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Preparation of YBa2Cu3O7−x superconducting films: influence of the chemical composition on the sintering
Surface and Coatings Technology 122 (1999) 24–27 www.elsevier.nl/locate/surfcoat
Preparation of YBa Cu O superconducting films: influence of the 2 3 7−x chemical composition on the sintering E. Benavidez a, N. Quaranta a, *,1, C.J.R. Gonza´lez Oliver b,2, R. Caruso c,2, O. de Sanctis c, A. Frattini d a Desarrollo y Tecnologı´a de Materiales, FRSN, UTN, Colo´n 332 (2900), San Nicola´s, Argentina b Centro Ato´mico Bariloche, CNEA, Av. Bustillo Km 9600 (8400), Bariloche, Argentina c Laboratorio de Materiales Cera´micos, FCEIyA, UNR-IFIR, Av. Pellegrini 250 (2000) Rosario, Argentina d Area Fı´sica-Dpto de Quı´mica Fı´sica–FCByF-UNR, Suipacha 531(2000) Rosario, Argentina
Abstract The influence of the cationic ratio Y:Ba:Cu on the kinetics of densification of the YBa Cu O ( YBCO) superconductor 2 3 7−x ceramic was studied. The powders were prepared from organic precursors and the nominal compositions were: (A) stoichiometric composition YBa Cu O , (B) composition on the tie line joining YBa Cu O –BaCuO phases, and (C ) composition in the 2 3 7−x 2 3 7−x 2 triangle YBa Cu O –Y BaCuO –CuO. The phases present in these powders heated to 890°C were detected by X-ray diffraction 2 3 7−x 2 5 ( XRD), while the ratio Y:Ba:Cu was measured by neutronic activation analysis (NAA). The reactions between 900°C and 1070°C under O atmosphere were determined by differential thermal analysis (DTA) at a heating rate of 10°C/min. Compacts of different 2 powders were obtained by uniaxial pressing and were then studied by dilatometric experiments under similar conditions as established in DTA measurements. The dilatometric curves were applied to different sintering models to obtain the validity ranges and activation energies corresponding to the different densification mechanisms that operate in the initial and intermediate stages of sintering. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Densification; Sintering; Superconductor; YBa Cu O 2 3 7−x
1. Introduction In a previous work of the authors [1], yttrium deficient chemical solutions prepared via organometallic routes were used to form YBa Cu O ( YBCO) films 2 3 7−x by dipping ceramic substrates of strontium titanate. The sintering of these coatings, rich in barium and copper, promoted the formation of the BaCuO phase or an 2 oxicarbonate compound derived from BaCuO that 2 increased the densification rate in the Y–Ba–Cu–O system due to the generation of a transient liquid phase. This fast densification formed closed pores which retained the carbon incorporated in the system from the organic precursors. YBCO materials with closed poros* Corresponding author. E-mail addresses: [email protected] (N. Quaranta), [email protected] (O. de Sanctis) 1 CICPBA researcher. 2 CONICET researcher.
ity exhibited broad and suppressed superconducting transitions [2]. This behaviour was attributed to the difficulty of oxygenating these materials uniformly. However, this is not the only cause of degradation in superconducting properties observed in high-density, organically prepared materials: retention of carbon in the YBCO structure also has a deleterious effect on the superconducting transition [3]. Because the densification behaviour of the Y–Ba– Cu–O system has a direct influence on the superconducting properties, in this work we study the influence of chemical composition on the sintering behaviour of the YBCO superconductor ceramic. Three compositions varying in cationic ratio Y:Ba:Cu were analysed, taking into account the reactions that arise in the thermal treatments. The kinetics of sintering of compacts derived from powders produced by a metallorganic route were analysed by dilatometric experiments, and the collected data were fitted to different sintering models.
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 40 5 - 3
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E. Benavidez et al. / Surface and Coatings Technology 122 (1999) 24–27
2. Experimental Details on the starting precursors and chemical solution preparations were reported elsewhere [1]. Basically, the organometallic solutions were obtained starting from: Y acetate, Ba methoxyethoxide and Cu butyrate, and were incorporated with nominal compositions: (A) stoichiometric composition YBa Cu O , (B) composi2 3 7−x tion on the tie line joining YBa Cu O –BaCuO 2 3 7−x 2 phases, and (C ) composition in the triangle YBa Cu O –Y BaCuO –CuO, as shown in Fig. 1. 2 3 7−x 2 5 The three solutions were dried and the resulting agglomerates were crushed in an agate mortar and pestle. These powders were heated to 890°C, for 12 h, in flowing oxygen and then pressed uniaxially (100 MPa) to form cylindrical compacts. These compacts were presintered to 750°C, for 4 h, under an oxygen atmosphere, after which no shrinkage was detected. The powders heated to 890°C were characterised by X ray diffraction ( XRD, Philips PW1700) and by neutronic activation analysis (NAA, nuclear reactor RA-6 Bariloche Atomic Center). Their thermal evolutions were monitored by differential thermal analysis (DTA, Netzsch STA-409) at a constant heating rate of 10°C/min to 1070°C under O flow. Densification studies 2 of compacts were analysed with a differential dilatometer ( Theta Instruments Inc., Dilatronic II ) under similar conditions to those described for the DTA measurements.
Table 1 Experimentally determined phases and compositions Sample
Ratio Y:Ba:Cu
Phase
A B C
1.00:1.99:3.04 0.66:2.19:3.00 1.00:1.62:3.05
YBa Cu O 2 3 7−x YBa Cu O +BaCuO 2 3 7−x 2 YBa Cu O +Y BaCuO+CuO 2 3 7−x 2 5
the nominal values corresponding to the compositions A, B and C. It can be seen from Fig. 2 that all peaks in DTA curves are endothermic. As in previous works [4,5], it is possible to establish that these peaks correspond to the following reactions: 925°C: YBa Cu O +BaCuO +CuOL(e ) 2 3 7−x 2 1 965–975°C: YBa Cu O +CuOY BaCuO +L(p ) 2 3 7−x 2 5 1 1002°C: YBa Cu O Y BaCuO +L(c ) 2 3 7−x 2 5 3 1012°C: YBa Cu O +BaCuO Y BaCuO +L(p ) 2 3 7−x 2 2 5 3 1025–1035°C: YBa Cu O Y BaCuO +L(m ) 2 3 7−x 2 5 1 In line with Aselage and Keefer’s work [4], the reactions were identified as eutectics (e ), pseudo-peritectics or x equilibrium between four phases (p ), and peritectics or x incongruent melts (m ). The c symbol corresponds to x 3 a particular composition of the liquids, where the liquid and two solid phases are in equilibrium [5].
3. Results and discussion The NAA results and phases detected by XRD are shown in Table 1. These data were in agreement with
Fig. 1. Location of the nominal composition in the phase diagram of the Y O –BaO–CuO system. 2 3
Fig. 2. DTA curves corresponding to powders heated at 10°C/min under O flow. 2
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E. Benavidez et al. / Surface and Coatings Technology 122 (1999) 24–27
Fig. 3. Lineal contraction (DL/L ) against temperature for compact A. o
Fig. 4. Lineal contraction (DL/L ) against temperature for compact B. o
Fig. 5. Lineal contraction (DL/L ) against temperature for compact C. o
Some models were applied, corresponding to the initial and intermediate stages of sintering [6–8], to the results of the dilatometric study carried out at constant heating rates (CHR). This allowed the calculation of the activation energy associated with the sintering process in the presence of a liquid phase and also the sintering processes by grain boundary diffusion or by volume diffusion. The activation energies corresponding
to the initial (Qini) and intermediate stages (Qint), grain boundary (Q ) or volume (Q ) diffusion as well as the b v activation energy for the sintering assisted by the liquid phase (Q ) corresponding to the samples A, B and C, L are shown in Figs. 3–5, respectively. The fractional contraction, DL/L , of compact A is o shown in Fig. 3. The initial stage of sintering starts at 824°C and ends at about 900°C; immediately the densification process enters an intermediate stage controlled by grain boundary diffusion. Between 922°C and 970°C the rate of shrinkage decreases and no models can be applied to this range. The reaction at about 970°C generates a peritectic liquid L(p ) which results in a shrinkage curve that can 1 be accommodated by both the sintering model involving a liquid phase (starting from 974°C ) and the sintering model involving material diffusion by volume in the intermediate stage (starting from 978°C ). Finally, the DTA data in the range 1026°C to 1050°C is in total agreement with a sintering mechanism involving liquid and an energy of 142 kcal/mol with the liquid L(m ) 1 resulting from peritectic decomposition of YBCO. Densification of compact B can be seen to start from 800°C as shown in Fig. 4. Then, between 900 and 930°C the grain boundary diffusion is the controlling mechanism for densification, followed by sintering involving a liquid phase as the control mechanism up to 950°C. This behaviour coincides with the small reaction detected in the DTA curve at about 930°C, corresponding to the occurrence of liquid L(e ). It is worth noting that the 1 greatest rate of shrinkage, d(DL/L )/dT, attributed to o the transient presence of liquid, is in the temperature range 932–933°C. This is due to the fact that atomic transport through the liquid phase is faster than through the solid phase [9]. From 957°C to 970°C, the intermediate stage of sintering of compact B has volume diffusion as the dominant mechanism of densification. From 970°C to 1006°C, the shrinkage can be attributed to any of the mechanisms of densification. The contraction above this range can be related to liquid phase sintering (LPS) up to 1020°C. This occurrence of liquid is in keeping with the reaction: YBa Cu O +BaCuO =Y BaCuO +L(p ), corre2 3 7−x 2 2 5 3 sponding to the DTA curve near 1012°C. The initial stage of sintering of compact C starts at about 815°C, and the intermediate stage is dominated first by grain boundary diffusion and second by volume diffusion, see Fig. 5. The temperature range between 980 and 1015°C corresponds to both the intermediate stage of sintering and the LPS mechanism with an activation energy of 396 kcal/mol, but the percentage of lineal shrinkage (>30%) corresponds to densities higher than the theoretical density. Therefore, it is only possible to obtain the value of DL/L in the presence of deformation and/or o
E. Benavidez et al. / Surface and Coatings Technology 122 (1999) 24–27
the transient occurrence of liquid, which in this case is rich in copper at 1002°C, in the compact.
4. Conclusions In the cases studied it was observed that the initial stage of sintering is dominated by volume diffusion, while the presence of liquid phase was established from both the dilatometric curves as well as from evidence regarding changes in the reactions involved from DTA. All compositions showed YBCO as the main phase. The yttrium deficient compound showed BaCuO and 2 CuO as secondary phases, which encouraged early densification from about 930°C. The barium deficient compound showed that the YBCO–CuO reaction stopped the shrinkage at about 970°C. The liquid, rich in copper, generated in the same composition at 1000°C resulted in the deformation of the compact. Initial densification in compact A was conducive to closed porosity and subsequent carbon retention. This situation can be deleterious to the superconducting properties of the YBCO films.
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Acknowledgement The authors gratefully acknowledge Mr. Sergio Ribeiro and Mr. Daniel Quattrini (Bariloche Atomic Center) for NAA and DTA measurements, respectively.
References [1] E. Benavidez, C.J.R. Gonza´lez Oliver, R. Caruso, O. de Sanctis, Mater. Chem. Phys. (1999) in press. [2] D.R. Clarke, T.M. Shaw, D. Dimos, J. Am. Ceram. Soc. 72 (1989) 1103. [3] T.M. Shaw, D. Dimos, P.E. Batson, A.G. Schrott, D.R. Clarke, P.R. Duncombe, J. Mater. Res. 5 (1990) 1176. [4] T. Aselage, K. Keefer, J. Mater. Res. 3 (1988) 1279. [5] K. Lay, G.M. Renlund, J. Am. Ceram. Soc. 73 (1990) 1208. [6 ] W.S. Young, I.B. Cutler, J. Am. Ceram. Soc. 53 (1970) 659. [7] C. Genuist, J.M. Haussonne, Ceram. Int. 14 (1988) 169. [8] C.J.R. Gonza´lez Oliver, J.E. Fiscina, E.A. Oliber, D. Russo, D.A. Esparza, Thermochim. Acta 203 (1992) 1143. [9] N.M. Hwang, Y.K. Park, H.K. Lee, J.H. Khan, K. Lee, H.G. Moon, J.C. Park, J. Am. Ceram Soc. 71 (1988) C210.
Preparation of YBa Cu O superconducting films: influence of the 2 3 7−x chemical composition on the sintering E. Benavidez a, N. Quaranta a, *,1, C.J.R. Gonza´lez Oliver b,2, R. Caruso c,2, O. de Sanctis c, A. Frattini d a Desarrollo y Tecnologı´a de Materiales, FRSN, UTN, Colo´n 332 (2900), San Nicola´s, Argentina b Centro Ato´mico Bariloche, CNEA, Av. Bustillo Km 9600 (8400), Bariloche, Argentina c Laboratorio de Materiales Cera´micos, FCEIyA, UNR-IFIR, Av. Pellegrini 250 (2000) Rosario, Argentina d Area Fı´sica-Dpto de Quı´mica Fı´sica–FCByF-UNR, Suipacha 531(2000) Rosario, Argentina
Abstract The influence of the cationic ratio Y:Ba:Cu on the kinetics of densification of the YBa Cu O ( YBCO) superconductor 2 3 7−x ceramic was studied. The powders were prepared from organic precursors and the nominal compositions were: (A) stoichiometric composition YBa Cu O , (B) composition on the tie line joining YBa Cu O –BaCuO phases, and (C ) composition in the 2 3 7−x 2 3 7−x 2 triangle YBa Cu O –Y BaCuO –CuO. The phases present in these powders heated to 890°C were detected by X-ray diffraction 2 3 7−x 2 5 ( XRD), while the ratio Y:Ba:Cu was measured by neutronic activation analysis (NAA). The reactions between 900°C and 1070°C under O atmosphere were determined by differential thermal analysis (DTA) at a heating rate of 10°C/min. Compacts of different 2 powders were obtained by uniaxial pressing and were then studied by dilatometric experiments under similar conditions as established in DTA measurements. The dilatometric curves were applied to different sintering models to obtain the validity ranges and activation energies corresponding to the different densification mechanisms that operate in the initial and intermediate stages of sintering. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Densification; Sintering; Superconductor; YBa Cu O 2 3 7−x
1. Introduction In a previous work of the authors [1], yttrium deficient chemical solutions prepared via organometallic routes were used to form YBa Cu O ( YBCO) films 2 3 7−x by dipping ceramic substrates of strontium titanate. The sintering of these coatings, rich in barium and copper, promoted the formation of the BaCuO phase or an 2 oxicarbonate compound derived from BaCuO that 2 increased the densification rate in the Y–Ba–Cu–O system due to the generation of a transient liquid phase. This fast densification formed closed pores which retained the carbon incorporated in the system from the organic precursors. YBCO materials with closed poros* Corresponding author. E-mail addresses: [email protected] (N. Quaranta), [email protected] (O. de Sanctis) 1 CICPBA researcher. 2 CONICET researcher.
ity exhibited broad and suppressed superconducting transitions [2]. This behaviour was attributed to the difficulty of oxygenating these materials uniformly. However, this is not the only cause of degradation in superconducting properties observed in high-density, organically prepared materials: retention of carbon in the YBCO structure also has a deleterious effect on the superconducting transition [3]. Because the densification behaviour of the Y–Ba– Cu–O system has a direct influence on the superconducting properties, in this work we study the influence of chemical composition on the sintering behaviour of the YBCO superconductor ceramic. Three compositions varying in cationic ratio Y:Ba:Cu were analysed, taking into account the reactions that arise in the thermal treatments. The kinetics of sintering of compacts derived from powders produced by a metallorganic route were analysed by dilatometric experiments, and the collected data were fitted to different sintering models.
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 40 5 - 3
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E. Benavidez et al. / Surface and Coatings Technology 122 (1999) 24–27
2. Experimental Details on the starting precursors and chemical solution preparations were reported elsewhere [1]. Basically, the organometallic solutions were obtained starting from: Y acetate, Ba methoxyethoxide and Cu butyrate, and were incorporated with nominal compositions: (A) stoichiometric composition YBa Cu O , (B) composi2 3 7−x tion on the tie line joining YBa Cu O –BaCuO 2 3 7−x 2 phases, and (C ) composition in the triangle YBa Cu O –Y BaCuO –CuO, as shown in Fig. 1. 2 3 7−x 2 5 The three solutions were dried and the resulting agglomerates were crushed in an agate mortar and pestle. These powders were heated to 890°C, for 12 h, in flowing oxygen and then pressed uniaxially (100 MPa) to form cylindrical compacts. These compacts were presintered to 750°C, for 4 h, under an oxygen atmosphere, after which no shrinkage was detected. The powders heated to 890°C were characterised by X ray diffraction ( XRD, Philips PW1700) and by neutronic activation analysis (NAA, nuclear reactor RA-6 Bariloche Atomic Center). Their thermal evolutions were monitored by differential thermal analysis (DTA, Netzsch STA-409) at a constant heating rate of 10°C/min to 1070°C under O flow. Densification studies 2 of compacts were analysed with a differential dilatometer ( Theta Instruments Inc., Dilatronic II ) under similar conditions to those described for the DTA measurements.
Table 1 Experimentally determined phases and compositions Sample
Ratio Y:Ba:Cu
Phase
A B C
1.00:1.99:3.04 0.66:2.19:3.00 1.00:1.62:3.05
YBa Cu O 2 3 7−x YBa Cu O +BaCuO 2 3 7−x 2 YBa Cu O +Y BaCuO+CuO 2 3 7−x 2 5
the nominal values corresponding to the compositions A, B and C. It can be seen from Fig. 2 that all peaks in DTA curves are endothermic. As in previous works [4,5], it is possible to establish that these peaks correspond to the following reactions: 925°C: YBa Cu O +BaCuO +CuOL(e ) 2 3 7−x 2 1 965–975°C: YBa Cu O +CuOY BaCuO +L(p ) 2 3 7−x 2 5 1 1002°C: YBa Cu O Y BaCuO +L(c ) 2 3 7−x 2 5 3 1012°C: YBa Cu O +BaCuO Y BaCuO +L(p ) 2 3 7−x 2 2 5 3 1025–1035°C: YBa Cu O Y BaCuO +L(m ) 2 3 7−x 2 5 1 In line with Aselage and Keefer’s work [4], the reactions were identified as eutectics (e ), pseudo-peritectics or x equilibrium between four phases (p ), and peritectics or x incongruent melts (m ). The c symbol corresponds to x 3 a particular composition of the liquids, where the liquid and two solid phases are in equilibrium [5].
3. Results and discussion The NAA results and phases detected by XRD are shown in Table 1. These data were in agreement with
Fig. 1. Location of the nominal composition in the phase diagram of the Y O –BaO–CuO system. 2 3
Fig. 2. DTA curves corresponding to powders heated at 10°C/min under O flow. 2
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E. Benavidez et al. / Surface and Coatings Technology 122 (1999) 24–27
Fig. 3. Lineal contraction (DL/L ) against temperature for compact A. o
Fig. 4. Lineal contraction (DL/L ) against temperature for compact B. o
Fig. 5. Lineal contraction (DL/L ) against temperature for compact C. o
Some models were applied, corresponding to the initial and intermediate stages of sintering [6–8], to the results of the dilatometric study carried out at constant heating rates (CHR). This allowed the calculation of the activation energy associated with the sintering process in the presence of a liquid phase and also the sintering processes by grain boundary diffusion or by volume diffusion. The activation energies corresponding
to the initial (Qini) and intermediate stages (Qint), grain boundary (Q ) or volume (Q ) diffusion as well as the b v activation energy for the sintering assisted by the liquid phase (Q ) corresponding to the samples A, B and C, L are shown in Figs. 3–5, respectively. The fractional contraction, DL/L , of compact A is o shown in Fig. 3. The initial stage of sintering starts at 824°C and ends at about 900°C; immediately the densification process enters an intermediate stage controlled by grain boundary diffusion. Between 922°C and 970°C the rate of shrinkage decreases and no models can be applied to this range. The reaction at about 970°C generates a peritectic liquid L(p ) which results in a shrinkage curve that can 1 be accommodated by both the sintering model involving a liquid phase (starting from 974°C ) and the sintering model involving material diffusion by volume in the intermediate stage (starting from 978°C ). Finally, the DTA data in the range 1026°C to 1050°C is in total agreement with a sintering mechanism involving liquid and an energy of 142 kcal/mol with the liquid L(m ) 1 resulting from peritectic decomposition of YBCO. Densification of compact B can be seen to start from 800°C as shown in Fig. 4. Then, between 900 and 930°C the grain boundary diffusion is the controlling mechanism for densification, followed by sintering involving a liquid phase as the control mechanism up to 950°C. This behaviour coincides with the small reaction detected in the DTA curve at about 930°C, corresponding to the occurrence of liquid L(e ). It is worth noting that the 1 greatest rate of shrinkage, d(DL/L )/dT, attributed to o the transient presence of liquid, is in the temperature range 932–933°C. This is due to the fact that atomic transport through the liquid phase is faster than through the solid phase [9]. From 957°C to 970°C, the intermediate stage of sintering of compact B has volume diffusion as the dominant mechanism of densification. From 970°C to 1006°C, the shrinkage can be attributed to any of the mechanisms of densification. The contraction above this range can be related to liquid phase sintering (LPS) up to 1020°C. This occurrence of liquid is in keeping with the reaction: YBa Cu O +BaCuO =Y BaCuO +L(p ), corre2 3 7−x 2 2 5 3 sponding to the DTA curve near 1012°C. The initial stage of sintering of compact C starts at about 815°C, and the intermediate stage is dominated first by grain boundary diffusion and second by volume diffusion, see Fig. 5. The temperature range between 980 and 1015°C corresponds to both the intermediate stage of sintering and the LPS mechanism with an activation energy of 396 kcal/mol, but the percentage of lineal shrinkage (>30%) corresponds to densities higher than the theoretical density. Therefore, it is only possible to obtain the value of DL/L in the presence of deformation and/or o
E. Benavidez et al. / Surface and Coatings Technology 122 (1999) 24–27
the transient occurrence of liquid, which in this case is rich in copper at 1002°C, in the compact.
4. Conclusions In the cases studied it was observed that the initial stage of sintering is dominated by volume diffusion, while the presence of liquid phase was established from both the dilatometric curves as well as from evidence regarding changes in the reactions involved from DTA. All compositions showed YBCO as the main phase. The yttrium deficient compound showed BaCuO and 2 CuO as secondary phases, which encouraged early densification from about 930°C. The barium deficient compound showed that the YBCO–CuO reaction stopped the shrinkage at about 970°C. The liquid, rich in copper, generated in the same composition at 1000°C resulted in the deformation of the compact. Initial densification in compact A was conducive to closed porosity and subsequent carbon retention. This situation can be deleterious to the superconducting properties of the YBCO films.
27
Acknowledgement The authors gratefully acknowledge Mr. Sergio Ribeiro and Mr. Daniel Quattrini (Bariloche Atomic Center) for NAA and DTA measurements, respectively.
References [1] E. Benavidez, C.J.R. Gonza´lez Oliver, R. Caruso, O. de Sanctis, Mater. Chem. Phys. (1999) in press. [2] D.R. Clarke, T.M. Shaw, D. Dimos, J. Am. Ceram. Soc. 72 (1989) 1103. [3] T.M. Shaw, D. Dimos, P.E. Batson, A.G. Schrott, D.R. Clarke, P.R. Duncombe, J. Mater. Res. 5 (1990) 1176. [4] T. Aselage, K. Keefer, J. Mater. Res. 3 (1988) 1279. [5] K. Lay, G.M. Renlund, J. Am. Ceram. Soc. 73 (1990) 1208. [6 ] W.S. Young, I.B. Cutler, J. Am. Ceram. Soc. 53 (1970) 659. [7] C. Genuist, J.M. Haussonne, Ceram. Int. 14 (1988) 169. [8] C.J.R. Gonza´lez Oliver, J.E. Fiscina, E.A. Oliber, D. Russo, D.A. Esparza, Thermochim. Acta 203 (1992) 1143. [9] N.M. Hwang, Y.K. Park, H.K. Lee, J.H. Khan, K. Lee, H.G. Moon, J.C. Park, J. Am. Ceram Soc. 71 (1988) C210.