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Sensitive determination of secondary phases in YBa2Cu3O7 − δ materials
June 1995
Materials Letters 24 (1995) 189-193
Sensitive determination of secondary phases in YBa,Cu307 _ 6 materials U. Wiesner, W. Bieger, G. %-abbes Institutfiir Festtirper- und Werkstofforschung Dresden e.V.. D-01 I71 Dresden, Germany Received 23 March 1995; accepted 11 April 1995
Abstract With the aid of determined polythermal phase diagrams in the Y-Ba-Cu-0 system,a sensitivemethod for the determination of the main impuritiesCuO, BaCuOzand BaC03 in YBa&u,O, _ s using DTA is described.
1. Introduction
Since the discovery of the high-temperature superconducting phase Y13a&u,O, _ 6, much information dealing with the preparation of YBa2Cu30, _ s of a pure quality has been published. The favoured method for the synthesis of YBa2Cu30,_s is the solid-state sintering of a stoichiometric mixture of YZ03, CuO and BaCO,. The formation of YBazCu30,_8 without any residual components depends on several processing parameters, such as sintering temperature and time, oxygen partial pressure and the purity of the ambient atmosphere. The purity of the components of the mixture and the exactness and homogeneity of the mixture tie important as well. The main impurities in the synthesized YBa$u,O, _ s are CuO and BaC03, as well as BaCuO, and YzBaCuO,. These are formed during the sintering process as a result of local inhomogeneities or from solidified liquid phases. In most cases, the analysis of the synthesized YBaJ&O, _ s is carried out using the XRD technique. In general, the sensitivity of the XRD analysis is not better than about 5%. Therefore, it is possible that an ‘ ‘XRD-single phase” YBa2Cu30, _ s may be contaminated with the impurities mentioned 0167-577x/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10167-577x(95)00075-5
above, which impair the electrical properties of the YBazCu307 _ s material. For this reason a more sensitive method of analysis of the main impurities (CuO and BaCuO,) is required. Differential thermal analysis ( DTA) and thermogravimetry (TG) are known as standard techniques to detect impurities in ceramic materials. Whereas it is easy to identify the impurity phases in simple binary or quasibinary eutectic and peritectic systems, this task becomes much more complicated in the investigated four-component system involving several ternary or quatemary eutectic and peritectic reactions. This paper describes a method for detection of impurity phases in YBa$&O, _ s by thermoanalytic measurements on the basis of an analysis of recently determined phase diagrams in the Y-Ba-Cu-0 system [ 11.
2. Fundamentals Phase relationships in the Y-Ba-Cu-0 system have been determined using DTA/TG, XRD, EDX and emf investigations [ l-31. Ifp( 0,) is adjusted to a constant value, the phase relations can be described by a set of univariant reactions, the temperature of which is fixed
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U. Wiesner et al. /Materials
L.&em 24 (1995) 189-193
perature it will be decomposed in a peritectic-like reaction: a YBa2Cu307-6+b
Y,BaCuO,+c
L+d Oz.
(ml)
Therefore, pure YBa&O, _ a does not show any thermal signal below 102O”C,and consequently, the detection of only one sharp DTA peak at 1020°C (accompanied by a mass loss of about 0.9% [ 21) is a sensitive proof for pure YBa#&O, _ s material. Subsequent reactions arising from the decomposition of the formed YzBaCuOS phase occur only at T> 1200°C. CuO (Fig. 2a): According to the univariant reaction: a YBa,t&O,
_ s + b CuO
+cY,BaCuOS+dL+eO,, Fig. 1. Quasitemary phase diagram YO,,rBaO-CuOx at 875°C for ~(0,) =0.21X 10’ Pa (123: YBqCu30,_~, 211: Y,BaCuOS, 011: BaCuO*, 001: CuO, 202: Y2Cu205. 210: Y,BaO,, 220: Y,BqO,, 143: YBa,Cu,O,, 163: YB~,&u~O~~, “023”: “BazCu,OS”, “035”: “Ba&O,“.)
under these conditions. Typically (and provided in this treatment), DTA/TG measurements will be performed under conditions of constant ~(0,). Therefore, the temperature is unambiguously defined for those DTA/ TG events which are caused by univariant reactions. According to the Gibbs phase rule, four condensed phases can coexist in such an univariant equilibrium state, i.e. four condensed phases are included in the considered reactions (either as reactants or products) ‘. These four phases cannot be represented by a binary or quasibinary phase diagram. Fig. 1 represents a part of the quasiternary projection of the subsolidus phase diagram near 875°C for ~(0,) ~0.21 X 105 Pa. The considered secondary phases are marked by “ + “. Assuming that the content of secondary phases (impurities) to be detected by DTA/TG remains small, the DTA/TG behaviour will be represented by the four selected polytbermal sections in Fig. 2a-2d (as derived from the results in Ref. [ 1 ] ) . The charge of the DTA/TG curve of YBazCu30,_s in the presence of several impurities can be explained as follows: Pure YBa,Cu,07_ 6: Pure YBa2Cu307 _ s is stable up to 1020°C at p( 0,) = 0.21 X lo5 Pa. Above this tem-
(PI)
CuO additions cause a primary thermal event already
at 940°C [ 1,4] which is very sensitive even for small amounts of CuO. The complete solution of the YBa2Cu30,_s phase in the liquid is indicated by a subsequent event which is not caused by an univariant reaction, and consequently, this event does not occur at a distinct temperature. Its upper limit is 1020°C (the peritectic-like decomposition of YBazCu307_ *). However, especially in the case of small additions the temperature does not change significantly. BaCuO, (Fig. 2b): A DTA signal at 991°C indicates BaCuO, impurities. At this temperature, BaCuOz reacts with YBa2Cu307_ 6 forming YBa.,Cu,O, and a liquid phase according to: a YBa,Cu,O, _ s + b BaCuO, + c YB~.,CU~O~.~ + d L + e O2 .
(P3)
The
following events at 1010 and 1020°C which have been found to be independent of the amount of BaCuO, correspond to the decomposition of YBa&u309 and YBa2Cu30,_ 6, respectively [ 31. Y,BuCuOs (Fig. 2~): Y,BaCuO, coexists with YBazCu307 _ 6 up to the peritectic decomposition temperature of YBa2Cu30,_, at 1020°C [ 11. No additional reactions occur below 102O”C,and consequently, Y,BaCuOs impurities cannot be detected by DTA. CuO + BuCuO, (Fig. 2~): The DTA peak at 899°C resulting from the ternary eutectic reaction a YBa2Cu30, _ 6+ b CuO + c BaCuOl + d L + e O2
’ This number can be reduced by n if there are n further restricting conditions.
(e,)
U. Wiesner et al. /Materials Letters 24 (1995) 189-193
(a)1100 ri
A
\ 'k________ I/
191
e
I I i 2ll+L
MllOO9
lL
\ \
;
f.
-- - _ i 123+143+L
loo&.
i
f 123+011
900 i 0 20 $02331
40
60
mol%
80
100
CUO
I
: I
0 20 11231
d
0 y"l.5
20
40
60
mol%
40
60 mol %
80
IdO
3 (OIlI
80 ,!da ~'0237
Fig. 2. Schematic polythermal sections (signed in Fig. 1) involving the 123-phase and the considered impurities.
unambiguously indic.ates the presence of both BaCuOz and CuO secondary .phases. No further peak has to be observed below the decomposition temperature of YBa&u307_8 if the molar ratio CuO: iBaCuOz is about 3 : 4, which corresponds to the ratio in the eutectic liquid (CuO: iBaCuOz: iYBaZCu30,_6= 1: 1.3125: 0.1875). For larger lCu0 amounts, a subsequent peak can be observed between 899°C (ei) and 940°C (pi), whereas the right parts of Figs. 2c and 2d represent the situation of a BaCuO, excess. Here, the eutectic peak will subsequently be followed by a DTA event at T’, in which T’ is between 899 and 991°C ( p3). A third peak observed at 1020°C refers to the reaction (m, ) . YzBaCuO, + CuO (Fig. 2d): The common influence of both YZBaCu05 and CuO impurities can be clarified on the basis of an imagined Yz03-YBazCu307-s
“Ba,Cu,O,” section as represented in Fig. 2d. In comparison with Fig. 2a, it becomes evident that it is impossible to distinguish between the contamination by CuO or by a mixture of CuO + Y,BaCuO, by reason of the first and second DTA events, which occur at 94O’C and less than 1020°C in both cases. BaCO,: Free BaC03 shows a phase transition at 806°C from a rhombic to hexagonal structure without any weight change [ 51. In air, BaC03 decomposes at about 114oOC. Carbonate on lattice positions in the YBa,Cu,O, _ 6 phase cannot be detected in this way. 3. Experiments and results All DTA investigations have been carried out using a Setaram TG/DTA 92 apparatus with a heating rate
192
U. Wiesner et al. /Materials
Letters 24 (1995) 189-193
of 10 K/min from room temperature to 1200°C. The oxygen partial pressure (p( 0,) = 0.2 1 X lo5 Pa) in the flowing argon-oxygen mixture was controlled by a zirconia-emf cell. Both DTA and TG were recorded and the peak onset temperatures have been applied in the analysis. Fig. 3 represents the DTA plots of several YBa,Cu,O,_s samples mixed with the secondary phases discussed above. The adequate powders were intimately mixed in an agate mortar. All synthetic mixtures used in these experiments are marked in the corresponding polythermal sections. The main events of the curves in Fig. 3 are in agreement with the reaction temperatures deduced from Fig. 2. Fig. 4 represents the DTA plots of YBa&u&_, materials from different origins. Fig. 4a shows the DTA
800
1000
1200 TPC
Fig. 4. DTA plots of several 123 materials (see text). (a) Synthesized 123, (b) thismaterialafteradditionalheattreatment, (c) + (d) commercial 123 materials.
800
1000
1200 TPC
Fig. 3. DTA plots of several mixtures representing 123 and different impurities. (The positions of the mixtures are. indicated by capital letters in Fig. 2.) 0: pore 123 (see Fig. 4b), A: 123+CuO, B: 123+Oll,C: 123+211,D: 123+011+CuO,E: 123+211+CuO.
plot of a YBa,Cu,O, _ s sample which was synthesized in the following way. Stoichiometric amounts of Y203, BaCOs and CuO were intimately mixed and heat treated several times and ground (2 times for 17 h at 900 to 920°C in air, followed by a 24 h annealing at 930°C in 1 X 10s Pa 0,). Though the XRD analysis indicates the presence of a single phase material, the DTA plot (T( on) = 901°C) indicates a material which is contaminated with CuO and BaCuO*. After an additional heat treatment (20 h at 930°C in 1 X 10s Pa O2 and 20 h at 380°C in 1 X 10’ Pa 0,)) only a single DTA event is recorded at 1023°C corresponding to pure YBa,Cu,O, _ s (Fig. 4b). The plots of two commercially available materials are represented in Figs. 4c and 4d. Both samples contain small quantities of BaCuOz and CuO (T( on) = 907°C on both cases), though only in the first material CuO has been detected by XRD. Due to the oxygen deficiency of the melt all univariant reactions at ~(0,) =0.21 X lo5 Pa are accompa-
U. Wiesner et al. /Materials Letters 24 (1995) 189-193
nied with a mass change [ 21. TG of the melting process of the ternary eutectics (e,) ( 1.25 mol% Y0,.5, 28.75 mol% BaO, 70 mol% CuO) indicates for example a mass loss of 3.5%. However, these changes will be reduced to less than 0.05% if only 1% of secondary phases have to be detected. Therefore, the sensitivities of the TG signals are not sufficient with respect to the purpose discussed in the present paper.
4. Conclusions DTA is shown to be a fast and sensitivity method to detect CuO, BaCuOz and BaC03 impurities, as well as mixtures of these compounds in YBa#&O, _ s materials on the basis of’ well-defined univariant reactions which are characterized by a fixed temperature (independent of the composition) atp( 0,) = 0.21 X 10’ Pa. DTA is not convenient to indicate Y,BaCu05 additions. Both the onset temperatures of the DTA events subsequent to the univariant reactions, as well as all detected peak areas, depend on the amounts of the appropriate compounds thus opening the possibility for a quantitative interpretation. However, the temperature shift of the subsequent DTA events is very limited for
193
small quantities of secondary phases, and furthermore, it strongly depends on the oxygen partial pressure of the environment. Therefore, the analysis of the DTA peak areas is expected to be the more promising technique.
Acknowledgement The authors wish to express their thanks to Mrs. B. Thaut and Mrs. W. Hiippner for careful experimental work. This paper results from a project granted by the Federal Minister of Research and Technology (BMPT) under No. 13 N 5897 A.
References Ill G. Krabbes, W. Bieger, U. Wiesner, M. Ritschel and A. Teresiak, J. Solid State Chem. 13 (1993) 420. [ 21 G. Krabbes, U. Wiesner, W. Bieger andM. Ritschel, Z. Metallkd. 85 (1994) 70. [3] G. Krabbes, W. Bieger and U. Wiesner, 3rd IUMRS-ICAM, Tokyo, 1993. [4] T. A&age, preprint, submitted to Physica C. [5] I. Barin and 0. Knacke, Tehrmochemical properties of inorganics substances (Springer, Berlin, 1973).
Materials Letters 24 (1995) 189-193
Sensitive determination of secondary phases in YBa,Cu307 _ 6 materials U. Wiesner, W. Bieger, G. %-abbes Institutfiir Festtirper- und Werkstofforschung Dresden e.V.. D-01 I71 Dresden, Germany Received 23 March 1995; accepted 11 April 1995
Abstract With the aid of determined polythermal phase diagrams in the Y-Ba-Cu-0 system,a sensitivemethod for the determination of the main impuritiesCuO, BaCuOzand BaC03 in YBa&u,O, _ s using DTA is described.
1. Introduction
Since the discovery of the high-temperature superconducting phase Y13a&u,O, _ 6, much information dealing with the preparation of YBa2Cu30, _ s of a pure quality has been published. The favoured method for the synthesis of YBa2Cu30,_s is the solid-state sintering of a stoichiometric mixture of YZ03, CuO and BaCO,. The formation of YBazCu30,_8 without any residual components depends on several processing parameters, such as sintering temperature and time, oxygen partial pressure and the purity of the ambient atmosphere. The purity of the components of the mixture and the exactness and homogeneity of the mixture tie important as well. The main impurities in the synthesized YBa$u,O, _ s are CuO and BaC03, as well as BaCuO, and YzBaCuO,. These are formed during the sintering process as a result of local inhomogeneities or from solidified liquid phases. In most cases, the analysis of the synthesized YBaJ&O, _ s is carried out using the XRD technique. In general, the sensitivity of the XRD analysis is not better than about 5%. Therefore, it is possible that an ‘ ‘XRD-single phase” YBa2Cu30, _ s may be contaminated with the impurities mentioned 0167-577x/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10167-577x(95)00075-5
above, which impair the electrical properties of the YBazCu307 _ s material. For this reason a more sensitive method of analysis of the main impurities (CuO and BaCuO,) is required. Differential thermal analysis ( DTA) and thermogravimetry (TG) are known as standard techniques to detect impurities in ceramic materials. Whereas it is easy to identify the impurity phases in simple binary or quasibinary eutectic and peritectic systems, this task becomes much more complicated in the investigated four-component system involving several ternary or quatemary eutectic and peritectic reactions. This paper describes a method for detection of impurity phases in YBa$&O, _ s by thermoanalytic measurements on the basis of an analysis of recently determined phase diagrams in the Y-Ba-Cu-0 system [ 11.
2. Fundamentals Phase relationships in the Y-Ba-Cu-0 system have been determined using DTA/TG, XRD, EDX and emf investigations [ l-31. Ifp( 0,) is adjusted to a constant value, the phase relations can be described by a set of univariant reactions, the temperature of which is fixed
190
U. Wiesner et al. /Materials
L.&em 24 (1995) 189-193
perature it will be decomposed in a peritectic-like reaction: a YBa2Cu307-6+b
Y,BaCuO,+c
L+d Oz.
(ml)
Therefore, pure YBa&O, _ a does not show any thermal signal below 102O”C,and consequently, the detection of only one sharp DTA peak at 1020°C (accompanied by a mass loss of about 0.9% [ 21) is a sensitive proof for pure YBa#&O, _ s material. Subsequent reactions arising from the decomposition of the formed YzBaCuOS phase occur only at T> 1200°C. CuO (Fig. 2a): According to the univariant reaction: a YBa,t&O,
_ s + b CuO
+cY,BaCuOS+dL+eO,, Fig. 1. Quasitemary phase diagram YO,,rBaO-CuOx at 875°C for ~(0,) =0.21X 10’ Pa (123: YBqCu30,_~, 211: Y,BaCuOS, 011: BaCuO*, 001: CuO, 202: Y2Cu205. 210: Y,BaO,, 220: Y,BqO,, 143: YBa,Cu,O,, 163: YB~,&u~O~~, “023”: “BazCu,OS”, “035”: “Ba&O,“.)
under these conditions. Typically (and provided in this treatment), DTA/TG measurements will be performed under conditions of constant ~(0,). Therefore, the temperature is unambiguously defined for those DTA/ TG events which are caused by univariant reactions. According to the Gibbs phase rule, four condensed phases can coexist in such an univariant equilibrium state, i.e. four condensed phases are included in the considered reactions (either as reactants or products) ‘. These four phases cannot be represented by a binary or quasibinary phase diagram. Fig. 1 represents a part of the quasiternary projection of the subsolidus phase diagram near 875°C for ~(0,) ~0.21 X 105 Pa. The considered secondary phases are marked by “ + “. Assuming that the content of secondary phases (impurities) to be detected by DTA/TG remains small, the DTA/TG behaviour will be represented by the four selected polytbermal sections in Fig. 2a-2d (as derived from the results in Ref. [ 1 ] ) . The charge of the DTA/TG curve of YBazCu30,_s in the presence of several impurities can be explained as follows: Pure YBa,Cu,07_ 6: Pure YBa2Cu307 _ s is stable up to 1020°C at p( 0,) = 0.21 X lo5 Pa. Above this tem-
(PI)
CuO additions cause a primary thermal event already
at 940°C [ 1,4] which is very sensitive even for small amounts of CuO. The complete solution of the YBa2Cu30,_s phase in the liquid is indicated by a subsequent event which is not caused by an univariant reaction, and consequently, this event does not occur at a distinct temperature. Its upper limit is 1020°C (the peritectic-like decomposition of YBazCu307_ *). However, especially in the case of small additions the temperature does not change significantly. BaCuO, (Fig. 2b): A DTA signal at 991°C indicates BaCuO, impurities. At this temperature, BaCuOz reacts with YBa2Cu307_ 6 forming YBa.,Cu,O, and a liquid phase according to: a YBa,Cu,O, _ s + b BaCuO, + c YB~.,CU~O~.~ + d L + e O2 .
(P3)
The
following events at 1010 and 1020°C which have been found to be independent of the amount of BaCuO, correspond to the decomposition of YBa&u309 and YBa2Cu30,_ 6, respectively [ 31. Y,BuCuOs (Fig. 2~): Y,BaCuO, coexists with YBazCu307 _ 6 up to the peritectic decomposition temperature of YBa2Cu30,_, at 1020°C [ 11. No additional reactions occur below 102O”C,and consequently, Y,BaCuOs impurities cannot be detected by DTA. CuO + BuCuO, (Fig. 2~): The DTA peak at 899°C resulting from the ternary eutectic reaction a YBa2Cu30, _ 6+ b CuO + c BaCuOl + d L + e O2
’ This number can be reduced by n if there are n further restricting conditions.
(e,)
U. Wiesner et al. /Materials Letters 24 (1995) 189-193
(a)1100 ri
A
\ 'k________ I/
191
e
I I i 2ll+L
MllOO9
lL
\ \
;
f.
-- - _ i 123+143+L
loo&.
i
f 123+011
900 i 0 20 $02331
40
60
mol%
80
100
CUO
I
: I
0 20 11231
d
0 y"l.5
20
40
60
mol%
40
60 mol %
80
IdO
3 (OIlI
80 ,!da ~'0237
Fig. 2. Schematic polythermal sections (signed in Fig. 1) involving the 123-phase and the considered impurities.
unambiguously indic.ates the presence of both BaCuOz and CuO secondary .phases. No further peak has to be observed below the decomposition temperature of YBa&u307_8 if the molar ratio CuO: iBaCuOz is about 3 : 4, which corresponds to the ratio in the eutectic liquid (CuO: iBaCuOz: iYBaZCu30,_6= 1: 1.3125: 0.1875). For larger lCu0 amounts, a subsequent peak can be observed between 899°C (ei) and 940°C (pi), whereas the right parts of Figs. 2c and 2d represent the situation of a BaCuO, excess. Here, the eutectic peak will subsequently be followed by a DTA event at T’, in which T’ is between 899 and 991°C ( p3). A third peak observed at 1020°C refers to the reaction (m, ) . YzBaCuO, + CuO (Fig. 2d): The common influence of both YZBaCu05 and CuO impurities can be clarified on the basis of an imagined Yz03-YBazCu307-s
“Ba,Cu,O,” section as represented in Fig. 2d. In comparison with Fig. 2a, it becomes evident that it is impossible to distinguish between the contamination by CuO or by a mixture of CuO + Y,BaCuO, by reason of the first and second DTA events, which occur at 94O’C and less than 1020°C in both cases. BaCO,: Free BaC03 shows a phase transition at 806°C from a rhombic to hexagonal structure without any weight change [ 51. In air, BaC03 decomposes at about 114oOC. Carbonate on lattice positions in the YBa,Cu,O, _ 6 phase cannot be detected in this way. 3. Experiments and results All DTA investigations have been carried out using a Setaram TG/DTA 92 apparatus with a heating rate
192
U. Wiesner et al. /Materials
Letters 24 (1995) 189-193
of 10 K/min from room temperature to 1200°C. The oxygen partial pressure (p( 0,) = 0.2 1 X lo5 Pa) in the flowing argon-oxygen mixture was controlled by a zirconia-emf cell. Both DTA and TG were recorded and the peak onset temperatures have been applied in the analysis. Fig. 3 represents the DTA plots of several YBa,Cu,O,_s samples mixed with the secondary phases discussed above. The adequate powders were intimately mixed in an agate mortar. All synthetic mixtures used in these experiments are marked in the corresponding polythermal sections. The main events of the curves in Fig. 3 are in agreement with the reaction temperatures deduced from Fig. 2. Fig. 4 represents the DTA plots of YBa&u&_, materials from different origins. Fig. 4a shows the DTA
800
1000
1200 TPC
Fig. 4. DTA plots of several 123 materials (see text). (a) Synthesized 123, (b) thismaterialafteradditionalheattreatment, (c) + (d) commercial 123 materials.
800
1000
1200 TPC
Fig. 3. DTA plots of several mixtures representing 123 and different impurities. (The positions of the mixtures are. indicated by capital letters in Fig. 2.) 0: pore 123 (see Fig. 4b), A: 123+CuO, B: 123+Oll,C: 123+211,D: 123+011+CuO,E: 123+211+CuO.
plot of a YBa,Cu,O, _ s sample which was synthesized in the following way. Stoichiometric amounts of Y203, BaCOs and CuO were intimately mixed and heat treated several times and ground (2 times for 17 h at 900 to 920°C in air, followed by a 24 h annealing at 930°C in 1 X 10s Pa 0,). Though the XRD analysis indicates the presence of a single phase material, the DTA plot (T( on) = 901°C) indicates a material which is contaminated with CuO and BaCuO*. After an additional heat treatment (20 h at 930°C in 1 X 10s Pa O2 and 20 h at 380°C in 1 X 10’ Pa 0,)) only a single DTA event is recorded at 1023°C corresponding to pure YBa,Cu,O, _ s (Fig. 4b). The plots of two commercially available materials are represented in Figs. 4c and 4d. Both samples contain small quantities of BaCuOz and CuO (T( on) = 907°C on both cases), though only in the first material CuO has been detected by XRD. Due to the oxygen deficiency of the melt all univariant reactions at ~(0,) =0.21 X lo5 Pa are accompa-
U. Wiesner et al. /Materials Letters 24 (1995) 189-193
nied with a mass change [ 21. TG of the melting process of the ternary eutectics (e,) ( 1.25 mol% Y0,.5, 28.75 mol% BaO, 70 mol% CuO) indicates for example a mass loss of 3.5%. However, these changes will be reduced to less than 0.05% if only 1% of secondary phases have to be detected. Therefore, the sensitivities of the TG signals are not sufficient with respect to the purpose discussed in the present paper.
4. Conclusions DTA is shown to be a fast and sensitivity method to detect CuO, BaCuOz and BaC03 impurities, as well as mixtures of these compounds in YBa#&O, _ s materials on the basis of’ well-defined univariant reactions which are characterized by a fixed temperature (independent of the composition) atp( 0,) = 0.21 X 10’ Pa. DTA is not convenient to indicate Y,BaCu05 additions. Both the onset temperatures of the DTA events subsequent to the univariant reactions, as well as all detected peak areas, depend on the amounts of the appropriate compounds thus opening the possibility for a quantitative interpretation. However, the temperature shift of the subsequent DTA events is very limited for
193
small quantities of secondary phases, and furthermore, it strongly depends on the oxygen partial pressure of the environment. Therefore, the analysis of the DTA peak areas is expected to be the more promising technique.
Acknowledgement The authors wish to express their thanks to Mrs. B. Thaut and Mrs. W. Hiippner for careful experimental work. This paper results from a project granted by the Federal Minister of Research and Technology (BMPT) under No. 13 N 5897 A.
References Ill G. Krabbes, W. Bieger, U. Wiesner, M. Ritschel and A. Teresiak, J. Solid State Chem. 13 (1993) 420. [ 21 G. Krabbes, U. Wiesner, W. Bieger andM. Ritschel, Z. Metallkd. 85 (1994) 70. [3] G. Krabbes, W. Bieger and U. Wiesner, 3rd IUMRS-ICAM, Tokyo, 1993. [4] T. A&age, preprint, submitted to Physica C. [5] I. Barin and 0. Knacke, Tehrmochemical properties of inorganics substances (Springer, Berlin, 1973).