, Dubna Moscow, Rusia; Costel PETRACHE, University Paris-Sud XI, France; University of Moscow, Russia; Maria KORDAKI, University of Patras, Grece; Marina V. FRONTASYEVA, Joint Institute for Nuclear Research, Michel WAUTELET, Mons-Hainaut University, Belgium; Dubna Moscow, Russia; Gheorghe UDUBAŞA, University of Bucharest, Grzegorz KARCH, University of Wroclaw, Poland; Romanian Academy Member. Jacques LANGLOIS, University Bretagne Occidentale Brest, France; Advisory board: Ana Maria ALBU, Politehnica University of Bucharest, Organic Chemistry Institut C. Nenitescu, Bucharest; Bakki AKKUŞ, University of Istambul, Turkey; Dimiter L. BALABANSKI, University of Sofia, Bulgaria, IKS, Katholieke Universiteit Leuven, Belgium; Alexander Vladislavovich BELUSHKIN, Joint Institute for Nuclear Research, Dubna Moscow, Rusia; University of Moscow, Russia; Vasile BERINDE, Nord University of Baia Mare; Constantina BOGHICI, Valahia University of Targoviste; Marian BOGHICI, Valahia University of Targoviste; Florin BUCESCU, University of Arts George Enescu, Iasi; Adrian CARABINEANU, University of Bucharest; Gheorghe CATA-DANIL, University of Bucharest; Andrei Florin DĂNEŢ, University of Bucharest; Delia DIMITRIU, Manchester Metropolitan University, England; Crinela Dumitrescu, Valahia University of Targoviste; Marina V. FRONTASYEVA, Joint Institute for Nuclear Research, Dubna Moscow, Russia; Constantin GHIŢĂ, Valahia University of Targoviste; Laura Monica GORGHIU, Valahia University of Targoviste; Iacob GUŢU, State University of Moldova, Chisinau, Republica Moldova; Florin IORAŞ, Buckinghamshire Chilterns University, England; Silviu JIPA, Valahia University of Targoviste; Wilhelm KAPPEL, INCDIE, ICPE CA Bucharest; Grzegorz KARCH, University of Wroclaw, Poland; Maria Kordaki, University of Patras, Grece; Jacques LANGLOIS, University Bretagne Occidentale Brest, France; Vasile MAGEARU, University of Bucharest; Tatiana MOROŞANU, Valahia University of Targoviste; Cristinel MORTICI, Valahia University of Targoviste; Gheorghe OPREA, The National University of Music, Bucharest; Călin OROS, Valahia University of Targoviste; Costel PETRACHE, University Paris-Sud XI, France; Sabin PAUŢA, Emanoil University of Oradea; Valahia University of Targoviste; Silviu SBURLAN, Ovidius University of Constanta; Radu SETNESCU, Valahia University of Targoviste; Gheorghe UDUBAŞA, University of Bucharest, Romanian Academy Member; Traian ZAHARESCU, INCDIE, ICPE CA Bucharest; Michel WAUTELET, Mons-Hainaut University, Belgium. Editors: Claudia STIHI Valahia University of Targoviste, Physics Section Ionica IONITA Valahia University of Targoviste, Chemistry Section Ingrid OANCEA Valahia University of Targoviste, Mathematics Section Constantina BOGHICI Valahia University of Targoviste, Arts Section Editorial Office address: Unirii Avenue, , Targoviste, Dambovita office@josa.ro, Phone/Fax: Editorial Contact person: Cristiana RADULESCU, radulescucristiana@yahoo.com ISSN Bibliotheca Publishing HouseTargoviste Certificated by the Ministry of Culture and Cults decision nr / Recognized by the National University Scientific Research Council (CNCSIS) - decision nr / and May 2006 Member of Romanian Publishers Association RPA Publisher contact person: Mihail STAN Nicolae Radian street, KB 2/3 Târgovişte cod Phone/fax: Phone: biblioth@gmail.com EDITED WITH SUPPORT OF NATIONAL AUTHORITY FOR SCIENTIFIC RESEARCH Copyright 2009 Bibliotheca Publishing House All rights reserved Papers may not be reproduced without the permission of the Organizing Committee. Papers are published under responsibility of the author(s). Year 9 No. 2 (11) July 2009 Bi-Annual 200 copies

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87 JOURNAL OF SCIENCE AND ARTS THE UV-VIS CHARACTERIZATION OF AZO DYE BY IRRADIATION IONIŢĂ IONICA 1, ANA-MARIA ALBU 2,3, RĂDULESCU CRISTIANA 1, MOATER ELENA IRINA 1 1 University "Valahia" from Targoviste, Faculty of Sciences, Department of Chemistry, Unirii Bdvl, Targoviste, Romania 2 Politehnica University of Bucharest, Department of Chemistry, 149 Calea Victoriei, Bucharest 3 Centre for Organic Chemistry, Romanian Academy, 202B Spl Independentei, Bucharest, Romania Abstract: The stereochimical factor, which play a significant role about colour azo dyes, lead at Z(sin)-E(anti) isomerism. The E(anti) form are much more stable at temperature rooms, while the Z(sin)-isomer can be generated only by photochemical methods. Keywords: azo dyes, isomerism, photochrom. 1. Introduction Photoresponsive systems are compounds which respond to light energy beyond only absorbing and releasing it as heat. The storage of the light energy for varying lengths of time, the reappearance of fluorescence and chemical or even mechanical energy are properties of photoresponsive system [1,2]. The absorption spectra of both two isomer form of azo dye are always differed. The detectable quantities of Z(sin)-isomer are obtained at exhibit of dye to light, lead to colour change, which will be reversible to the removal lights [3,4]. This phenomenon is named as photochromia or phototropy and it isn t wanted in the dyes with commercial applications [5]. The photocromic degree depends decisive of the quantic yield of photoisomerization and of Z(sin) forms thermostability. Fortunately, the strong donor groups from electrons azobenzene system reduce sensible life duration of Z(sin) isomers and thus the photochroma is a grand question for dyes producers [6]. 2. Experimental part In this work synthesis of azo dye is present. This dye a will be polymerization and study photochromic proprieties [1,2]. The synthesis follows two steps: diazotizing of p-aniline and the coupling salt of diazonium with the phenylaminomethylensulphonic acid. A mixture of NaHSO 3 solution and 28% formaldehyde solution for synthesis of phenylaminomethylensulphonic acid is heated at 70 0 C, where it is maintained for 0,5h. After that the resulted solution is cooled at 25 0 C, then the aniline is added and the mixture is heated for 3h at C and maintained for 1h at 35 0 C. The mixture was diluted with water, cooled to 20 0 C and precipitate product is formed [7]. NH 2 + CH 2 O + NaHSO 3 NH-CH 2 -SO 3 -Na+ H 2 O The preparation of the azo dye is presented in the following scheme (A): NaNO 2 NH 2 N N - H + 2 O ] Cl+ NH-CH 2 -SO 3 -Na N N NH HCl 2 SO 2 Cl Ni NO 2 +3H 2-2H2 O NaNO Cl NH 2 Cl N N]HSO 4 H 2 SO 4 H 2 O Cl N N NH 2 SO 2 NH-CH 2 -SO 3 -Na (1) Scheme A. The preparation of azo dye + 89

88 JOURNAL OF SCIENCE AND ARTS The purification of azo dye (1) through recrystallization from solvents was achieved. The solvents utilized are 1,4 dioxane and N,N dimethyleformamide, which are purified by anhydrization procedure. The purity was checked by thin-layer chromatography. The physico-chemical characterization of the synthesised compound was done by UV, IR and NMR spectroscopy [8]. The spectra UV was recorded on a SECOMAM S750 spectrophotometer, quartz vat used for all solution of concentration mol/l in DMF (Table 1). The synthesized compound (1) was characterized by 13 C-NMR and 1 H-NMR, using deuterated dimethyl-sulfoxide as a solvent. H 10 H H J(meta) 7 Cl N N NH 2 H 9 8 H H H H J(meta) J(orto) J(orto) The 1 H- NMR spectra: (H 6 )H 2 split doublet H : 7.56 ppm with the coupling constants orto J(2,3) = 8.64 Hz; (H 5 )H 3 split doublet H : 7.80 ppm with the coupling constants orto J(3,2) = 8.64 Hz; (H 12 )H 8 split doublet H : 7.67 ppm with the coupling constants orto J(8,9) = 8.64 Hz; (H 9 )H 11 split doublet H : 6.83 ppm with the coupling constants orto J(9,8) = 8.64 Hz The results of the elemental analysis proved the presence of nitrogen and chloride atoms (Table 2) Table 2. Elemental analysis of the azo dye 2 1 N Cl N NH 2 %C calculated found %H calculated 4.31 found 3.76 %N calculated found %Cl calculated found %Br calculated - found - The irradiation of the azo dye (1) was made with a experimental installation with Hg lamp (250 W) [1] to record the UV-VIS spectra before and after irradiation. The intensity of absorption before and after irradiation was measured with the same UV VIS spectrophotometer (Table 3). The irradiated dye samples were transported immediately in the dark after irradiation. The solutions were exposed to UV light to increase the rate of Z(sin)-E(anti) isomerization. Compound N Cl N NH 2 Table 1.Results of UV-VIS analysis of azo dye Concentration max Absorbance lg [I 0 /I] [mol/l] [nm] ,

89 JOURNAL OF SCIENCE AND ARTS H N N Cl H 2 N N N Cl 2 N Fig.1.The UV-Vis spectra after 2 h from irradiation Fig.2.The UV-Vis spectra after 24 h from irradiation Table 3. Results of UV-VIS analysis of azo dyes solutions H 2 N N N R, R = Cl R* Cl Conc (mol/l) 3.00 Before irradiation A (4693) A (4337) After 2 h from irradiation After 24 h from irradiation A (4960) A (3033) A (4606) A (2853) *in bracket are the molar extinction coeficient (L/mol. cm). It has been observed a change in the absorption spectral domain after irradiation (Fig.1, Fig.2) of the chromophor solution with UV radiation for a short time (Table 1). The absorption spectra of both two form isomer of azo dye are differed and thus, if exhibit of a dye to light produce the detectable of Z(sin)-isomer, noticed changed it and colour will be reversible to the removal lights. 3. Conclusions In all the cases a decrease in optic absorbance is observed, subsequently to the irradiation, with a tendency to come back at initial value (sometimes just outrunning in UV radiation absence, after 24 hours). Similarly to the absorption evolution behavior was molar extinction coefficient, too. The both parameters render default, in the fact, the frequency of Z(sin)-E(anti) transitions concomitantly the concentration of both isomers in system.the irradiation of the chromophor solution 91

90 JOURNAL OF SCIENCE AND ARTS with UV radiation for a short time, it can be observed a change in the spectral absorption domain. The DMF solvent selection solvent was needed for the later utilization in polymer analogue reactions [2]. The preference for DMF is justified because of this is solvent as much for dye and for polymeric substrate. If the intermediaries present two absorbtion maximums in the UV area which are characterizated by order 10 4 molar extinction coeficient, their coupling in azo colour stuctures determine a movement of second maximum towards bigger wavelenghts in visible zone. The bathochrome movement or hypsochromy which appears in the dye case is a direct consecquence of azo chromofor which is responsable for the maximum movement in visible area and respectivly, the nature and the volume of the substituent produce these phenomenons. The spectra analysis show for all dyes the existence of two maximums of absorption on covering curve in visible area. All these prove the existence of two isomeric forms Z(sin)-E(anti). References [1] Ioniţă I., Albu A-M, Tărăbăşanu Mihăilă C., Bădulescu R., Rădulescu C., International Conference Polymeric Materials 2004, 29 septembrie 1 octombrie 2004, Halle/Saale Germany ISBN , PI 40, [2] Ioniţă, I., Albu, A.M., Rădulescu Cristiana, Hossu, A.M., Moater E. I., Dyes with potential application in photochromic materials, Ovidius University Annals of Chemistry, vol. XVII, , [3] I. Popov, V.T. Skripkina, S.P. Protsyik, A.A. Skrynikova, B.M. Krasovitskii, M. Yagupol skii, Ukrain. Khim. Zhun, 57/8, 843, [4] Durr H., Bous-Laurent H., Photochromism Molecules and Systems, Elsevier, Amsterdam, [5] Ioniţă I., Dumitrescu C., The Annals of Valahia University of Târgovişte, Session: Fundamental Science nr.12, 2002, Târgovişte, [6] Ioniţă I., Albu A-M, Rădulescu C., Hossu A-M., Moater E.I., Scientific Study & Research, vol. VI(1), , [7] Floru L., Langfeld H.W., Tărăbăşanu-Mihăilă C., Coloranţi azoici, Ed. Tehnică, Bucureşti, [8] Balaban A.T., Banciu M., Pogany I., Aplicaţii ale metodelor fizice în chimia organică, Ed. Ştiinţifică şi Enciclopedică, Bucureşti, Manuscript received: / accepted:

91 JOURNAL OF SCIENCE AND ARTS AAS AND TDS MEASUREMENTS FOR WATER QUALITIES ANALYSIS Anca GHEBOIANU 1, Ion V. POPESCU 1,2, Claudia STIHI 3, Iulian BANCUTA 1, Ioana DULAMA 4 1 Multidisciplinary Research Institute for Science and Technologies, Valahia University of Targoviste, No 2 Carol I Street, Targoviste, Romania, anca_b76@yahoo.com 2 Academy of Romanian Scientists, 54 Splaiul Independentei, Bucharest050094, Romania 3 Physics Department, Faculty of Science and Arts, Valahia University of Targoviste 4 PhD School, Faculty of Physics, University of Bucharest, Romania Abstract: Water quality is affected by the many substances water contacts during its movement through the hydrologic cycle. Water dissolves a wide variety of minerals, nutrients, and other substances from soils, rocks, and the atmosphere, and carries them in solution. The rapid increase of pollution in Dambovita County, due to the industrial processes, thermal power station and domestic sewage, modify the quality of water along the county. In this paper are presented the results of atomic absorption spectrometry (AAS) and total dissolved solids (TDS) measurements of water samples from two affluent of Arges River and underground waters from the same zone of Dambovita County, Romania. All samples of water were collected in successive three weeks and were measured with a GBC Avanta Atomic Absorption Spectrometer (AAS) from Valahia University of Targoviste laboratory. The TDS measurements were performed at the sampling sites by means of HACH CO150 conductometer, three weeks running. The obtained results reveal an evident seasonal dynamic of quality of Arges River along the Dambovita County. Keywords: AAS, TDS, pollution, water quality 1 Introduction In Dambovita zone, the significant impact of the extract of petroleum and natural gases activities for environment is the salting phenomenon and pollution with petroleum of the surface and subterranean waters and of the soil. The most impurities from the potable water are dissolved anorganics salts. So, TDS parameter is relevant. The potable water sources which contain big concentrations of anorganics salts (over 1000 mg/l) are improper because the anorganics salts. These types of waters are improper for agriculture because of the negative effect on the plants [1]. In this paper are presented the analysis of some surface and underground waters from the River basin of Arges Vedea, which is a zone that is affected by the activities of oil extraction. The analysis techniques that are used in our studies of the pollution of waters with heavy metals have been: determination of the electric conductivity and TDS and atomic absorption spectrometry, all this analysis have been effectuated in the research laboratories of University Valahia of Targoviste. 2 Experimental determinations The water probes from Potop River have been collected from the same place at an interval of one week. In figure 1 are shown the collecting points of water probes. In the monitoring action of the zones affected by the oil extraction, we sampling underground water probes from 4 fountains from the same zone a zone with a big impact of SC PETROM, deposits Ludesti Hulubesti. 93

92 JOURNAL OF SCIENCE AND ARTS Figure 1. Collecting points of water samples 2.1 The determination of TDS in water samples Determination of TDS of water samples have been realize at the sampling places with the portable conductometer HACH CO150, with the conductometric cell have platinum electrodes. The experimental results have been correlated with the heavy metals concentration that was determinate in the water probes. 2.2 The analysis of water probes with the atomic absorption spectrometer To determine the concentrations of Fe, Zn and Cd we used the atomic absorption spectrometry method [2]. We use the calibration method for determination of elemental concentration in sample: some sample solutions at known concentrations (three or more) are measured for draw the calibration curve of concentration like a function of absorbance. The absorbance of one sample unknown is determinate by extrapolation in the calibration curve. The standard sample is prepared so his concentration will include the concentration value of the unknown sample The samples have been analyzed with the atomic absorption spectrometer with flame AVANTA GBC from the University Valahia of Targoviste. This system is used to elemental analysis of a variety samples (solids, liquids). It can determinate almost all chemical elements. It has the measurements limit at 1 ppm. The water samples have been kept in polypropylene bottle, filtrated by filter paper and the level of ph has been increase at 4 5 with HNO Results and discussion The results that have been obtained by TDS measurements of the surface water samples are shown in table 1 and of the underground water samples are shown in table 2. 94

93 JOURNAL OF SCIENCE AND ARTS Sample location Potop 1 Potop 2 Potop Hulubesti Valea Hotarului Valea Gaterului Valea Banului Potocelu Prodila Table 1. Values of TDS in surface water probes, TDS (mg/l) TDS(mg/L), Average TDS(mg/L), Average TDS(mg/L), Average TDS (mg/l) POTOP 1 POTOP 2 POTOP HULUBESTI VALEA HOTARULUI VALEA GATERULUI VALEA BANULUI POTOCELU PRODILA Figure 2. TDS variation in surface water probes From the obtained dates it can be seen that in the Potop Hulubesti collecting point have been obtained the biggest values of TDS. In figures 2, 3 are shown the diagrams of TDS in all the water samples. 95

94 JOURNAL OF SCIENCE AND ARTS Table 2. Values of TDS for fountain water samples Samples TDS(mg/L), TDS(mg/L), TDS(mg/L), Fountain Fountain Fountain Fountain (mg/l) TDS 4/3/2008 4/10/2008 4/17/ FOUNTAIN 1 FOUNTAIN 2 FOUNTAIN 3 FOUNTAIN 4 Figure 3. TDS variation in fountain water samples As we can see in figure 3, the high value of TDS was obtained for water sample from fountain no. 2 ( mg/l), but not exceeded the maximum admissible value (1000 mg/l) [6, 7]. Standard values of surface water for class 1, 2 and 3 are presented in the Table 3 which means: class1 - very clean fresh surface water, ecosystem consummation where basic organisms can breed naturally, resources used for consumption which requires ordinary water treatment processes before use; aquatic organism of conservation, fisheries an recreation; class 2 - medium clean fresh surface water sources used for: consumption, but passing through and ordinary treatment process before use and agriculture; fairly clean fresh surface water resources used for consumption which requires special water; treatment process before use and industry [3, 4, 5]. Table 3. Standard values of surface water for Class1, 2 and 3 Element Class 1 Class 2 Class 3 (mg/l) (mg/l) (mg/l) Ca Fe Cu Ni Mn Zn Cd Cr Pb Na

95 JOURNAL OF SCIENCE AND ARTS The obtained results for the concentrations of Fe, Zn and Cd in surface water and fountain water samples by the atomic absorption spectrometry are shown in tables 4 and 5. Standard error was less than 5%. Table 4. The concentration of Fe, Zn and Cd in surface water measured by AAS method, mg/l Point Fe Zn Cd collection Potop Potop Potop Hulubesti Valea Hotarului Valea Gaterului Valea Banului Potocelu Prodila collected date of samples: ; 2. collected date of samples: ; 3. collected date of samples: In figures 4, 5 and 6 are shown the concentrations variation of Fe, Zn and Cd in surface water samples for a period of 3 weeks compared with the admissible value for water samples from the first class category. Figure 4. The variation of Fe concentration in surface water during April 2008 Figure 5. The variation of Zn concentration in surface water during April

96 JOURNAL OF SCIENCE AND ARTS Figure 6. The variation of Cd concentration in surface water during April 2008 Table 5. The concentrations of Fe, Zn and Cd in fountains water measured by AAS method, mg/l Samples Fe Zn Cd Fountain <LD* Fountain <LD Fountain <LD Fountain <LD *LD Detection Limit 1. collected date of samples: ; 2. collected date of samples: ; 3. collected date of samples: In figures 7, 8 and 9 are shown the concentrations variation of Fe, Zn and Cd in fountain water samples for a period of 3 weeks compared with the admissible value for water samples from the first class category. Fe concentration 1 3 mg/l Class 1 Figure 7. The variation of Fe concentration in fountain water samples during April Fountain 1 Fountain 2 Fountain 3 Fountain 4 98

97 JOURNAL OF SCIENCE AND ARTS mg/l Zn concentration Class 1, 2 and 3 Figure 8. The variation of Zn concentration in fountain water samples during April Fountain 1 Fountain 2 Fountain 3 Fountain mg/l Cd concentration Class 1, 2 and Figure 9. The variation of Cd concentration in fountain water samples during April Fount ain 1 Fountain 2 Fountain 3 Fountain 4 From the experimental data obtained we can observe that: In we have obtained the maximum values for concentrations of Fe, Cd and Zn in all water fountains samples. All obtained values are over the standard values for quality of potable water. The smallest concentration values of Zn and Cd have been recorded in and are over the maximum admitted value. 4. Conclusions The big concentration of Fe in water is due to the precipitations from the third week. The water with a big concentration of Fe and Zn don t cause big problems for health but is unpleasant at taste, appearance and smell. The rivers from the zone where are presents the activities of oil extraction, are affected by a pollution with heavy metal and by a salting phenomenon. We observe that the values of heavy metal concentration are over the maximum admissible values. The obtained results reveal an evident seasonal dynamic of quality of Arges River along the Dambovita County. 99

98 JOURNAL OF SCIENCE AND ARTS References [1] Davies, B.R., Walker, K. F., The ecology of river systems, 1986, John Wiley&Sons, New York, [2] Sperling, M. B, Welz, B., Atomic Absorbtion Spectroscopy, Weinheim, Wiley-VCH. ISBN [3] Dix, H.M., Environmental Pollution, John Wiley, Chichester, , (1981). [4] Koski-Vahala, J., Hartikainen, H., Tallberg, P., Journal of Environmental Quality, 30: , [5] National standards STAS 4706 for surface waters, quality categories and conditions. [6] Singh, T, Kalra, Yp., Journal of the American Water Works Association, [7] Stihi, C, Popescu, I.V., Apostol, S., Vlaicu, Revista de Chimie, 58(12), , [8] Stihi, C, Popescu, I.V., Bancuta, A., Stihi, V., Vlaicu, Gh., Rom. Journ. Phys., 50, Nos. 9 10, , Manuscript received: / accepted:

99 JOURNAL OF SCIENCE AND ARTS A STUDY OVER ELECTROCHEMICAL DEPOSITION OF NANOSTRUCTURES ANDREEA STANCU 2, DORIN LEŢ 1 1 Valahia University of Targoviste - Multidisciplinary S&T Research Institute, Dambovita 2 Valahia University of Targoviste Materials Science PhD School POS-DRU program, Dambovita Abstract: This paper treats the problem of nanostructured materials manufactured through electrodeposition and electroless processes for industrial applications in order to demonstrate the vast richness promised by electrochemistry. The fundamental concern that dictate the control of the reaction rate and ensuing thickness and composition of the deposit is argued in relation to compositionally modulated nanolayers in a variety of architectures: core-shell nanoparticles, nanowires, pillars, tubes, and composite materials. The electrochemical processing technique is, in some cases, an alternative to other techniques like chemical vapor-deposition CVD methods, but it also finds an exclusive niche for the deposition of nanostructured materials of high aspect ratio geometries. Keywords: electrochemical deposition, nanomaterials, MCM system 1. Introduction The process of electrochemical deposition is old and had become an integrated technique for the fabrication of nanosize devices. Electrodeposition, electroless deposition, and displacement reactions can be used to deposit metals, alloys, and metal-matrix composite materials, governed by electrochemical reactions. Nanomaterials prepared by electrodeposition are differentiated by at least one dimension in the nanometer range, they include nanostrutured thin-film multilayers, nanowires, nanowires with nanometric layers, nanotubes, nanosize particles embedded into metal matrices, and discrete or pressed nanoparticles with metal shells. Given the nanometric nature of the structures, the physical properties of nanomaterials can be considerably different from bulk materials having the same layout. Electrochemical deposition consists in the reduction of metal ions with an impressed current or potential. On the other hand, electroless and displacement processes occur without an impressed current. Magnetic compositionally modulated multilayered thin films and nanowires are typical examples of electrodeposited nanostructured materials, while coatings around nanoparticles and lining nanoporous walls have been carried out by displacement and electroless processes. Electrochemical deposition had become known not only as a costeffective alternative to vapor-deposition methods for thin films, but also as preferred method to deposit nanostructured layers onto irregular substrates and into deep indentations, allowing the fabrication of materials such as nanotubes and nanowires. 2. Multilayers compositionally modulated Multilayer compositionally modulated MCM materials are synthesized by deposition, in a sandwich-like manner, of alternate layers having different compositions. One of the first examples of MCMs was demonstrated by Blum in 1921, when alternate Cu and Ni layers, tens of microns thick, were deposited from two different electrolytes. The resulting Cu/Ni multilayer improved the tensile strength of the electrodeposit compared to elemental copper deposits. Today, MCM materials of interest in other systems include not only mechanical properties (e.g., fracture and tensile strength, hardness) but also magnetic properties. Magnetic multilayers separated by paramagnetic layers on the nanoscale give rise to giant 101

100 JOURNAL OF SCIENCE AND ARTS magnetoresistance GMR, characterized by a decrease in resistance (>1%) with an applied magnetic field, as the magnetic domains change from an anti-ferromagnetic alignment to one that is in the direction of the magnetic field. Cobalt copper, multilayer, Co/Cu, is one example that has been thoroughly examined in literature, both in the form of a thin film and nanowire. Figure 1 shows the MR response of an electrodeposited Co/Cu multilayer film with 1000 repeat bilayers, deposited from a ph 3 electrolyte containing 0.005M CuSO 4, 0.5M CoSO 4, and 0.54M boric acid. The copper was deposited with a low-current density of 0.2mA/cm 2 and the cobalt at a high-current density of 20mA/cm 2, under quiescent conditions. Figure 1. Co/Cu multilayers with 1000 bilayers, deposited on Si (111), with 2.5 nm Co layer thickness and variable Cu layer thicknesses: (a) 1 nm, (b) 1.5 nm, (c) 2 nm. Resistance decrease is dependent upon a diversity of factors, including the choice of electrolyte, bilayer number and layer sizes, as represented in Figure 1. Higher room temperature GMR-percentage changes have been reported with thinner electrodeposited films having less than 100 bilayers, although the values are still 2 4 times lower than vapordeposited counterparts. Multilayer electrochemical deposition can be carried out using a single or dual bath approach. Dual-bath electrodeposition requires that either the substrate or electrolyte be transferred during the deposition of each layer. To simplify the process, a single bath is more desirable and the compositional modulation of the layers to be obtained by current pulsing. In Figure 2 the steady-state partial current densities are represented for two depositing metals, M 1 and M 2 at the working electrode from a single electrolyte. The total polarization is shown as the dotted line. At point A, the reduction of M 1 occurs. At more negative potentials, such as at point B, both M 1 and M 2 deposit simultaneously. At point B, the rate of deposition of M 2 is larger than that of M 1, as a result of which the deposit is rich in the second component. In order to obtain layers with the highest purity, it is desirable for the rate of deposition of M 1 to be as small as possible, while depositing the M 2 -rich layer. The trade-off, however, is that the low M 1 rate extends the processing time. Either the potential or current can be modulated in a square-wave pulse to fabricate the multilayers. Scanning tunneling microscopy of cleaved cross-sections of lead-thallium-oxygen nanometer size deposits showed that potentiostatic pulses resulted in more discrete layers than galvanostatic pulses. In order to limit the reaction rate of the more noble species, its concentration in the electrolyte is maintained at a value that is orders of magnitude lower than the other metal ion 102

101 JOURNAL OF SCIENCE AND ARTS species. In the example given in Figure 1, the electrolyte composition of Cu(II), M 1, is 100 times lower than Co(II), M 2, which permits a layered deposit when the current is pulsed. Thus, at point B, the reaction rate of M 1 reaches a mass-transport-limiting current density, i lim. At steady state, an estimate of the limiting current density can be readily determined, assuming that diffusion is the dominant mode of transport with a Nernstian boundary layer approximation, where D (cm 2 /sec) is the diffusion coefficient of the reacting species, C b (mol/cm 3 ) its bulk concentration, δ (cm) the mass transport boundary layer thickness, n (equiv/mol) the number of electrons transferred, fixed by the reaction under consideration, and F (As/equiv) the Faraday's constant (96485 C/equiv). The boundary layer thickness is the most difficult to determine as it is dependent on the hydrodynamic environment surrounding the electrode. It is typically several hundred microns, while it has only tens of microns in well-mixed electrolytes. (1) Figure 2 - Two electrodepositing metals with disparate reaction rates. M 2, the less noble reactant, is in excess in the electrolyte, and generally deposits under kinetic control during multilayer fabrication. Deposition far from equilibrium can be approximated by a Tafel equation: this requires knowledge of two kinetic parameters, the exchange-current density, i 0 (A/cm 2 ) and the cathodic-transfer coefficient, α c - (dimensionless). These values have been reported for a wide variety of reactions; however, i 0 is dependent on the species' activity, i.e. concentration, and is highly specific to a particular electrolyte. The surface overpotential, s - (V), is the polarization away from the equilibrium potential, when no concentration gradients occur. If the polarization is large, kinetic control of the M 2 reaction gives way to a transport control as described by Equation 1, and depicted at point C in Figure 2. Deposition, however, is not desirable at this region because of the loss in efficiency; both species are dominant by mass transport, due to the development of rough-surface deposits and excess-side reactions from the solvent. (2) 103

102 JOURNAL OF SCIENCE AND ARTS The kinetic behavior is differentiated by a uniform concentration distribution of reactants near the electrode surface. During the multilayer fabrication, if the much noble species, M 1, is deposited below point A (Figure 2), under kinetic control, and then is codeposited at point B, under mass transport control, there will be a change in the noble metalconcentration gradient that is time-dependent, resulting in a compositional gradient within the alloy layer. Classical descriptions of a single reacting species under a time-dependent diffusion control for a galvanostatic or potentiostatic pulse have been developed by Sands and Cottrell. Due to the enhanced compositional gradient of a reactant during pulsing, its limiting current is subsequently enhanced during the pulse. An expression for the larger pulse limiting current density, i p, compared with the DC, unpulsed, counterpart (Equation 1) for a galvanostatic pulse, where r is the ratio of the first current-density pulse (point A, Figure 2) to the second value (point B, Figure 2), a (= 2 D/4δ 2 ) the diffusion parameter, the second period duration, and θ the cycle time. Equation (3) predicts that for the CoCu/Cu system, the amount of Cu codeposited with Co in the alloy layer of a multilayer will be larger than Cu co-deposited with Co as a DC plated bulk alloy. A portion of the anodic component of the partial-current density from the less noble metal species is shown in the lower left-hand corner of Figure 2. Depending on the kinetics of this reaction, there can be a simultaneous displacement of M 2 deposited metal by the more noble species, M 1, occurring at the start of the M 1 pulse during multi-layer electrodeposition. The displacement for the CoCu/Cu system: is controlled, so that even when the impressed current is cathodic (negative), there can still be a small anodic contribution, given by the following general expression: (3) (4) (5) where α a is the anodic transfer coefficient. The outcome of displacement reactions occurring during the processing of the multilayer not only results in a change in the composition, but can also exaggerate the concentration gradients at the layer interfaces. To suppress displacement reactions, strategies such as adding additives to alter the kinetic parameters in Equation 6 or alloying the magnetic layer with a corrosion resistance material can be considered. Larger GMR values were observed with a small amount of Ni incorporated into the Co-alloy layer as shown in Figure 3. (6) 104

103 JOURNAL OF SCIENCE AND ARTS Figure 3. Giant magnetoresistance of two comparable electrodeposited multilayer films with one film containing 3.5 wt% Ni in the Co alloy layers, with 2000 bilayers The partial-current density Equations 1, 3 and 6 are vitally important to the design and prediction of the layer at composition, x, and thickness, Δ, in conjunction with Faraday's law, which relates the mass plated to the charge passed, (7) for species j with atomic weight M, density ρ, and time of deposition, t. In the case of electrolytes containing mixtures of iron group (Co, Ni, Fe) ions, the kinetic information should be ascertained from the alloy electrolyte rather than from elemental, single-metal deposition, on account of the anomalous-co-deposition behavior. Anomalous co-deposition refers to the preferential deposition of the less noble metal species, and has been widely reviewed with regard to bulk alloy electrodeposition. 3. Nanomaterials 3.1. Nanowires To minimize concentration gradients within a multilayer, deposition into recessed geometries is inherently agreeable to improved electrodeposits. Template syntheses in nanoporous membranes are prepared in anodic-aluminum oxide, polycarbonate, and diblockcopolymer membranes. Nano dimensional layers within a nanowire are of particular interest for GMR studies when the current flow is perpendicular to the plane of the layers, referred to as CPP-GMR. In contrast, current-in-plane giant magnetoresistance CIP-GMR is the preferred mode of measurement for thin-film multilayers because of the extremely small values of resistance in the perpendicular direction, which precludes accurate and easy analysis. In CIP- GMR, the characteristic scaling length is the electron mean-free path, typically a few nanometers. However, for CPP-GMR, the critical length scale is the spin-diffusion length, which is generally larger than 10nm, and thus larger multilayer sizes can be tolerated. (8) 105

104 JOURNAL OF SCIENCE AND ARTS In Figure 4 a transmission electron micrograph image of a CoNiFeCu/Cu nanowire is represented, showing multilayer s that are evident near the edge of a 200-nm diameter wire. The layer sizes of the Co-rich alloy and Cu layers are 4 and 2nm, respectively. The CPP- GMR of an array of nanowires is compared with a thin-film CIP-GMR with comparable layer sizes and chemical composition. While the GMR value is about the same, the magnetic field sensitivity clearly differs. Nanowires of multilayers have also been studied in literature for a variety of other systems. For example, room-temperature CPP-GMR of Co/Cu multilayer nanowires was reported to be 14-15% and 20%. The CoNi/Cu nanowires have been reported as possessing significantly larger GMR, 55% at room temperature. These features render magnetic nanowires of interest for high-density magnetic recording. Earlier researches with Fe deposition into alumina-nanoporous templates also reported similar findings with an easy axis parallel to the wires with enhanced coercivity. Magnetic-alloy nanowires from CoFe, CoNi, NiFe, and CoNiFe have been reported as being not layered. Figure 4. Thin film CPP-GMR and CIP-GMR of a CoNiFeCu/Cu 200nm nanowire 3.2. Nanotubes They represent another type of nanoscale architecture fabricated inside nanoporous membranes for advanced catalytic and sensory materials. Electro-less deposition of metals inside the pores of membranes requires use of a chemical reducing agent to deposit the metal onto a surface from solution. A sensitizer which binds to the pore wall is usually used at the beginning. The sensitized surface is activated by exposure to a catalyst, resulting in the formation of discrete nanoscopic-catalyst particles. Finally, the catalyst-covered surface is immersed in the electrolyte containing the ions of the metal to be deposited and the reducing agent, for a surface-constrained deposition on the pore walls. Therefore, at the pore surface, the cathodic reaction (metal deposition) is equal to the anodic reaction (oxidation of the reducing agent) without any external power supply. Dissolution of the membrane can release an array of deposited tubes if needed. Nanowires can also be achieved in this fashion by immersing the nanoporous membrane for longer periods of time. Unlike electrochemical deposition, electro-less method provides no control over modulating the composition for alloy deposition. Also, the electro-less deposition cannot control the length of the nanowires or the nanotubes. By knowing the rates of each reaction there can be determined the rate of electro-less deposition. Figure 5 schematically represents the partial current density of the cathodedeposition reaction (M n+ + ne - M), and the anodic reaction of the reducing agent (Red Ox + ne - ). The equilibrium reduction/oxidation potential of the metal deposition, equilibrium 106

105 JOURNAL OF SCIENCE AND ARTS potential, E 1, must be more noble, thus larger, than the reducing agent equilibrium potential, E 2, thus E 1 > E 2. The schematic in Figure 5 is often referred to as an Evan's diagram, depicting the absolute value of the partial current density with potential in order to view both the anodic and cathodic reactions in the same quandrant of the graph. A logarithm scale is used for the current-density axis, since kinetic expressions are exponential in nature (i.e., Equations 2 and 6), and thus these regions will appear as potential-dependent straight lines. Potential-independent regions identify mass-transport control. The intersection of the two partial-current densities provides the resulting reaction rate, or current density of the electroless process at a potential intermediate between the two equilibrium half-cell potentials. The Evan's diagram illustrates this concept of the "mixed-potential theory" where the resulting current density and potential of the electro-less process depends on the kinetics or transport limitations involved in both the "mixed" anodic and cathodic systems. Pt and Ni tubes and wires electrodeposition has been observed in aluminum oxide membranes, the tube formation was a result of a nonuniform electric field concentrated at the pore wall. CoNiCu tubes were deposited from a boric acid electrolyte at a constant potential at efficiencies lower than 50%. An increase in the current efficiency, by a change in the electrolyte composition, resulted in the wire formation. Thus, gas-evolving side reactions in tube formation are suspected to be an additional controlling feature. Figure 5. Schematic of the Evan's diagram showing the electroless process Pillars The difficulty of measuring a single nanowire and the random array of the nanowires represents a disadvantage of nanowire deposition of multilayers. An alternative architecture is micro-size pillars that still render GMR to be measured in the CPP mode. Using vapor-deposition techniques etching through thin films of multilayers, it had been shown in literature, that the fabrication of CPP-GMR pillars is possible. Optical lithography and reactive-ion-etching techniques are also used to fabricate pillars of vacuum-sputtered Fe/Cr multilayers, as well as molecular beam epitaxy evaporation deposition of Co/Cu multilayers. Over 50% CPP-GMR was observed and the researchers showed that the CPP- GMR mode measurements were often larger than measurements in the CIP mode. Multilayered material electrodepostion into a lithographic pattern, rather than by etching it, can offer a cost-effective alternative. In addition, the deposition of materials into deep recesses is problematic with vapor deposition, line-of-sight type of techniques. The advantage of using electrodeposition for pillar fabrication is the same as for nanowire generation; it can offer conformal deposition in deep recesses. On planar electrodes in unagitated electrolytes typical diffusion layer thicknesses are of the order of 100 Lim. Thus, electrodeposition into deep recesses >100 μm, will extend the 107

106 JOURNAL OF SCIENCE AND ARTS nonsteady-state regime for diffusion-controlled reactions, compared with planar electrodes. The convective hindrance of the recess further increases the diffusion boundary layer thickness by roughly the size of the recess depth. An estimate of the change of the limitingcurrent density with time of a species inside a recess, where the outside pore-mouth region of the recess is well mixed, is given in two limits. Figure 6. Array of CoCu microposts fabricated by x-ray lithography. A short-time approximation for a reduction reaction follows where L is the recess depth. The leading term describes the Cottrell behavior on an unrecessed surface. At long times, the limiting current density is given as (9) and at steady state this reduces to Equation (1) where δ = L. From Equation 10 can be noticed that for typical metal ions, the time to achieve steady state in a recess hundreds of microns deep can be several seconds to minutes. During multilayer pulse plating, the diffusion-controlled species, often Cu, is depositing under a nonsteady regime at the start of deposition, during both the magnetic alloy deposition layer and the elemental Cu layers. As the deposition proceeds, the limiting-current density of the transport-controlled species will increase, due to a shortened boundary-layer thickness. In addition, the transient change in its concentration near the electrode surface is also altered so that steady state is achieved sooner. Regardless of how the deposition is controlled, galvanostatically or potentiostatically the consequence of this dynamic effect will result in more of the transport-controlled species in the second, magnetic layer. 4. Considerations Fabrication of nanomaterials via electrochemical deposition is a wide area, and only a short overview has been presented in this study just to list the leading topics and challenges in this field. Emphasis has been placed on MCM systems area of industrial interest for a wide variety of applications. Compositionally modulated multilayer s, having nanosize dimensions, can be deposited as thin films, nanowires, pillars, and nanotubes and are governed by the (10) 108

107 JOURNAL OF SCIENCE AND ARTS electrodeposition process. Compositional control of the process requires knowledge of the kinetic and mass-transport regions of the depositing metals. Nanoparticles used as discrete particles may rely on electro-less fabrication methods for the generation of shells around the nanoparticle core. Using template synthesis methods, such as alumina-porous membranes and track-etched polycarbonate porous membranes, to electrochemically deposit metal nanoparticles inside the pores has become popular in the recent years. These deposits have been studied in the context of a wide spectrum of scientific goals ranging from catalysis to magnetic properties and magnetic-data storage. Attention has also been focused on the application of small metal particles in surface-enhanced spectroscopy, photocatalysis and selective solar absorbers. Studies with atomic absorption have shown that iron, nickel, cobalt, and gold particles have equivalent areas per volume, with particle radii in the range 3 to 5 nm. Magnetic measurements on iron, nickel, and cobalt films reveal them to be highly anisotropic with magnetization perpendicular to the surface of the film. The unusual optical absorption of noble-metal nanoparticles such as copper, silver, and gold embedded in a dielectric medium such as alumina renders them of interest for optical applications. References [1] Weihnacht, V., Peter, L., Toth, J., Padar, J., Kerner, Z., Schneider, C.M., Bakonyi, I., J. Electrochem. Soc., 150: C507-C515, [2] Ohgai, T., Hoffer, X., Fabian, A., Gravier, L., Ansermet, J.P., J. Mater. Chem., 13: , [3] Chassaing, E., J. Electrochem. Soc., 148: C690-C694, [4] Kelly, J., Kern, P., Landolt, D., J. Electrochem. Soc., 147: , [5] Nabiyouni, G., Schwarzacher, W., Rolik, Z., Bakonyi, I., J. Magn. Magn. Mater., 253: 77-85, [6] Zhang, J., Moldovan, M., Young, D.P., Podlaha, E.J., Proceedings of the 205th Meeting of the Electrochemical Society on Electrochemical Processing in ULSI and MEMS, San Antonio, TX, [7] Huang, Q., Podlaha, E.J., J. Electrochem. Soc., 151: C119-C126, [8] Khan, H.R., Petrikowski, K., Mater. Sci. Eng. C, 19: , [9] Yoo, W.C., Lee, J.K., Adv. Mater., 16: , [10] Davis, D., Podlaha, E.J., Electrochem. Solid-State Lett.,8: D1-D4, [11] Platt, M., Dryfe, R.A.W., Roberts, E.P.L., Electrochim. Acta, 49: [12] Platt, M., Dryfe, R.A.W., Roberts, E.P.L., Chem. Commn., 20: , [13] Johans, C., Konturri, K., Schiffrin, D.J., J. Electroanalyt. Chem., 526: [14] B. Ravel, E.E. Carpenter, and V.G. Harris, J. Appl. Phys., 91: , [15] Lin, J., Zhou, W., Kumbhar, A., Wiemann, J., Fang, J., Carpenter, E.E., O'Connor, C.J., J. Solid State Chem., 159: (2001). [16] W.L. Zhou, E.E. Carpenter, J. Lin, A. Kumbhar, J. Sims, and C.J. O'Connor, Eur. Phys. J. D: At., Molecular Optical Phys., 16: , [17] Carpenter, E.E., Sims, J.A.,Wienmann, J.A., Zhou, W.L., O'Connor, C.J., J. Appl. Phys., 87: , [18] Salazar-Alvarez, G., Mikhailova, M. Toprak, M., Zhang, Y., Muhammed, M., Materials Research Society 2001 Fall Meeting Symposium Proceedings, Vol. 707, , [19] Son, S.U., Jang, Y., Park, J., Na, H.B., Park, H.M., Yun, H.J., Lee, J., Hyeon, T., J. Am. Chem. Soc., 126: , [20] Park, J.-I., Kim, M.G., Jun, Y.-W., Lee, J.S., Lee, W.-R., Cheon, J., J. Am. Chem. Soc., 126: , [21] Guo, Z., Kumar, C.S.S.R., Henry, L.L., Doomes, E.E., Hormes, J., Podlaha, E.J., J. Electrochem. Soc., 152: D1-D5,

108 JOURNAL OF SCIENCE AND ARTS [22] Lozano-Morales, A., Podlaha, E.J., J. Electrochem. Soc., 151: C478-C483, [23] Panda, A., Podlaha, E.J., Electrochem. Solid-State Lett., 6: C149-C152, [24] Erler, F., Jakob, C., Romanus, H., Spiess, L., Wielage, B., Lampke, T., Steinhauser, S., Electrochim. Acta, 48: , [25] Wang, T., Kelly, K., J. Micromech. Microeng., 15: 81-90, Manuscript received: /

109 JOURNAL OF SCIENCE AND ARTS CARACTERIZATION OF THE IONIC NITRING IN THE PROCESS APPLICATION ON 12% Cr STEELS FOR TOOLS AND COLD DEFORMATION DEVICES MANUFACTURING GHEORGHE VLAICU 1,3, IULIAN BANCUTA 1, VALERICA Gh. CIMPOCA 1, IOANA DULAMA 1 ANCA GHEBOIANU 1, MARIANA BAHRIM 4, ION V. POPESCU 1,2 1 Valahia University of Targoviste, Multidisciplinary Research Institute for Sciences and Technologies, , Targoviste, Romania 2 Valahia University of Targoviste, Faculty of Sciences and Arts, Sciences Department, , Targoviste, Romania 3 S.C. MECHEL Targoviste S.A., , Targoviste, Romania 4 High School Ovidius Constanta, Constanta, Romania Abstract: The ionic nitriding found a good application in the treatment of cold deformation devices that, because of the hard work conditions, must stand complex loads and show superior phzsicomechanical properties. For example, the working rolls of plate cold rolling mills must stand loads with specific tensions of N/m 2, must present hardness of 60 HRC, a very good toughness (a high rotatingb bending fatigue strength), must stand to outstanding dynamic tests (impacts, torsion) and must present a high wer hardness and outstanding tribological properties. The good behaviour in exploitation of these devices is conditioned by severe adjustments, high dimensional accuracy and the absence of clearance in gear system, reason for that the dimensional restrictions to the manufacturing of deformation devices are very exigent. So, for cold working are used special high-speed steels, maraging steels and very often (being more accessible) steels with 12% Cr. This article analyses the possibility of applyng the ionic nitriding to steels with 12% Cr, referring to VMoC120, pursuing the way of hardening by this supplemental treatment the working surface of the deformation devices, so that they meet the quality requirements. Keywords: VMoC120 steel, ionic nitriding, tribological properties 1. Introduction The final mechanical properties of the cold working devices are the result of the technological interdependent factors, which interfere on the technological flow: processing, casting, forging, preliminary heat treatment, cold treatment, secondary heat treatment, rectifing and surface hardening and tempering by ionic nitriding process applications. The studied steel has the next chemical composition: C Mn Si Cr Mo V S P 1,63 0,42 0,30 11,5 0,51 0,29 0,014 0,01 The hardening is made at o C and tempering at the temperature of o C. After this treatment it is obtained a hardening structure with HRC hardness. 2. Experimental part Samples with = 15mm and h = 8 mm where token for establishing of VMoC120 steel ionic nitriding process parameters. After the secondary heat treatment, samples are subjected to the ionic nitriding process in the experimental equipment of the Central and Research Laboratory of the S.C. MECHEL Targoviste S.A. Fig. 1. The nitriding process parameters: discharge power, gaseous mixture ratio (x = nitrogen/hydrogen) nitring temperature, exposing time were varied between imposed limits by the initial martensitic structure that must no be destroyed and by the final nitrided structure that must be obtained. 111

110 JOURNAL OF SCIENCE AND ARTS The final structure must have high hardness and high tribological properties in combined layer and tenacious diffusion layer. Figure 1. Bloc scheme of the experimental installation. The micro hardness method for establishing of the nitriding layer thickness is used. The curve HV 0.3 vs. distance by the sample edge was piked up. The d value (from the micro hardness profile) at which the layer hardness is: H V H 2,72 is considered to be the layer thickness. Where: H VM is the maximum hardness of the layer and H VB the basis material hardness (core). The way in wich the combined layer mean hardness varies with nitriding temperature was analysed within the framework of the experiments made for the establishing steels (with 12% Cr) ionic nitriding technological parameters. The curves H V f T are shown in Fig. 2. VM H VB H VB Figure 2. Dependence H V f T Figure 3. Variation of hardness vs. distance 112

111 JOURNAL OF SCIENCE AND ARTS The micro hardness curve (the profile) vs. distance by the sample edge is presented in Fig. 3. The nitrided layer thickness variation (obtained by the above mentioned method) vs. exposing time for various temperature values is presented in Fig. 4. Figure 4. The nitrided layer thickness variation vs. exposing time for various temperature values. The formation s kinetic of layers at the nitriding of this kind of steel was pursued on samples at the following parameters: I = 12 ma, U = 1400 V, N 2 /H 2 = 1/1, T = 500 o C and exposing times t = 1h, 1.5h, 2h, 2.5h, 3h. The qualitative and quantitative structural analyse was made by X-ray diffraction. The unit normalization and ASTM card index comparison where used for qualitative analyse. The integral intensity calculation method was used for quantitative structural analyse. Only samples with the same quantity of co-existing phases where take in account and only the representative peacks for the formed nitrided phases: (200) for '' structure and (100) for ε structure were analysed. 3. Results and conclusions The next conclusions can be drawn from the experiments: - the tempering must be done at a temperature above t = 500 o C to assure for the ionic nitriding treatment (that must be done at temperatures under the tempering one) enough temperature. - the ionic nitriding made on austenitic structure of VMoC120 steel leads to the obtaining of a combined layer realized by '' and ε phases with a maximum hardness of H V0,3 = the nitriding layer thickness for various temperatures varies with the exposing time after a t law (for exposing times among 1-3h) - the mean hardness of the obtained combined layer is lessening with the increase of the working temperature: from H V0,3 =1200 at T = 500 o C to H V0,3 = 750 at 700 o C. So, the o indicated temperature to obtain a hard combined layer is T = 500 C. The next conclusions are resulted from the diffractograms analyses: - the configuration of the obtained layers at the ionic nitriding carried out on VMoC120 steel in hardened-tempered state is complex. The superficial layer contains (Fe, Cr, Mo, V) 4 N and (Fe, Cr, Mo, V) 2,3 N nitrides that together with the diffusion layer confers outstanding mechanical properties to the metal [1]. 113

112 JOURNAL OF SCIENCE AND ARTS With the increasing of the exposing time (from t = 1h to t = 3h) the ε nitriding quantity formed initially in the superficial layer decrease and the '' nitriding quantity increase enough. So, at an exposing of 2-3h it will be obtained a combined layer formed by '' nitride and a smaller quantity (percentage) of ε nitride. The ε nitride has a low hardness but shows superior tribological properties and rust-resisting properties [2, 3]. From these experimental results we can say that the ionic nitriding of VMoC120 steel must be done in the next technological conditions: temoerature T = o C, exposing time t = 2 ½-3h, gaseous mixture N 2 /H 2 = 1/1 and pressure p = 3 torr. Referrence [1] Edenhofer B., Traitement thermique, no. 80, [2] Pourprix Y., Traitement thermique, no. 141, [3] A. Chalaa, L. Chekourb, C. Nouveauc, C. Saieda, M.S. Aidab and M.A. Djouadid, Surface and Coatings Technology, 200(1-4), , Manuscript received: / accepted :

113 JOURNAL OF SCIENCE AND ARTS NANOMATERIALS PHYSICAL PROPERTIES SIMULATION AND MODELING APPLICATIONS DORIN LEŢ 1, ANDREEA STANCU 2, ZORICA BACINSCHI 2, VALERICĂ CIMPOCA 1 1 Valahia University of Targoviste - Multidisciplinary S&T Research Institute, Dambovita 2 Valahia University of Targoviste Materials Science PhD School POS-DRU program, Dambovita Abstract: Modeling of nanomaterials requires realistic description of the system across various length and time scales. In order to illustrate how atomistic modeling is being used to determine the structure, physical, and chemical properties of materials at the nanoscale, molecular dynamics (MD) simulations will be presented for nanoscale assemblies based on carbon nanotubes, diamond surfaces, metal alloy nanowires, and ceramics. Some possible applications of atomistic modeling to carbon nanotubes, diamond surfaces, metallic nanowires, and organic liquids confined to nanoscale slits and structural transitions in ceramics will be also presented. These simulations use recent developments in force fields for metals, alloys, ceramics, and various phases of carbon which are also summarized here. This gives some glimpse of the enormous role that theory and modeling is likely to play as nanoscale science becomes a central theme in the 21st century technology. Keywords: nanomaterial, Quantum Mechanics, Molecular Dynamics simulation 1. Introduction Nanotechnology, as defined in literature, is concerned with the structures, properties, and processes involving materials having organizational features on the spatial scale of nm. At this scale devices may lead to dramatically enhanced performance, sensitivity, and reliability with dramatically decreased size, weight, and cost. Indeed these scales can lead to new phenomena providing opportunities for new levels of sensing, manipulation, and control. However, being much smaller than the wavelength of visible light but much larger than simple molecules, it is difficult to characterize the structure and to control the processes involving nanomaterials. From the experimental point of view, the fundamental problem in the nanoscale region is that the units are too small to see and manipulate and too large for single pot synthesis from chemical precursors. Because it is difficult to see what we are doing at the nanoscale, it is essential to develop theoretical and computational approaches sufficiently fast and accurate that the structure and properties of materials can be predicted for various conditions (temperature, pressure, concentrations) as a function of time. In this regard, a multiscale modeling approach of the system starting from quantum mechanical modeling to describe the electronic structure and optical properties, atomistic molecular dynamics (MD) calculations to describe diffusion processes, and meso scale simulation to describe the morphology of soft materials takes prominence Multiscale, hierarchical approach Using preemptive theoretical predictions over the properties of new materials represents a particular advantage before experimental approaches. This allows the system to be designed (adjusted and refined) so as to obtain the optimal properties before the arduous experimental task of synthesis and characterization. However, there are significant challenges in using theory to predict accurate properties for nanoscale materials. Thus, a cube of polyethylene (PE) 100nm on side would have -64 million atoms, much too large for standard classical molecular dynamics (MD) and enormously too large for quantum mechanics (QM). Thus, the usage of multiscale (MSC) hierarchical strategy is better suited (Figure 1). The idea is to start with accurate first principles QM on small system (10s or 100s of atoms) at a level sufficient to describe bond breaking and formation processes (reactions). Based on the QM results, we then find force fields (FF) to replace the electrons in terms of springs. Using the 115

114 JOURNAL OF SCIENCE AND ARTS FF allows classical MD simulations with 1000s of atoms. With current methods and hardware, MD is practical up to -1 million atoms, but a million atoms of PE is a cube of only - 25 nm on a side. To treat much larger systems, it is essential to average the atoms into collective units (segments, grains, pseudoatoms). This is the mesoscale region at the heart of nanoscale technology. Progress is being made in mesoscale simulations, but the demands of nanoscale technology require many additional advances. The simulations at the mesoscale can then be used to determine the parameters for finite element grids used in continuum calculations. Figure 1. Multiscale computational hierarchy of materials simulations 1.3. Nanoscale approach Nanomaterials may include carbon nanotubes, fullerenes, dendrimers, ceramics, zeolites, semiconductors, metals, polymers, and liquid crystals. These might be in the vapor/gas, liquid or solid phase (or all three phases may be present and interacting through vapor liquid, solid liquid, vapor solid interfaces). The properties needed to predict include: Structural properties: Internal structure (bond topology, distances and angles), morphology, microstructure Mechanical properties: Vibrational modes, elastic moduli, yield limits, strength, toughness, temperature and pressure effects on mechanical properties (plasticity, yield, fracture, creep) Surface properties: Reconstruction, oxidation, adhesion, friction, and wear Transport properties: Diffusion and thermal conductivity Rheological properties: Viscosity and flow of fluids in the nanoscale regime, non- Newtonian behavior, flow and transport properties of nanoparticles to make electrorheological or magneto-rheological fluids, structure fluid interactions and their effect on transport properties, time, and frequency dependence of the flow properties. 116

115 JOURNAL OF SCIENCE AND ARTS Forecasting the structure, dynamics, and properties at the nanoscale require substantial improvements in theory (FF and simulation methodologies) and the software (the algorithms implementation to do the calculations). 2. Methods for simulating nanoscale materials 2.1. Quantum mechanics QM is the foundation for the theoretical description of materials. QM is particularly important for describing processes in which bonds are broken and formed. Only with QM accurate barrier heights and bond energies can be obtained. In most recent studies there has been remarkable progress in such first principles electronic structure methods; however, the calculations are often far too slow for studying the applications of interest to nano-technology. Quantum mechanical methods capable of giving accurate barriers for the reactions of nanoscale materials: Generalized valence bond GVB, Psuedospectral generalized valence bond PS-GVB, Multireference configuration interaction MR-CI, Gaussian dual space density functional theory GDS-DFT 2.2. Force fields For convenient calculations on large systems, its need to average over electrons from QM to obtain a FF that can describe the energy and forces in terms of atomic positions. Using results from QM we can develop FF adequate for predicting the energetics needed to simulate the structure and properties of nanomaterials. The FF must be accurate enough to obtain the correct energy differences for different phases of materials but must also describe the intermediate structures involved in phase transformations and interfacial phenomena. Standard FF generally use simple springs to represent bonds and angles in describing structures and vibrations of molecules. Such simple FF generally does not accurately describe the vibrational properties of molecules, which require cross terms and more complicated springs. The best FF are fit to the QM using the Hessian-Biased FF (HBFF) approach, which allows experimental information (about frequencies, polarizabilities, etc.) to be combined with normal mode information from QM. This HBFF approach has been used to develop accurate FF for polymers, ceramics, semiconductors, and metals. For fast qualitative considerations of new systems, generic FF is suitable for general classes of systems. This includes the DREIDING FF (for the main group elements) and the Universal FF (UFF) (all elements: any inorganic, organo-metallic, or organic molecules). In recent years, critical advances have been made in developing FF for describing: metals, where many body interactions play critical role on their physical properties; oxides, ceramics, and zeolites, where competition between ionic and covalent bonding is very important, especially in describing polymorphic phase transitions, reactions, defects, surface, and interface properties; covalent bonded system such as carbon, hydrocarbons, silicon, germanium and their behavior far from equilibrium where the description of bond breaking and formation must be included to obtain an accurate description Molecular dynamics Using FF to predict the forces, the coupled sets of Newton s equations can be solved to describe the motion of the N interacting atoms this being referred as MD. The trajectories (R i, V i ; i = 1, N) can be connected (generated by MD) to obtain macroscopic properties through the use of statistical mechanics and thermodynamics. MD simulations of hetero- 117

116 JOURNAL OF SCIENCE AND ARTS geneous nanomaterials may require thousands to millions of atoms to be considered explicitly. Accurate evaluation of the long-range interactions (electrostatic and dispersion), which decrease slowly with distance, is the most time-consuming aspect for MD simulations of such large systems. This cost is order (N 2 ) for N particles. Thus, a system of 10 million atoms requires the evaluation of terms each time step. The standard approach to simplifying such calculations, for finite systems, has been to ignore the interaction beyond some nonbond cutoff. However, for one million particles this requires maintaining an enormous nonbond list and also leads to errors orders of magnitude too large. For periodic systems such cutoffs lead to unacceptable errors, requiring Ewald approaches which require Fourier transforms. This leads to a scaling of N 1.5, totally impractical for systems with million atoms. Because nanoscale simulations require simulations of millions of atoms, methods and optimized parallel computer programs (MPSim) were developed for efficient high capacity MD. Special features include: Cell Multipole Method (CMM) which dramatically reduces the cost of long-range Coulomb and van der Waals interactions while retaining high accuracy. The cost scales linearly with size, allowing atomic-level simulations for million atom systems. Reduced Cell Multi-pole Method (RCCM) which handles the special difficulties with long-range Coulomb interactions for crystals by combining a reduced unit cell plus CMM for interaction of the unit cell with its adjacent cells. The cost scales linearly with size while retaining high accuracy, allowing simulation of crystals having a million atoms per unit cell (the major use is for models of amorphous and semi-crystalline materials). Newton Euler Inverse Mass Operator method (NEIMO) for internal coordinate dynamics (e.g., treats torsions only). This allows the solution of the dynamical equations for internal coordinates without inverting the mass tensor (moment of inertia tensor). The cost of NEIMO is linear in the number of degrees of freedom and small compared to other costs for million atom systems. More recently we also developed a new constrained force algorithm (CFA) for massively parallel MD simulation of polymers and dendrimers. Steady state MD methods are used to simulate nonequilibrium processes such as friction and wear in diamond, metals, and metal oxides. Here, the external work is dissipated through material and coupled to a thermal bath using Langevin equation. 3. Applications of modeling and simulation for nanomaterials To summarize some recent applications in nanoscale systems illustrating the role of new developments in simulation technology: 1. Characterization of SWNTs with accurate (QM derived) FF using MD. 2. MD studies of alkali doped single-walled nanotubes. 3. Plastic deformations and mechanical behavior of multi-walled nanotubes. 4. MD Simulation of Friction and Wear Processes on Diamond. 5. MD Simulation of Friction and Flow Process for iron oxide slabs separated by 8 nm covered by a SAM and lubricated with n C 16 H Plastic deformation behavior of metallic nanowires. 7. Phase behavior of ceramics under compressive loads. Single-Walled Carbon Nanotubes energetics and structure properties Carbon nanotubes were discovered in 1991 by Iijima. Since then, there have been many advances in synthesis, in characterization, and in the theoretical understanding of such nanotubes. The novel mechanical and electronic properties of these nanotubes suggest many applications to nanotechnology. The single-walled carbon nanotubes (SWNT) are the simplest carbon nanotubes which and they were discovered simultaneously by the Iijima group and an IBM-Caltech team. These SWNT, which can be regarded as a graphite sheet rolled-up into a cylinder, show 118

117 JOURNAL OF SCIENCE AND ARTS remarkable mechanical and electrical properties. They present tremendous potential as components for use in nanoelectronic and nanomechanical device applications or as structural elements in various composite materials. Tightly bundled linear ropes of SWNT are expected to have remarkable mechanical properties, as well as superior electronic and magnetic properties. Various levels of theory have been used to characterize properties of the SWNT. This includes classical molecular mechanics (MM), lattice dynamics, MD, tight binding QM, and ab initio QM methods. Assessing the mechanical stability of SWNT, tubes with three different chiral forms (n, n) armchair, (n, 0) zigzag, and (2n, n), must be taken into consideration. Thus for diameters less than 60 Å it was demonstrated that circular SWNT are most stable, but that larger SWNT collapse into a shape in which the opposite walls in the middle section are 3.4 Å apart (the van der Waals distance) while each end has a nearly circular diameter of ~10.7 Å. To mimic long isolated nanotubes, periodic boundary conditions in the c-direction (tube direction) have to be imposed. To eliminate the inter tube interactions, the cell parameter a and b must be set as 50 times of the circular tube diameter. Energy and structural optimizations can be resolved using MPSim. To extract mesoscale parameters characterizing the basic energetics of tubes, the tube can be approximated as a membrane with a radius of curvature R and a bending modulus of κ. Using continuum theory, a tube with wall thickness a and length L has an elastic energy stored of: thus, the energy per atom becomes: (1) (2) where N is the number of carbon atoms per slab and E 0 is energy per carbon atom for tubes with R = (i.e. flat sheets). Letting, where ρ is the number of carbon atoms per unit area of tube wall, we obtain: The armchair SWNT is expected to have the lowest energy for a growing exposed edge. Good crystals of bucky tubes have not been reported experimentally. However, the mechanical properties can be predicted for crystalline (n,n) armchair, (n,0) zigzag, and (2n,n) chiral tubes. These all have similar cross section radii. The MD and MM studies lead to a hexagonal closest packing as the most stable form for all three forms. (3) K-doped SWNT crystals the structure Development of methods to control the catalytic synthesis of SWNT to form ordered ropes containing 100s 1000s of tubes gives hope for developing structures useful for new generations of nanoscale devices. Recent reports that these SWNT ropes can be doped to form metallic conductors give further hope for interesting devices. There is no data on the structure for such doped systems. To provide this data the predicted minimized crystal structure for armchair (10, 10) SWNT is used. Supposing up to 6 independent SWNT per unit cell and appropriate numbers of K atoms distributed in various ways to which 20 ps of MD is applied to equilibrate the system and quench the structures by 119

118 JOURNAL OF SCIENCE AND ARTS minimizing the energy. Then analyzing each case to see if the pattern of K binding sites would suggest new structures to build and minimize. For triangular crystals with n up to 2, the K intercalates between three tubes, leading to essentially the same spacing as in pristine SWNT. Figure 3 shows the energy per carbon atom as a function of number of intercalated K ions for two different packing schemes (square and triangular) and different doping types: exo (K atoms allowed only outside the tubes) and endo (K atoms allowed only inside the tubes). Assuming exo K, the global minimum is the triangular structure for K 5 C 80 = KC 16. The optimum structure has the K packed in the same (2 x 2) pattern observed for intercalated graphite, KC 8. The difference for the KC 16 SWNT is that the K are only on the outside of the tube (vide infra), leading to half the amount of K. For n > 7, the K causes significant distortions in the tube shells. Figure 2. Energy per carbon atom for triangular and square packing of K doped SWNT Diamond surfaces MD simulation of friction Theoretical investigation of wear and friction is especially important in the design of micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS). In order to describe wear, it is essential that the calculations properly describe bond formation and breaking. Thus, we use the GEE-BOD FF. Low friction is a crucial factor in determining the performance, efficiency, and durability of MEMS. Since silicon is available in large single crystals which can be etched easily to form micron scale devices, most MEMS are made of silicon. However, studies have shown that the friction between such Si systems is very high, leading to very rapid wear. As a result, the use of diamond is proposed for MEMS involving moving parts. In addition to diamond being the hardest material, polycrystalline diamond (PCD) has a friction coefficient several times smaller than silicon. Considering two diamond crystals with hydrogenated surfaces put in contact, the ratio between the external force applied for maintaining the moving block at a constant velocity and the average calculated normal force of the static block gives the friction coefficient. 120

119 JOURNAL OF SCIENCE AND ARTS Figure 3. Time evolution of structure under tensile load Figure 4 shows the running average of the force in the normal direction with respect to sliding direction with the sliding velocity maintained at a constant value. The initial oscillations represent the approach a steady state. The average normal force is very similar for different sliding directions. This indicates that differences in the differential friction coefficients arise from differences between the surfaces. Figure 5 presents the running averages of calculated friction coefficients. As expected, the xy direction has the lowest friction coefficient while the x and y directions are nearly identical. If the two surfaces were perfectly aligned, sliding in the x and y directions would have yielded identical friction coefficients. Figure 4. Average of the normal force for constant sliding speed Figure 5. Average of friction coefficients calculated for different sliding di ti 121