Phase quantification of mullite zirconia and zircon commercial powders using PAC and XRD techniques

Transcription

1 Hyperfine Interact DOI /s Phase quantification of mullite zirconia and zircon commercial powders using PAC and XRD techniques Nicolás M. Rendtorff Maria S. Conconi Esteban F. Aglietti Cecilia Y. Chain Alberto F. Pasquevich Patricia C. Rivas Jorge A. Martínez María C. Caracoche Springer Science+Business Media B.V Abstract The short range technique of the Perturbed Angular Correlation (PAC) and x-ray diffraction (Rietveld) methods have been employed to determine the phase content in commercial mullite zirconia and zircon raw materials that are ordinarily used to produce ceramic materials. The PAC technique, which probes zirconiumcontaining compounds at nanoscopic level, showed that zircon contains crystalline ZrSiO 4 and an important amount of a structurally distorted zircon, which is also observed accompanying monoclinic zirconia in mullite zirconia. This particular zircon phase was not detected by the long range x-ray diffraction Rietveld technique. After an annealing treatment, important changes in crystalline contents of the powders allow confirming, by the x-ray diffraction Rietveld method, the preexistence of this particular zircon phase. This fact must be taken into account when preparing multicomposites based on the present raw materials. Keywords Ceramics XRD techniques Gamma ray PAC N. M. Rendtorff M. S. Conconi E. F. Aglietti Centro de Tecnología de Recursos Minerales y Cerámica (CETMIC: CONICET-CIC), Camino Centenario y 506, C. C. 49 (B1897ZCA), M. B. Gonnet, Buenos Aires, Argentina N. M. Rendtorff E. F. Aglietti P. C. Rivas CONICET, Buenos Aires, Argentina C. Y. Chain A. F. Pasquevich J. A. Martínez (B) M. C. Caracoche Departamento de Física, IFLP, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 1900 La Plata, Argentina toto@fisica.unlp.edu.ar C. Y. Chain A. F. Pasquevich J. A. Martínez M. C. Caracoche CIC-BA, Buenos Aires, Argentina P. C. Rivas IFLP, Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata, 1900 La Plata, Argentina

2 N.M. Rendtorff et al. 1 Introduction Mullite zirconia (MZ) and zircon (Z) are significant raw materials for the development of refractories and traditional and advanced ceramics. Though zircon is a natural raw material, synthetic zirconium silicate is also available in the market. Mullite zirconia grains, in turn, can be obtained through an electrofusion process due to its high melting point. These two powders are ordinarily used in refractories fabrication [1]. Also ceramics containing mullite zirconia grains and zircon are mainly employed in the glass industry [2]. Finally, for advanced ceramics, mullite zirconia composites require an efficient sintering process. In this case, fine-sized powders and a sintering aid as zircon are needed. It has been reported that mullite zirconia zircon composites result in the improvement of the thermal shock resistance of the material [3]. In the ceramic processing, the quality of the starting raw materials defines the microstructure and properties of the final material. Commonly, the raw materials have a technical sheet where the main crystalline phases and the glassy phase content are reported. Composition is one of the most commonly used processing variable when designing a certain composite material. The perturbed angular correlation (PAC) technique is highly efficient to investigate zirconium containing-compounds at nanoscopic scale. The hyperfine quadrupole interaction at Zirconium sites of monoclinic zirconia [4] and zircon [5, 6] crystalline lattices are perfectly known. In this work, the PAC short range technique and the x-ray diffraction (XRD) along with the Rietveld method have been employed to determine the actual phase content of commercial mullite zirconia and zircon raw materials that are ordinarily used to produce sintered mullite zirconia zircon multicomposites [7]. Results are discussed together with those obtained on the materials after an annealing treatment. 2 Experimental 2.1 Powders Commercial mullite zirconia (MZ) grains produced by electrofusion of Al 2 O 3 and ZrSiO 4 by Elfusa LT, Brazil were attrition milled to reduce particle size to 5 μm. Synthetic zircon (z) powder of d 50 = 2 μm was acquired from Mahlwerke Kreuts, Mikron, Germany. 2.2 XRD analyses XRD patterns were obtained at room temperature using the Cu Kα radiation in a Philips 3020 diffractometer. Data were collected in the range of 10 2θ 70 in steps of 0.04 with a counting time of 2 s per step. Quantitative Rietveld analysis [8, 9]was carried out to characterize the crystalline phases present using the FullProf program [10, 11]. A micronized (d 50 = 0.7 μm) fully crystalline fluorite (CaF 2 ) wasusedasinternal standard, in order to quantify the non crystalline phase present in the raw materials [12, 13]. The overall non crystalline content (NCC %wt.) is obtained from the

3 Phase quantification of MZ and zircon commercial powders following equation, which can be easily derived by considering a standard free of amorphous phase: NCC = 1 Ws/Rs 100 Ws 104 % where Ws (%wt.) is the weighted concentration of the internal standard and Rs (%wt.) is the Rietveld analyzed concentration of the internal standard. The amount of CaF 2 employed was 50 wt.% for both materials. 2.3 PAC analysis The PAC method [14] is an efficient tool to investigate zirconia-based ceramics since it allows the hardly resolvable by other techniques determination of different atomic configurations around zirconium sites. Randomly distributed natural hafnium impurities at zirconium sites become the very sensitive 181 Hf hyperfine radioactive probes by the thermal neutron activation of some of the 180 Hf isotopes in the sample. Briefly, the method consists in the examination of the angular correlation of the kev γ γ cascade emitted during the 181 Hf to 181 Ta β decay. The comparison of the measured angular correlation against that of the isolated probe, known from the nuclear physics, informs about the electric field gradient (EFG) created by the neighboring charges around the nuclear probe. The information is drawn from the so-called spin rotation curve A 2 G 2 (t) by fitting the quadrupole parameters that describe the EFG at the Zr(Hf) site, i.e. its intensity through the quadrupole frequency ω Q and its departure from axial symmetry via the asymmetry parameter η. In addition, also the degree of local disorder often associated to the presence of lattice defects can be taken into account through the distribution width or spread δ of the hyperfine quadrupole frequency. On account of the r 3 dependence of the quadrupole interaction, slightly different Zr surroundings and their relative fractions can be well distinguished. The neutron irradiated powders for PAC were investigated using a four BaF 2 detectors setup with a time resolution of about 1 ns at Hafnium energies. 2.4 Annealing The annealing treatment is meant to evidence the crystallization of any noncrystalline phases present in a certain powder. It consists in a thermal treatment of several hours at a temperature below ( 100 C) the sintering temperature or melting point of the material. Both PAC measurements and XRD experiments were performed before and after an annealing treatment of 15 h at 1,500 C. The powders before and after annealed were named respectively Z and ZA (for zircon) and MZ and MZA (for mullite zirconia). 3Results 3.1 XRD analysis Figure 1 shows the x-ray diffractograms of the as-acquired and after heated zircon and mullite zirconia powders. Diffractograms 1a and 1b of the Z powder revealed

4 N.M. Rendtorff et al. Fig. 1 Powder diffraction patterns obtained for the zircon powder (a), for the zircon powder after annealing (b), for the mullite zirconia powder (c) and for the mullite zirconia powder after annealing (d); (M = mullite, B = m-zro 2,T= t- ZrO 2, z = ZrSiO 4 ) only the diffraction lines of zircon (ZrSiO 4 ). The crystalline phases detected by XRD for the MZ powder (1c) were mullite (3Al 2 O 3.2SiO 2 ) and monoclinic zirconia (m- ZrO 2 ). Once annealed (1d), ZrSiO 4 diffraction lines appeared in the diffractogram of this powder. The evaluation of the non crystalline phase using the internal standard method (Rietveld) for Z showed Rs 51.5% for the CaF 2 content, indicating that the non crystalline phase content NCC was below 4.3wt.%. For MZ, Rs was 51.1% and NCC 3.5wt.%. Hence, the non crystalline phase content for the two powders is rather low when evaluated by this simple method. Table 1 shows the crystalline content of the powders derived from the Rietveld quantification. Data confirmed the apparent changes mentioned before. In zircon the heating treatment causes the narrowing of the peaks due to the thermally induced growth of crystallite size with no important change in the peaks areas. Regarding mullite zirconia, the heating treatment produced some substantial changes. It is known that monoclinic zirconia (m-zro 2 ) undertakes a martensitic transformation to a tetragonal form (t-zro 2 ) over 1,100 C on the heating cycle, reverting around 900 C in the cooling cycle. This reversibility depends on several conditions [15]. In this work almost 5% of the total amount of zirconia in the powder after the annealing treatment remained in the metastable tetragonal zirconia form. On top of this, the zircon (ZrSiO 4 ) diffraction lines appeared, indicating a 9 wt.% contribution of crystalline zircon. This experimental result could be interpreted in two ways: zircon was already present in the original powder in a nondiffractable form (suggesting that the internal standard method for the non crystalline phase evaluation gave underestimated amounts) or zircon formed out of the zirconia and silica components of the mullite zirconia powder upon annealing. 3.2 PAC analysis Figure 2 shows the PAC spin rotation curves of the as-acquired and annealed powders. Fittings reveal the existence of several zirconium sites whose quadrupole parameters are listed in Table 2.

5 Phase quantification of MZ and zircon commercial powders Table 1 Crystalline phase content of the powders as determined by the Rietveld method Powder Mullite m-zro 2 t-zro 2 ZrSiO 4 (wt.%) (wt.%) (wt.%) (wt.%) Z ZA MZ MZA Fig. 2 Spin rotation curves determined for the zircon powder (a), for the zircon powder after annealing (b), for the mullite zirconia powder (c) and for the mullite zirconia powder after annealing (d). Full curves are the fitting result. Individual fitted interactions are also plotted Remembering that the PAC technique only probes zirconium containingcompounds, Table 3 presents the fitted relative fractions of the hyperfine components in molar fractions for the original and after annealed- powders. For zircon, a two-site hyperfine pattern (Fig. 2a) had to be assumed to achieve a satisfactory fit: the interaction describing crystalline zircon [5, 6], hereinafter named cz, and an unknown and highly asymmetric interaction, xz in the following. The abundance ratio found for these sites was 1:1. The spread (δ 15%) of the last interaction reflects that half of the zircon raw material involves a significant local disorder. Figure 2b shows the PAC spin rotation curves for the mullite-zirconia. The original material exhibits three zirconium containing- hyperfine components, corresponding to monoclinic zirconia, distorted monoclinic zirconia (m -ZrO 2 ) [16] and the unknown and highly asymmetric interaction xz of zircon.

6 N.M. Rendtorff et al. Table 2 Quadrupole parameters of the hyperfine interactions Structure Name ω Q (Mrad/s) η δ (%) Monoclinic zirconia m-zro Distorted monoclinic m -ZrO zirconia Crystalline zircon cz (ZrSiO 4 ) Distorted zircon xz Table 3 Fitted relative fractions of the hyperfine components in molar fraction for the original and after annealed powders Powder m-zro 2 m -ZrO 2 t- ZrO 2 cz (ZrSiO 4 ) xz (ZrSiO 4 ) Z ZA MZ MZA The fitting of the spin rotation curve of ZA (Fig. 2c) evidences that upon heating an important recovery of crystalline zircon has occurred: a transformation of about half the xz interaction has given place to the cz interaction. Hyperfine results in MZA (Fig. 2d) indicate that upon annealing the distorted monoclinic form has become the regular form and that a very small amount of the high temperature t-zro 2 phase has not reverted on cooling from 1500 C to room temperature. However, within the errors, the initial amount of ZrO 2 (72 ± 6mole%) has not been thermally altered (75 ± 4 mole%). On the other hand, only half of the xz interaction has given place to cz. 4 Discussion Being crystalline zircon the only phase detected by the XRD-Rietveld analysis in the original Z powder, the transformation of disordered surroundings (xz interactions) into regular ones (cz interaction) revealed by PAC in ZA could not be disclosed. On the other hand, since MZ was produced by electrofusion of zircon and alumina, the existence of zircon residues in the material, depicted by the zx interaction determined by PAC, is not unexpected. This result, however, does not match XRD information unless guessing that the involved locally disordered Zirconium surroundings do not exhibit the long range order needed for any XRD signal. Regarding MZA, two relevant results can be derived. On one side, the appearance of crystalline zircon determined by XRD-Rietveld is supported by the xz to cz conversion process revealed by PAC. On the other side, as the total zirconia content is not diminished after annealing, zircon formation out of silica and zirconia must be disregarded and zircon had to be present in the original powder. It seems hence consistent to interpret the xz interaction as depicting disordered regions of zircon with no XRD manifestations. The change from the xz interaction to the cz one, observed in both materials at nanoscopic scale, could be related to the aperiodic to crystalline transformation reported to occur in other synthetic zircons near 850 C[17, 18]. In an EXAFS study of the zirconium oxygen coordination in zircon [18] this transformation was

7 Phase quantification of MZ and zircon commercial powders Table 4 Comparison between XRD-Rietveld and PAC techniques normalized relative fractions (%mole) Technique m-zro 2 t-zro 2 Cz MZA Rietveld PAC interpreted as due to a not detectable by XRD change of the oxygen coordination from 7 to 8. Summing up, crystalline zircon grew in zircon and mullite zirconia raw materials due to the thermal rearrangement of a nondiffractable, aperiodic zircon present in both as-acquired powders. The mismatch between the minor amounts of amorphous zircon in the as-acquired powders (i.e. that determined by the internal standard method as well as that suggested from the Rietveld analysis performed on MZA) and the relevant amount of locally disordered zircon drawn from PAC is clearly indicating the different capabilities of the used techniques. A final support to the existence of this nondiffractable form and to its thermal transformation to crystalline zircon is evidenced in Table 4, where the phase quantification from both techniques is compared for MZA. In fact, a good agreement between the molar relative fractions is achieved if it is taken into account that PAC does not sense mullite (with no Zirconium atoms) and that XRD is not sensitive to the aperiodic domains of zircon described by the xz interaction. 5 Conclusions The Perturbed Angular Correlation technique, along with XRD Rietveld analyses, were used for quantifying the phase content of two commercial powders commonly used for processing refractory ceramic composites. The PAC technique allowed determining a locally disordered zircon that was not detected by XRD Rietveld internal standard (50 wt.% of CaF 2 ) method and whose existence was only suggested when interpreting the XRD Rietveld data of the strongly annealed samples. The thermal treatment of 1,500 C for 15 h caused a partial transformation of these disordered regions into crystalline zircon. Composition is sometimes one of the more commonly used processing variables for materials design. The fact that both as-acquired materials investigated in this work contain important amounts of nondiffractable zircon must then be taken into account when preparing or designing ceramic composite materials based on these raw materials. References 1. Moya, J.S., Osendi, M.I.: Effect of ZrO2 (ss) in mullite on the sintering and mechanical properties of mullite/zro2 composites. J. Mater. Sci. Lett. 2, (1983) 2. Parkinson, D.E.: Feeder and forehearth refractories. Glass Technol. 29, (1988) 3. Rendtorff, N., Garrido, L., Aglietti, E.: Mullite zirconia zircon composites: properties and thermal shock resistance. Ceram. Int. 35, (2009) 4. Caracoche, M.C., Dova, M.T., López García, A.R., Martínez, J.A., Rivas, P.C.: Hyperfine interaction of ZrO2 tetragonal phase. Hyperfine Interact. 39, (1988)

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