Preparation and Dielectric Properties of Mn-Doped Ba0.6Sr0.4TiO3-MgTiO3 Composite Ceramics

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1 ACTA PHYSICO-CHIMICA SINICA Volume 24, Issue 8, August 2008 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2008, 24(8): ARTICLE Preparation and Dielectric Properties of Mn-Doped Ba0.6Sr0.4TiO3-MgTiO3 Composite Ceramics Mingli Li, Mingxia Xu*, Hui Liang, Xiaolei Li, Tingxian Xu Key Laboratory for Advanced Ceramics and Machining Technology of the Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin , P. R. China Abstract: Barium strontium titanate (Ba 0.6 Sr 0.4 TiO 3, BST) nano-powders were prepared using Ba(NO 3 ) 2, Sr(NO 3 ) 2, oxalic acid dehydrate, and tetrabutyl titanate (Ti(OC 4 H 9 ) 4 ) as precursors by the chemical co-precipitation method. The product was characterized by thermogravimetry-differential scanning calorimetry (TG-DSC) thermal analyses, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The experimental results indicated that the resulting Ba 0.6 Sr 0.4 TiO 3 nano-powders were homogeneous with agglomerated nature. The Ba 0.6 Sr 0.4 TiO 3 -MgTiO 3 (BST-MT) bulk composite ceramics doped by Mn were obtained by the traditional solid phase method. The XRD patterns demonstrated that Mn-doped BST was unable to change the perovskite crystalline structure of BST materials. SEM photographs revealed that the crystalline grains became larger with increasing the content of doping Mn (<1.5% (x, molar fraction)) and then the size of grains decreased after the Mn content exceeded 1.5% in the BST ceramics, suggesting the effect of Mn doping on the morphologies of BST-MT composites. The dielectric properties of BST-MT composite ceramics doped with 0.1% 2.0% (x) Mn were investigated systematically. Two effects of Mn doping on the dielectric properties of the BST-MT composite ceramics were observed. At low Mn doping concentrations (<1.5%), Mn mainly acted as an acceptor dopant to replace Ti at the B site of ABO 3 perovskite structure, leading to a diffused phase transition. It was also observed that the grain size increased drastically as the Mn content increased and thus caused the decrease of dielectric loss. At higher Mn doping concentrations (>1.5%), the grain size decreased and the suppression of permittivity and the drastic increase of the dielectric losses were observed, which indicated a composite mixing effect. Key Words: Barium strontium titanate; Mn doping; Microstructure; Dielectric property Barium strontium titanate (Ba 1 y Sr y TiO 3 ) ferroelectric materials have attracted considerable attention for more than ten to twenty years owing to their chemical stability, high permittivity, high tunability, and low dielectric losses, etc. BST materials have shown great promise for application as phase shifting elements in phased array antennas and as tuning elements in devices operating at microwave frequencies [1 8]. Generally, BaTiO 3 and SrTiO 3 are the representative ABO 3 -type perovskite materials. BaTiO 3 is usually a ferroelectric material with the Curie temperature of 120 C. SrTiO 3 is a paraelectric one with no ferroelectric phase transition [8]. Nevertheless, the combined production of BaTiO 3 and SrTiO 3, Ba 1 y Sr y TiO 3, is a solid solution system between BaTiO 3 and SrTiO 3, i.e., Ba 1 y Sr y TiO 3 simultaneously has the advantages of high electric constant of BaTiO 3 and the structural stability of SrTiO 3 [8,9]. In view of their merits, the investigation on BST solid solution is therefore significantly important [7 10]. Various innovative approaches, like sol-gel [11 13], spray pyrolysis [14,15], combustion synthesis [16], chemical co-precipitation [17,18], hydrothermal method [19], etc., have been used to synthesize undoped and doped BST. The chemical co-precipitation method has attracted special interest owing to its simple procedure and low reaction temperature, etc. In addition, because the doped atoms can affect the crystalline structure and chemical stability of BST, it is required to modify their electrical properties. The selection of dopants is a key factor in the preparation of ABO 3 -type perovskite BST materials [20 25]. It has been reported that MgO addition can be used Received: February 29, 2008; Revised: May 4, *Corresponding author. xumingxia@tju.edu.cn, lml@tju.edu.cn; Tel/Fax: The project was supported by the National Natural Science Foundation of China ( ). Copyright 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at

2 in BST ceramics and thin films to suppress the permittivity and losses [26 29]. Ba 1 y Sr y TiO 3 -MgTiO 3 based composite ceramics maybe the most promising applicant for the phase shifting elements and tuning elements. Also, it is well known [30 32] that addition of 3d transition metals, e.g., Mn, Fe, Cr, and Cu, improves the dielectric properties of BST, and that Mn is the most effective among them. Mn ions are believed to substitute Ti and act as acceptors. Liu et al. [33] recently investigated Ca and Mn co-doping effects on the structure and dielectric properties of sol-gel derived BST ceramics. However, no detailed results of the Mn effects on the microstructure of the BST ceramics have been given, and very few reports have investigated systematically the effect of different Mn contents on the microstructure and dielectric properties of BST ceramics. In this article, we choose Ba 0.6 Sr 0.4 TiO 3 (y=0.4) since its Curie temperature can be shifted to room temperature (RT), and report a systematic study of the microstructure and dielectric properties of Ba 0.6 Sr 0.4 TiO 3 -MgTiO 3 (BST-MT) based composite ceramics with Mn doping ranging from 0 to 2.0% (x). The mechanisms of Mn doping on the microstructure and dielectric properties of BST-MT composites are discussed. 1 Experimental The starting materials used for the present synthesis of barium-strontium titanyl oxalate (Ba 1 y Sr y TiO(C 2 O 4 ) 2 4H 2 O (y= 0.4), BSTO) were barium nitrate (>99.0%), strontium nitrate (>99.0%), titanium tetrabutoxide (>98.0%), and oxalic acid dihydrate (SD fine, 99.5%). Initially, 102 ml of titanium tetrabutoxide (0.3 mol L 1 ) was dissolved in 500 ml of ethanol. Then, 0.6 mol L 1 solution of oxalic acid was made by dissolving g of oxalic acid in 350 ml of ethanol and 350 ml of distilled water. To this solution, titanium tetrabutoxide solution in ethanol was added with continuous stirring, and the white precipitate initially obtained got redissolved, resulting in clear solution of 0.3 mol L 1 oxalotitanic acid (H 2 TiO(C 2 O 4 ) 2, HTO). Accurately weighed quantities, g (0.18 mol L 1 ) of Ba(NO 3 ) 2 (SD fine, >99.5%) and g (0.12 mol L 1 ) of Sr(NO 3 ) 2 (SD fine, >99.5%), were dissolved in 1142 ml of distilled water. The nitrate solution and the above clear HTO solution were added dropwise into ammonia nitrate solution simultaneously and were vigorously stirred. The resulting reaction-mixture was stirred vigorously at 80 C for 2 h and then kept overnight for the reaction to complete. The precipitate obtained was filtered, washed, and dried in air. The pyrolysis of BSTO at 750 C for 4 h produced the Ba 0.6 Sr 0.4 TiO 3 (BST) powders. Then, the Ba 0.6 Sr 0.4 TiO 3 - MgTiO 3 (BST-MT) bulk composite ceramics doped by Mn were obtained by the traditional solid phase method using 5% (mass fraction) of ZnNb 2 O 6 Bi 2 O 3 as sintering aids. Mn doping was introduced from manganese carbonate (MnCO 3 ) (SD fine, ca 1 µm) with molar fractions of 0, 0.1%, 0.2%, 0.5%, 1.2%, 1.5%, and 2.0%. The mixtures were ball milled for 4 h and dried powders were pressed into cylindrical disks of about 2.0 cm diameter and 0.8 cm thickness by applying a pressure of 50 MPa. The compacts were then sintered at 1250 C in air for 2 h. The precursor (Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 4H 2 O) (BSTO) was used to conduct thermal analysis using thermogravimetry (TG) and differential scanning calorimetry (DSC) (WCT-2A, Beijing Optical Instrument Co., China), under air atmosphere using a heating rate of 10 C min 1 from room temperature to 1000 C. Powder X-ray diffraction (XRD) measurement was performed on an X-ray diffractometer (D/max 2500, Rigaku Co., Japan ) using a copper anticathode (Cu K α radiation, λ= nm) with 2θ ranging from 10 to 80, operated with a scan speed of 5 ( ) min 1 and a step of The morphology of the Ba 0.6 Sr 0.4 TiO 3 powders and Mn-doped BST-MT bulk composite ceramics was observed using a scanning electron microscope (SEM) (XL-30 TEP, PHILIPS, Netherlands). The grain size was estimated from polished and etched samples. The dependence of permittivity and dielectric loss on the curing temperature was determined using an impedance analyzer (E4980A intelligence LCR, Agilent, USA). For electrical contact, the flat surfaces of the bulk ceramic samples were coated with Ag alloy rubbing. To reduce the experimental errors, the mean values of three times testing were selected as the final results. 2 Results and discussion 2.1 TG-DSC analysis of BSTO To investigate the thermal degradation behaviors of the Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 4H 2 O (BSTO) powders, TG-DSC analyses were carried out and the corresponding curves are shown in Fig.1. In the TG curve, a three-stage mass loss is shown. Corresponding to these degradation steps, several exothermic and endothermic peaks were exhibited in the DSC curve. Owing to the two curves, the following three functions (1 3) were inferred, which were consistent with the three stages, respectively. A gradual mass loss below 300 C can be assigned to Fig.1 DSC and TG curves of Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 4H 2 O powders in air

3 the evaporation and elimination of the bonded water and residual organic solvent. A sharp mass loss in the temperature range from 300 to 650 C was mainly caused by the decomposition of the anionic TiO(C 2 O 4 ) 2 2. Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 4H 2 O Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 +4H 2 O (1) Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 +1/2O 2 Ba 0.6 Sr 0.4 CO 3 TiO 2 +CO +2CO 2 (2) Ba 0.6 Sr 0.4 CO 3 TiO 2 Ba 0.6 Sr 0.4 TiO 3 +CO 2 (3) The further mass loss beyond 650 C was owing to the crystallization of Ba 0.6 Sr 0.4 TiO 3 and release of CO 2. Moreover, it could be found in the DSC curve that there existed different exothermic and endothermic peaks. These exothermic and endothermic peaks clearly exhibited the formation of crystallization with an increase in annealing temperature. The first exothermic peak at C was related to the removal of bonded water and residual organic groups. The exothermic peaks at and C demonstrated the occurrence of the reaction shown in function (2), resulting in the production of Ba 0.6 Sr 0.4 CO 3 TiO 2. The endothermic peak at C can be ascribed to the crystallization of the Ba 0.6 Sr 0.4 TiO 3 powders. Moreover, it can be inferred from the TG curve that the reaction ended at about 750 C. A further increase in the sintering temperature promoted the crystallization of BST, but no new chemical reactions took place. According to the result, the precursor was calcined at 750 C to obtain the Ba 0.6 Sr 0.4 TiO 3 nano-powders. 2.2 SEM observation and XRD analysis of BST nanopowders Fig.3 XRD patterns of Ba 0.6 Sr 0.4 TiO 3 powders calcined at 750 C The precursor of Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 4H 2 O and the resultant Ba 0.6 Sr 0.4 TiO 3 powders obtained by calcining BSTO at 750 C were examined by SEM (Fig.2(a, b)). It can be seen from Fig.2(a, b) that both the precursor and Ba 0.6 Sr 0.4 TiO 3 powders are homogeneous with agglomerated nature. Further, the morphology is similar considering grain size (less than 100 nm in diameter) and spherical shape. The homogeneous nano-powders of Ba 0.6 Sr 0.4 TiO 3 obtained through the chemical co-precipitation method can be sintered at lower temperature owing to their high chemical activity and thus reduce the costs of fabrication. Fig.3 shows the XRD patterns of Ba 0.6 Sr 0.4 TiO 3 powders obtained by calcining Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 4H 2 O at 750 C for 2 h. All nine crystal faces were clearly indicated, which should be assigned to (110), (111), (200), (211), (100), (220), (221), (310), and (210) as marked in Fig.3. It was consistent with the standard XRD pattern, and obviously, no peak of any impurity was detected indicating that the homogenous phase of Ba 0.6 Sr 0.4 TiO 3 was synthesized. From Figs.2 and 3, it can be naturally concluded that Ba 0.6 Sr 0.4 TiO 3 nano-powders were successfully prepared by the chemical co-precipitation method. 2.3 SEM observation and XRD analysis of Mn doped BST-MT composites Fig.2 SEM images of Ba 0.6 Sr 0.4 TiO(C 2 O 4 ) 2 4H 2 O (a) and Ba 0.6 Sr 0.4 TiO 3 (calcined at 750 C) (b) powders Fig.4 shows the SEM images of the surface of BST-MT ceramics with different contents of Mn doping. The grain size increased drastically with the increase of Mn addition from 0.1% to 1.5% (x). However, when the addition reached up to 2.0%, the decrease of the grain size was detected. It is well known that Mn is an acceptor for BaTiO 3 and SrTiO 3. When Mn is coordinated by six oxygen ions, its ionic radius is nm, which is close to the ionic radius of Ti 4+, nm; Mn will replace Ti at the B site of perovskite ABO 3 structure and bring the local deformation of the perovskite unit cells, which promotes the grain growth as observed in Fig.4(a, b, c). The theoretical interpretation to the above trend may be attributed to the fact that the doping Mn ions have the tendency to rearrange and aggregate within limited space, leading to an increase in the size of particles and the distortion of crystalline

4 Mingli Li et al. / Acta Physico-Chimica Sinica, 2008, 24(8): Fig.4 SEM images of the surface of BST-MT ceramics with different contents of Mn doping xmn: (a) 0.1%, (b) 0.5%, (c) 1.5%, (d) 2.0% lattice. As the Mn content further increases (x>1.5 %), some of these may precipitate out as MnO discrete particles, which hinder the grain growth. The MnO nanoparticles may act as pinning sites on the grain boundaries. It may imply that the MnO pinning effect becomes predominating above the solution limit of Mn in BST ceramics. Moreover, both virgulate and equiaxed grains are observed in Fig.4. In our previous publication[34], the components of BST-MT composites were investigated through energy dispersive spectrometer (EDS). The analysis of the EDS results suggested that the virgulate and equiaxed grains were MgTiO3 and Ba0.6Sr0.4TiO3, respectively. The XRD patterns of Mn-doped BST-MT composite ceramics at room temperature are shown in Fig.5. Both the Ba0.6Sr0.4TiO3 and MgTiO3 phases were detected. The peaks of Ba0.6Sr0.4TiO3 phase were sharp, whereas those of MgTiO3 phase were rather weak and even weaker peaks of phases corresponding to sintering aids were detected. The relative intensity of peaks between Ba0.6Sr0.4TiO3 and MgTiO3 phases can be explained as that the molar ratio of Ba0.6Sr0.4TiO3 to MgTiO3 in the bulk composites was 4:1. It can be concluded that the crystalline structure of Mn doped BST still retains the perovskite phase structure after the replacement of B-site Ti4+ with Mn2+, implying that the doping of Mn2+ is unable to change the crystalline structure of BST materials. Moreover, careful analysis of the patterns shows that the peaks owing to (110) of BST shift gradually to higher angle when the dopant of Mn increases from 0 to 1.5% indicating that Mn enters the lattice of BST. Also, the c/a values of the BST phase with 0, 0.5%, 1.2%, 1.5%, and 2.0% of Mn were , , , , and , respectively. 2.4 Measurement and analysis of the dielectric properties of Mn-doped BST-MT composites Fig.6 shows the curves of permittivity vs temperature of the Mn-doped Ba0.6Sr0.4TiO3-MgTiO3 (BST-MT) composites. The permittivity is suppressed when Mn is doped. The Curie temperature (TC) is 40 C for undoped BST-MT, and it shifts to a higher temperature of 15 C for 0.5% Mn-doped BST-MT, and then shifts to a lower temperature of 25 C for 1.5% Mn-doped BST-MT. Also, the phase transitions at TC become more diffused as the Mn-doping content increases. As mentioned above, doped Mn replaced Ti at the B-site of perovskite ABO3 structure and a double ionized oxygen vacancy (VO) was formed simultaneously, i.e., MnO( TiO2) Mn"Ti+VO+OO (4) Fig.5 XRD patterns of BST-MT composite ceramics with different contents of Mn doping

5 Fig.6 Dielectric constant vs temperature measured at 1 MHz for BST-MT composites with different contents of Mn doping Charged oxygen vacancies, in combination with Mn ions, obviously give rise to the local deformation of the perovskite unit cells and promote the grain growth, and thus lead to the shift of the Curie temperature T C. Fig.7 shows the dielectric loss of the Ba 0.6 Sr 0.4 TiO 3 -MgTiO 3 (BST-MT) composites with different contents of Mn doping. The dielectric loss decreases sharply with increase of Mn dopant. It reaches the minimum value when the addition of Mn is 1.5% and then increases with the increase of Mn content. Also, it can be seen that the dielectric loss increases with the increase of testing temperature. As mentioned before, doped Mn replaces Ti at the B site of perovskite ABO 3 structure, thus giving rise to the local deformation of the perovskite unit cells. On one hand, the local deformation of the lattice promotes the growth of the grain size and thus decreases the percent of grain boundary, which can increase the permittivity. Both electric dipoles formed from Mn" Ti -V O complex and elastic dipoles owing to distortions caused by V O will be present in the BST ceramics. Furthermore, the ionic radius of Mn 2+ (0.067 nm) is smaller than that of Ti 4+ (0.068 nm), and therefore, Mn 2+ can move more freely in the octahedral interspace of the perovskite unit cells. Easier orientation of the electric and elastic dipoles contributes to the increase of the permittivity and the suppression of dielectric loss. On the other hand, the introduction of Mn 2+ extends the dissipation of microstructure and thus a reduction of the permittivity. Also, oxygen vacancies that reside at the corners of the octahedra are well interconnected and can therefore be regarded as relatively mobile. Mobile defect complexes migrate to domain boundaries, similar to that reported in hard ferroelectric lead zirconate titanate [35]. Orientation of the electric and elastic dipoles results in domain-wall pinning and thus a reduction of the dissipation in the ferroelectric state. In addition, the presence of oxygen vacancies and defects will cause larger dielectric losses. Another reason may be related to the changeable valence of Mn ions, which will bring about energy jump owing to the effect of outer force, resulting in the occurrence of multivalent Mn ions and the alteration of crystalline state. The complex factors brought the final results shown in Figs.6 and 7. The increases or decreases of the permittivity and dielectric loss are determined by the dominant one or two factors. Above 1.5% (x), the effect of Mn doping on the dielectric properties is more likely to be similar to a composite effect. The permittivity decreases, while the dielectric loss increases, with the increase in Mn doping, which is similar to that previously reported in BST-MT composites [28]. 3 Conclusions The homogeneous Ba 0.6 Sr 0.4 TiO 3 nano-powders with high chemical activity were successfully prepared by the chemical co-precipitation method. The Ba 0.6 Sr 0.4 TiO 3 -MgTiO 3 (BST- MT) bulk composite ceramics doped by Mn were obtained by the traditional solid phase method. The dielectric properties of Ba 0.6 Sr 0.4 TiO 3 -MgTiO 3 (BST-MT) composite ceramics doped with Mn ranging from 0.1% to 2.0% (x) were investigated systematically. The XRD patterns demonstrated that Mndoped BST was unable to change the perovskite crystalline structure of BST materials. SEM photographs revealed that the crystalline grains became larger with increasing the content of doping Mn when the Mn content was below 1.5% and then the size of grains decreased after the Mn content exceeded 1.5% in the BST ceramics, suggesting the effect of Mn doping on the morphologies of BST-MT composites. Two types of effects were found in the Mn-doped BST-MT composite ceramics. One is the doping effect when the Mn content was below 1.5%, where, Mn acted as an acceptor to replace Ti at the B site of perovskite ABO 3 structure of the BST grains, which led to the shift of Curie point to higher temperatures and significant suppression of losses. The other was the composite effect when the Mn content was above 1.5%. MnO may precipitate out and probably segregate at the grain boundaries, which will dilute the permittivity and hinder the grain growth. References Fig.7 Dielectric loss vs temperature measured at 1 MHz for BST-MT composites with different contents of Mn doping 1 Sengupta, L. C.; Sengupta, S. IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 1997, 44: 792

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