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Fabrication of stabilized ZrO2 by hot petroleum drying method
61
CERAMURGIA INTERNATIONAL, Vol. 5, n. 2, 1970
Fabrication of Stabilized ZrOz by Hot Petroleum Drying Method SU-I:L PYUN Department
of
Materials
Science,
Korea
A series of MgD- and CsD- partially and fully-stabilized 21-0~ bodies were produced from sulfate and acetate powders using a wet chemical drying method [K hot petroleum drying method s). The optimum sintering conditions for powders produced from sulfates and acetates were found: calcining at 1200°C for 2-5 h, followed by sintering for 5 h at 16oo’C. This drying process proved to be a very effective method for preparing stabillzed ZrO, without conventional ceramic process of mixing, milling, and granulating.
1 . INTRODUCTION Stabilized Zr01 is often used as an oxygen sensor in molten metals because of its high oxygen ion conductivity, but reliable methods to fabricate suitable samples are still needed. Two factors are of prime importance for oxygen sensor: al the electronic conductivity should be negligibly small compared to ionic conductivity, and bl the probe should possess good thermal shock resistance, and at the same time must have a low oxygen permeability. These conditions are, unfortunately, sometimes incompatible. For example, presently available fully-stabilized ZrOl probes exhibit high ionic conductivity but have poor thermal shock resistance. In part, this may be due to lack of optimization of fabrication conditions and inappropriate choice and amount of the appropriate stabilizer. This work has attempted to optimize fabrication of MgO- and CaO-stabilized Zr02. Because solid-state reactions are involved in powder preparation, the sintered quality is also dependent on the particle size and purity of the starting materials. For this reason, we used a wet chemical drying method I-’ for the starting materials and found that a sinterable. reactive, and homogeneous powder could be readily produced. Optimized fabrication requires concern with both calcining and final sintering: For example, too low a calcining temperature can lead to a large shrinkage during final sintering, causing fracture of the sintered product, while too high a calcining temperature can lead to particle coalescence, which can impede densiflcation. We have found that, with appropriate control of calcining, high quality stabilized zirconia ceramics can be readily produced.
2 - SPECIMEN
PREPARATION
The method of specimen preparation, which has been mentioned in previous publications ** ’ was employed: Zr-sulfate, Mg-sulfate, Ca-sulfate, and Al-sulfate (or the corresponding acetates] were dissolved in water to give 7 different compositions. A commercial emulsifier was added to the aqueous solution to lower
Advanced
Institute
of
Science,
Seoul,
Korea,
the surface tension; this allowed very small droplets to be added to a hot petroleum bath (17OCI which yielded small sulfate (acetate) particles. The homogeneous powder was isolated from the petroleum bath by filtration. The first series of samples was partially stabilized and contained 6 mole O/O of solute stabilizer (either MgO, CaO, or an equal mixture), and 8 mole % of while the solute (6 mole % CaO + 2 mole VO AL03), second series was fully stabilized and contained 12 mole O/O solute (again either MgO, CaO, or a mixture); both sulfate-based and acetate-based powders of each composition were produced. The chemical analysis of the Zr-sulfate was HfCk < O.Ol%, SiOz = 0.075%, Ti(X < 0.01 O/O, and Fe203 < 0.01%. The purity of the metallic acetates was similar. The dried powders were calcined at 800”, llOO“, and 1200°C and discs, 10 mm in diameter and 3 mm thick were prepared at a pressure of 196 MPa and were sintered at 1600“ and 1800°C in air.
3 - RESULTS AND
DISCUSSION
Characterization of the powder using differential thermal analysis (DTA), thermogravimetric analysis (TGA), and x-ray diffraction (XRD) and characterization of the sintered discs (density and grain size) are presented in the following sections.
3.1 - DTA and TGA DTA and TGA were performed on a series of samples with a heating rate of iO”C/min (Figs. l-2). The evaporation of physically and chemically absorbed water, the decomposition of hydrates, and oxidation of the adsorbed petroleum all occurred below 710°C. A strong endothermic reaction occurred in sulfate-based powders between 710” and 750°C and was accompanied by a large weight loss due to the decomposition of Zrsulfate. The large magnitude of the endothermic peak at a narrow temperature range for the Mg-doped samples suggested rapid crystallization and XRD showed that by 800°C cubic phase of ZrCL had already formed. The cubic ZrO* produced at 800% is not believed to be stabilized by MgO solute, or any solute stabilizer. It is well-known’ that ZrcX with very fine particle size formed by decomposition of either organic or inorganic salts can have either tetragonal or cubic symmetry, even if the equilibrium structure is monoclinic. That the cubic ZrOl produced at 800°C has a very fine particle size was evident from the peak broadening in the XRD spectra. A second endothermic reaction appeared at 980.990% and 1150-I 180°C for the Mg- and Ca-doped samples
62
100 100
200
300400
600600
200
300
400
300
600
700
600900
loo0
SWIL
PYUN
1100
1200
Temperature OC-
7008009001ooo1100
Temperature OCFIGURE 1 Mg-sulfate.
-
DTA
and
TGA
curves
of i&sulfate
with
FIGURE 2 Ca-sulfate.
12 mole 94
-
DTA
and
TGA
curves
of
Zr-sulfate
with
6
mole %
and the corresponding weight losses respectively, indicate that Mg- and Ca-sulfates decompose at these temperatures; the decomposition of Mg- and Ca-sulfates in the mixed powders occurs at a lower temperature than in pure sulfates themselves (1127” and 145o”C, respectively). The lowering of the decomposition temperature of powders produced by petroleum drying method is attributed to the intimate mixing of the components. The enthalpy change due to the formation of cubic ZrO* is so small that it could not be observed by DTA.
the comparison of the intensity of various mixtures of cubic and monoclinic ZrO?. As noted above in the samples containing 6 and 12 mole % MgO, a metastable cubic phase has formed at 8OO”C, but transformed to a phase having tetragonal symmetry after heating at 1200°C. At 1600°C samples containing 6 and 12 mole % MgO consisted of both tetragonal and cubic phase, and a cubic solid solution, respectively. The samples with 6 mole % CaO were both tetragonat and cubic at 1200°C and 1600°C. The samples doped with 12 mole % CaO contained both tetragonal and cubic solid solution at 1200°C and transformed to a homogeneous cubic solid solution by heating at 1600°C.
3.2 - X-ray diffraction
The was
analysis
As shown in Tables I and II, the specimens were heated at various temperatures for times varying between 10 min and 20 h, cooled to room temperature, and the phases present were determined by XRD. The proportion of cubic to monoclinic ZrOz was determined using the standard curve which was calibrated by
TABLE
I
- XRD
results
of partially-stabilized
210,
prepared
6 MgO
800°C (powder)
1000°C (powder)
> 90% cubic < 10% monoclinic
from:
6 CaO
50% 50%
cubic monoclinic
6 CaO 2 Al203
1200°C (powder)
540°C (powder)
610°C (powder)
3 MgO 3 CaO 6 CaO
cubic
100% cubic
1600°C (sintered spcms)
40% 60%
cubic monoclinic
60-70% 30-40%
cubic monoclinic
50°% cubic 50% monoclinic
70-80% 20-30%
cubic monoclinic
>95% <5%
cubic monoclinic
>95% <5%
cubic monoclinic
50% 50%
cubic monoclinic
50%
50%
cubic monoclinic
50-60% 40-50%
cubic monoclinic
70% 30%
cubic monoclinic
.lOO% cubic
powders
1200°C (powder) 40% 60%
100%
1600°C (powder)
monoclinic
Acetate-based Stabilizer content (mole %)
powders
100%
3 MgO 3 CaO
relationship in the MgO-CaO-doped samples to that in the CaO-doped samples. All the
above-mentioned tetragonal forms which were stable at a high temperature were inverted to the monoclinic forms during cooling. The results of XRD on MgO- and CaO-doped tirconia were in accord with the phase relationship in the system ZrOrMg05 and ZrO1-CaO 6, respectively. If 1.72 wt % ALOJ impurity (2 mole %)
Sulfate-based Stabilizer content (mole %)
phase similar
cubic monoclinic
90% cubic 10% monoclinic
1600°C (powder) $O’+O cubic 50% monoclinic
1600°C (sintered spcms) 60-70% 30-40% 100%
cubic monoclinic cubic
FABRICATION
___
OF
STABILIZED
ZrOz
BY
HOT
PETROLEU~M
DRYING
63
METHOD
---
TABLE
ll
- XRD
results
of full-stabilized
ZrO,
prepared
from:
Sulfate-based Stabilizer content [mole %I I2 MgO
800°C
(powder)
1000°C (powder)
powders
1150°C (powder)
1’200°C (powder)
< 10% cubic >90% monoclinic
>90% cubic < 10% monoclinic
6 MgO 6 CaO > 90% cubic < 10% monoclinic
I2 CaO
Acetate-based Stabilizer content (mole %)
540°C (powder)
610°C (powder)
I2 CaO
100% cubic
100% cubic
cubic monoclinic
> 99% cubic
>90% cubic < 10% monoclinic
100% cubic
100% cubic
>90%
100% cubic
100% cubic
cubic monoclinic
1600°C (powder)
1200°C (powder)
>90%
>99%
1600°C (sintered specimens)
powders
> 90% cubic < 10% monoclinic
6 MgO 6 CaO
100% monoclinic
1600°C (powder)
cubic monoclinic
100% cubic
sI~~~r$e~$ntered 100% cubic 100% cubic
was added to partially CaO-stabilized sample, it was found to promote stabilization of zirconia. In the sample sintered at 1600°C free AL03 could not be detected by the XRD spectra. Finally, high temperature XRD showed that a metastable cubic zirconia had already formed at 540 and 610% in the acetate-based zirconia powders. The occurrence of a metastable cubic phase resulted from a fine particle size effect. It is interesting that sintered bodies generally has a higher cubic content than did powders of the same composition.
3.3 - Calcining In order to characterize the calcined powders, the specific surface areas were investigated using the BET method and the average particle size was calculated from the equation d = 6/s
p
a
where d is the average diameter of the particle in cm, s the specific surface in m’/g and p is the theoretical density of the sample in g/cm’ (Table IIII. As expected, coarser powders were found at the higher sintering temperatures.
TABLE heated
Ill - Determination of the average particle size at 800°C and 1200°C using the BET method:
Sulfate-based
Composition 6 6 I2 6 6 6 12
MgO MgO MgO CaO MgO CaO CaO
powders
Calcining
Specific surface
Particle size
800°C I h 1200°C 4h 800°C I h 1200°C 3h 1200°C 4 h
15.9 0.89 13.1 I .46 0.94
0.064 I.18 0.08 0.71 1.12
1200°C 4 h
0.67
1.57
Acetate-based 6 CaO I2 CaO
of powder
1200°C 4 h 1200°C 4 h
powders I .71
1.58
0.58 0.65
b FIGURE 3 - Microstructure of sintered body for 12 mole % CaO - stabilized ZrO, prepared from acetate-based powder. Calcined at 1200°C for 4 h and sintered at 16WC for 5 h. 300 x FIGURE 3a - Unpolished, unetched d - 22 p,m. FIGURE 3b - Polished. thermally etched d - 21 pm.
SU-IL PYUN
64
.__--
TABLE
IV
- Results
Sulfate-based
of the
powders
sintering
calcined
of
at
MgO
and
CaO-stabilized
for
1200°C
--
2-5 h and
Stabilizer content
Content of cubic ZrOl WI
Lattice constant of cubic ZrOz (AI
6 MgO
60-70
5.068
DO*:
sintered
at 1600°C
for
5 h
Gt$nt;ze Porosity
Density p experimental (g/cm’)
unpolished sample Wnl
5.31
10.6
Grain growth and grain size of the polished sample (pm1
13
disc;nt$uous
discontinuous
P oY8*= 12 MgO
99
5.047
4.76
18.6
3 MgO 3 CaO
95
5.082
5.39
10.2
15
100
5.082
4.71
18.5
8
6 CaO
95
5.095
5.24
12.2
17
discontinuous
6 CaO 2 AhO,
100
5.082
5.26
12.5
12
discontinuous
5.102
4.32
25.0
6
66 %
12 CaO
TABLE
V
- Results
Acetate-based
of
powders
the
slnterlng
calcined
of
at
MgO
and
1200°C
CaO-stabilized
ZrO,:
2-5 h and
sintered
for
at 1600%
for
12
5h
Content of cubic ZrO* (O/o1
Lattice constant of cubic Zr02 [AI
3 MgO 3 CaO
60-70
5.082
5.26
11.1
6 MgO 6 CaO
100
5.082
4.63
19.9 P o!Jen <0.8
6 CaO
100
5.104
5.29
11.1
31
12 CaO
100
5.095
5.10
11.8
22
Stabilizer content
TABLE
VI
- Results
Sulfate-based
of the
powders
sintering
calcined
of
at
MgO
and
1200°C for
CaO-stabilized 4 h and
27
discontinuous discontinuous
discontinuous 9;l 21
ZrOz:
sintered
at
1800°C for
4 h
Stabilizer content
Content of cubic Zr02 (%I
Lattice constant of cubic ZrOl [AI
6 MgO
>95
5.068
5.20
13.6
3 MgO 3 CaO
100
5.082
5.26
12.5
63
6 MgO 6 CaO
100
5.082
4.78
17.3 P open= 0.80
29
12 CaO
100
5.102
5.08
11.8
Acetate-based
powders
calcined
at
1200°C
for
4 h
and
Density p experimental
Porosity
[g/cm’1
sintered
at
1800°C
00 f:o:8:)
for
Grain size of the unpolished sample (pm)
4 h
6 MgO 6 CaO
100
5.082
4.73
18.2
20
6 CaO
100
5.104
5.30
10.9
69
FA6RtDATtDN _ _ --
OF STABILIZED
~. -----~
ZrDz BY HOT PETROLEUM DRYING
METHOD
__~~___-._---”
-
..---
____.-
65
.-
SfDterDd samples prepared from powders calcined at 800” and 1100°C were cracked; this resulted from the Hence, all undecomposed sulfates in these powders. powders were calcined at 1200°C.
3.4 - Sintered
bodies
Sintering was carried out at 1600” and 1800°C and the results are collected in Tables IV-VI. The total * of the sintered porosity * and the open porosity samples were determined using mercury porosimetry, while the theoretical density was determined using XRD. Even increasing the sintering temperature from 1600” to 1800°C brought no increase in the sintered density. In all measured cases, the open porosity was I 0.8%. Such materials are clearly desirable for oxygen sensor. Independent of the acetate or sulfate based starting material, the fully stabilized (12 mole % solute) bodies tended to be more porous than the partially stabilized bodies (6 mole % solute). It was reported in the literature’ that the sintering of zirconia partially stabilized with CaO was enhanced by SiO* impurity. However, addition of 1.72 wt % Al201 impurity brought no increase in the sintered density. A polished section was produced from every sample and thermally etched in air for 30-45 min at about 150°C lower than the sintering temperature. The grain size of the polished samples was compared to the surface grain size of the sintered bodies (Tables IV and VI, and were found to be nearly the same [Fig. 3). Discontinuous grain growth occurred on occasion (Fig. 41, but only in the partially stabilized samples. The grain size was 5-15 pm for the samples prepared from sulfate based powders and 20-30 urn for the samples prepared from acetate-based powders for a sintering temperature of 1600°C. Sintering at 1800% resulted in appreciable increase in grain growth [60-70 Irm grain size in the acetate samples); however, no further densification occurred (Fig. 5). The density and microstructural data indicate that sintering at 1600°C is satisfactory. ??
of sintered body for 6 mole % FIGURE 4 - Microstructure MgO -stabilized ZrO, prepared from sulfate-based powder. Calcined at 1200% for 4 h and sintered at 1600°C for 5 h. Polished, thermally etched. d < 1 pm for non-growing greins and d Y 8 pm for growing grains. 300x
4 - CONCLUSION A series of MgO- and CaO- partially and fully-stabilized Zr02 bodies were produced from sulfate and acetate powders using a wet chemical drying method. On the basis of the results of stabilization grade and sintered density of zirconia the optimum sintering conditions for powders produced from sulfate and acetate were; calcining at 1200°C for 2-5 h, followed by sintering for 5 h at 1600°C. A metastable cubic phase occurred at the low temperature (540”, 610”, and 800%). At a high temperature (1600°C) magnesia and calcia solute stabilizer were nearly equivalent SD far as sintering grade of zirconia was concerned.
AKNOWLEDGEMENT The author wishes to express his gratitude to Prof. Arthur H. Heuer, Case Western Reserve University, Cleveland, Ohio, for his helpful discussion in the preparation of this manuscript.
of sintered body for 6 mole % FIGURE 5 - Microstructure CaG - stabllized Zr@ prepared from acetate-based powder. Calcined at 1200% for 4 h and sintersd at 1800°C for 4 h. Unpolished. unetched. d - 69f.tm. 300x
REFERENCES and W.W. RHODES, Science 1. F.J. SCHNETTLER, F.R. MONFORTE of Ceramics, edited by G.H. Stewart, published by the British Ceramic Society, vol. 4 (1968) 79. Powder Met. Intern. 8 (1976) 2. P. REYNEN and H. BASTIUS. 91. M. FAIZULLAH and H.V. KAMPTZ, 3. P. REYNEN, H. BASTIUS. Ber. Dtsch. Keram. Ges. 54 (19771 63. 4. R.C. GARWE. J. Phys. Chem. 69 (19651 1238. and V.S. STUBICAN, J. Amer. Ceram. Sot. 5. D. VIECHNICKI 46 (1965) 292. STUBICAN and S.P. RAY. J. Amer. Ceram. Sot. 66 6. V.S. (1977) 534. P.S. NICHOLSON and W.W. SMELTZER, 7. J.F. SHACKELFORD, Amer. Ceram. Sot. Bull. 53 (1974) 865.
Received
May
29, 1978;
V pore * P0pm =
??
V .PnxPIC by H9.
,
revised
where
V,,,,
copy
is
received
the
pore
November
volume
28, 1978.
displaced
CERAMURGIA INTERNATIONAL, Vol. 5, n. 2, 1970
Fabrication of Stabilized ZrOz by Hot Petroleum Drying Method SU-I:L PYUN Department
of
Materials
Science,
Korea
A series of MgD- and CsD- partially and fully-stabilized 21-0~ bodies were produced from sulfate and acetate powders using a wet chemical drying method [K hot petroleum drying method s). The optimum sintering conditions for powders produced from sulfates and acetates were found: calcining at 1200°C for 2-5 h, followed by sintering for 5 h at 16oo’C. This drying process proved to be a very effective method for preparing stabillzed ZrO, without conventional ceramic process of mixing, milling, and granulating.
1 . INTRODUCTION Stabilized Zr01 is often used as an oxygen sensor in molten metals because of its high oxygen ion conductivity, but reliable methods to fabricate suitable samples are still needed. Two factors are of prime importance for oxygen sensor: al the electronic conductivity should be negligibly small compared to ionic conductivity, and bl the probe should possess good thermal shock resistance, and at the same time must have a low oxygen permeability. These conditions are, unfortunately, sometimes incompatible. For example, presently available fully-stabilized ZrOl probes exhibit high ionic conductivity but have poor thermal shock resistance. In part, this may be due to lack of optimization of fabrication conditions and inappropriate choice and amount of the appropriate stabilizer. This work has attempted to optimize fabrication of MgO- and CaO-stabilized Zr02. Because solid-state reactions are involved in powder preparation, the sintered quality is also dependent on the particle size and purity of the starting materials. For this reason, we used a wet chemical drying method I-’ for the starting materials and found that a sinterable. reactive, and homogeneous powder could be readily produced. Optimized fabrication requires concern with both calcining and final sintering: For example, too low a calcining temperature can lead to a large shrinkage during final sintering, causing fracture of the sintered product, while too high a calcining temperature can lead to particle coalescence, which can impede densiflcation. We have found that, with appropriate control of calcining, high quality stabilized zirconia ceramics can be readily produced.
2 - SPECIMEN
PREPARATION
The method of specimen preparation, which has been mentioned in previous publications ** ’ was employed: Zr-sulfate, Mg-sulfate, Ca-sulfate, and Al-sulfate (or the corresponding acetates] were dissolved in water to give 7 different compositions. A commercial emulsifier was added to the aqueous solution to lower
Advanced
Institute
of
Science,
Seoul,
Korea,
the surface tension; this allowed very small droplets to be added to a hot petroleum bath (17OCI which yielded small sulfate (acetate) particles. The homogeneous powder was isolated from the petroleum bath by filtration. The first series of samples was partially stabilized and contained 6 mole O/O of solute stabilizer (either MgO, CaO, or an equal mixture), and 8 mole % of while the solute (6 mole % CaO + 2 mole VO AL03), second series was fully stabilized and contained 12 mole O/O solute (again either MgO, CaO, or a mixture); both sulfate-based and acetate-based powders of each composition were produced. The chemical analysis of the Zr-sulfate was HfCk < O.Ol%, SiOz = 0.075%, Ti(X < 0.01 O/O, and Fe203 < 0.01%. The purity of the metallic acetates was similar. The dried powders were calcined at 800”, llOO“, and 1200°C and discs, 10 mm in diameter and 3 mm thick were prepared at a pressure of 196 MPa and were sintered at 1600“ and 1800°C in air.
3 - RESULTS AND
DISCUSSION
Characterization of the powder using differential thermal analysis (DTA), thermogravimetric analysis (TGA), and x-ray diffraction (XRD) and characterization of the sintered discs (density and grain size) are presented in the following sections.
3.1 - DTA and TGA DTA and TGA were performed on a series of samples with a heating rate of iO”C/min (Figs. l-2). The evaporation of physically and chemically absorbed water, the decomposition of hydrates, and oxidation of the adsorbed petroleum all occurred below 710°C. A strong endothermic reaction occurred in sulfate-based powders between 710” and 750°C and was accompanied by a large weight loss due to the decomposition of Zrsulfate. The large magnitude of the endothermic peak at a narrow temperature range for the Mg-doped samples suggested rapid crystallization and XRD showed that by 800°C cubic phase of ZrCL had already formed. The cubic ZrO* produced at 800% is not believed to be stabilized by MgO solute, or any solute stabilizer. It is well-known’ that ZrcX with very fine particle size formed by decomposition of either organic or inorganic salts can have either tetragonal or cubic symmetry, even if the equilibrium structure is monoclinic. That the cubic ZrOl produced at 800°C has a very fine particle size was evident from the peak broadening in the XRD spectra. A second endothermic reaction appeared at 980.990% and 1150-I 180°C for the Mg- and Ca-doped samples
62
100 100
200
300400
600600
200
300
400
300
600
700
600900
loo0
SWIL
PYUN
1100
1200
Temperature OC-
7008009001ooo1100
Temperature OCFIGURE 1 Mg-sulfate.
-
DTA
and
TGA
curves
of i&sulfate
with
FIGURE 2 Ca-sulfate.
12 mole 94
-
DTA
and
TGA
curves
of
Zr-sulfate
with
6
mole %
and the corresponding weight losses respectively, indicate that Mg- and Ca-sulfates decompose at these temperatures; the decomposition of Mg- and Ca-sulfates in the mixed powders occurs at a lower temperature than in pure sulfates themselves (1127” and 145o”C, respectively). The lowering of the decomposition temperature of powders produced by petroleum drying method is attributed to the intimate mixing of the components. The enthalpy change due to the formation of cubic ZrO* is so small that it could not be observed by DTA.
the comparison of the intensity of various mixtures of cubic and monoclinic ZrO?. As noted above in the samples containing 6 and 12 mole % MgO, a metastable cubic phase has formed at 8OO”C, but transformed to a phase having tetragonal symmetry after heating at 1200°C. At 1600°C samples containing 6 and 12 mole % MgO consisted of both tetragonal and cubic phase, and a cubic solid solution, respectively. The samples with 6 mole % CaO were both tetragonat and cubic at 1200°C and 1600°C. The samples doped with 12 mole % CaO contained both tetragonal and cubic solid solution at 1200°C and transformed to a homogeneous cubic solid solution by heating at 1600°C.
3.2 - X-ray diffraction
The was
analysis
As shown in Tables I and II, the specimens were heated at various temperatures for times varying between 10 min and 20 h, cooled to room temperature, and the phases present were determined by XRD. The proportion of cubic to monoclinic ZrOz was determined using the standard curve which was calibrated by
TABLE
I
- XRD
results
of partially-stabilized
210,
prepared
6 MgO
800°C (powder)
1000°C (powder)
> 90% cubic < 10% monoclinic
from:
6 CaO
50% 50%
cubic monoclinic
6 CaO 2 Al203
1200°C (powder)
540°C (powder)
610°C (powder)
3 MgO 3 CaO 6 CaO
cubic
100% cubic
1600°C (sintered spcms)
40% 60%
cubic monoclinic
60-70% 30-40%
cubic monoclinic
50°% cubic 50% monoclinic
70-80% 20-30%
cubic monoclinic
>95% <5%
cubic monoclinic
>95% <5%
cubic monoclinic
50% 50%
cubic monoclinic
50%
50%
cubic monoclinic
50-60% 40-50%
cubic monoclinic
70% 30%
cubic monoclinic
.lOO% cubic
powders
1200°C (powder) 40% 60%
100%
1600°C (powder)
monoclinic
Acetate-based Stabilizer content (mole %)
powders
100%
3 MgO 3 CaO
relationship in the MgO-CaO-doped samples to that in the CaO-doped samples. All the
above-mentioned tetragonal forms which were stable at a high temperature were inverted to the monoclinic forms during cooling. The results of XRD on MgO- and CaO-doped tirconia were in accord with the phase relationship in the system ZrOrMg05 and ZrO1-CaO 6, respectively. If 1.72 wt % ALOJ impurity (2 mole %)
Sulfate-based Stabilizer content (mole %)
phase similar
cubic monoclinic
90% cubic 10% monoclinic
1600°C (powder) $O’+O cubic 50% monoclinic
1600°C (sintered spcms) 60-70% 30-40% 100%
cubic monoclinic cubic
FABRICATION
___
OF
STABILIZED
ZrOz
BY
HOT
PETROLEU~M
DRYING
63
METHOD
---
TABLE
ll
- XRD
results
of full-stabilized
ZrO,
prepared
from:
Sulfate-based Stabilizer content [mole %I I2 MgO
800°C
(powder)
1000°C (powder)
powders
1150°C (powder)
1’200°C (powder)
< 10% cubic >90% monoclinic
>90% cubic < 10% monoclinic
6 MgO 6 CaO > 90% cubic < 10% monoclinic
I2 CaO
Acetate-based Stabilizer content (mole %)
540°C (powder)
610°C (powder)
I2 CaO
100% cubic
100% cubic
cubic monoclinic
> 99% cubic
>90% cubic < 10% monoclinic
100% cubic
100% cubic
>90%
100% cubic
100% cubic
cubic monoclinic
1600°C (powder)
1200°C (powder)
>90%
>99%
1600°C (sintered specimens)
powders
> 90% cubic < 10% monoclinic
6 MgO 6 CaO
100% monoclinic
1600°C (powder)
cubic monoclinic
100% cubic
sI~~~r$e~$ntered 100% cubic 100% cubic
was added to partially CaO-stabilized sample, it was found to promote stabilization of zirconia. In the sample sintered at 1600°C free AL03 could not be detected by the XRD spectra. Finally, high temperature XRD showed that a metastable cubic zirconia had already formed at 540 and 610% in the acetate-based zirconia powders. The occurrence of a metastable cubic phase resulted from a fine particle size effect. It is interesting that sintered bodies generally has a higher cubic content than did powders of the same composition.
3.3 - Calcining In order to characterize the calcined powders, the specific surface areas were investigated using the BET method and the average particle size was calculated from the equation d = 6/s
p
a
where d is the average diameter of the particle in cm, s the specific surface in m’/g and p is the theoretical density of the sample in g/cm’ (Table IIII. As expected, coarser powders were found at the higher sintering temperatures.
TABLE heated
Ill - Determination of the average particle size at 800°C and 1200°C using the BET method:
Sulfate-based
Composition 6 6 I2 6 6 6 12
MgO MgO MgO CaO MgO CaO CaO
powders
Calcining
Specific surface
Particle size
800°C I h 1200°C 4h 800°C I h 1200°C 3h 1200°C 4 h
15.9 0.89 13.1 I .46 0.94
0.064 I.18 0.08 0.71 1.12
1200°C 4 h
0.67
1.57
Acetate-based 6 CaO I2 CaO
of powder
1200°C 4 h 1200°C 4 h
powders I .71
1.58
0.58 0.65
b FIGURE 3 - Microstructure of sintered body for 12 mole % CaO - stabilized ZrO, prepared from acetate-based powder. Calcined at 1200°C for 4 h and sintered at 16WC for 5 h. 300 x FIGURE 3a - Unpolished, unetched d - 22 p,m. FIGURE 3b - Polished. thermally etched d - 21 pm.
SU-IL PYUN
64
.__--
TABLE
IV
- Results
Sulfate-based
of the
powders
sintering
calcined
of
at
MgO
and
CaO-stabilized
for
1200°C
--
2-5 h and
Stabilizer content
Content of cubic ZrOl WI
Lattice constant of cubic ZrOz (AI
6 MgO
60-70
5.068
DO*:
sintered
at 1600°C
for
5 h
Gt$nt;ze Porosity
Density p experimental (g/cm’)
unpolished sample Wnl
5.31
10.6
Grain growth and grain size of the polished sample (pm1
13
disc;nt$uous
discontinuous
P oY8*= 12 MgO
99
5.047
4.76
18.6
3 MgO 3 CaO
95
5.082
5.39
10.2
15
100
5.082
4.71
18.5
8
6 CaO
95
5.095
5.24
12.2
17
discontinuous
6 CaO 2 AhO,
100
5.082
5.26
12.5
12
discontinuous
5.102
4.32
25.0
6
66 %
12 CaO
TABLE
V
- Results
Acetate-based
of
powders
the
slnterlng
calcined
of
at
MgO
and
1200°C
CaO-stabilized
ZrO,:
2-5 h and
sintered
for
at 1600%
for
12
5h
Content of cubic ZrO* (O/o1
Lattice constant of cubic Zr02 [AI
3 MgO 3 CaO
60-70
5.082
5.26
11.1
6 MgO 6 CaO
100
5.082
4.63
19.9 P o!Jen <0.8
6 CaO
100
5.104
5.29
11.1
31
12 CaO
100
5.095
5.10
11.8
22
Stabilizer content
TABLE
VI
- Results
Sulfate-based
of the
powders
sintering
calcined
of
at
MgO
and
1200°C for
CaO-stabilized 4 h and
27
discontinuous discontinuous
discontinuous 9;l 21
ZrOz:
sintered
at
1800°C for
4 h
Stabilizer content
Content of cubic Zr02 (%I
Lattice constant of cubic ZrOl [AI
6 MgO
>95
5.068
5.20
13.6
3 MgO 3 CaO
100
5.082
5.26
12.5
63
6 MgO 6 CaO
100
5.082
4.78
17.3 P open= 0.80
29
12 CaO
100
5.102
5.08
11.8
Acetate-based
powders
calcined
at
1200°C
for
4 h
and
Density p experimental
Porosity
[g/cm’1
sintered
at
1800°C
00 f:o:8:)
for
Grain size of the unpolished sample (pm)
4 h
6 MgO 6 CaO
100
5.082
4.73
18.2
20
6 CaO
100
5.104
5.30
10.9
69
FA6RtDATtDN _ _ --
OF STABILIZED
~. -----~
ZrDz BY HOT PETROLEUM DRYING
METHOD
__~~___-._---”
-
..---
____.-
65
.-
SfDterDd samples prepared from powders calcined at 800” and 1100°C were cracked; this resulted from the Hence, all undecomposed sulfates in these powders. powders were calcined at 1200°C.
3.4 - Sintered
bodies
Sintering was carried out at 1600” and 1800°C and the results are collected in Tables IV-VI. The total * of the sintered porosity * and the open porosity samples were determined using mercury porosimetry, while the theoretical density was determined using XRD. Even increasing the sintering temperature from 1600” to 1800°C brought no increase in the sintered density. In all measured cases, the open porosity was I 0.8%. Such materials are clearly desirable for oxygen sensor. Independent of the acetate or sulfate based starting material, the fully stabilized (12 mole % solute) bodies tended to be more porous than the partially stabilized bodies (6 mole % solute). It was reported in the literature’ that the sintering of zirconia partially stabilized with CaO was enhanced by SiO* impurity. However, addition of 1.72 wt % Al201 impurity brought no increase in the sintered density. A polished section was produced from every sample and thermally etched in air for 30-45 min at about 150°C lower than the sintering temperature. The grain size of the polished samples was compared to the surface grain size of the sintered bodies (Tables IV and VI, and were found to be nearly the same [Fig. 3). Discontinuous grain growth occurred on occasion (Fig. 41, but only in the partially stabilized samples. The grain size was 5-15 pm for the samples prepared from sulfate based powders and 20-30 urn for the samples prepared from acetate-based powders for a sintering temperature of 1600°C. Sintering at 1800% resulted in appreciable increase in grain growth [60-70 Irm grain size in the acetate samples); however, no further densification occurred (Fig. 5). The density and microstructural data indicate that sintering at 1600°C is satisfactory. ??
of sintered body for 6 mole % FIGURE 4 - Microstructure MgO -stabilized ZrO, prepared from sulfate-based powder. Calcined at 1200% for 4 h and sintered at 1600°C for 5 h. Polished, thermally etched. d < 1 pm for non-growing greins and d Y 8 pm for growing grains. 300x
4 - CONCLUSION A series of MgO- and CaO- partially and fully-stabilized Zr02 bodies were produced from sulfate and acetate powders using a wet chemical drying method. On the basis of the results of stabilization grade and sintered density of zirconia the optimum sintering conditions for powders produced from sulfate and acetate were; calcining at 1200°C for 2-5 h, followed by sintering for 5 h at 1600°C. A metastable cubic phase occurred at the low temperature (540”, 610”, and 800%). At a high temperature (1600°C) magnesia and calcia solute stabilizer were nearly equivalent SD far as sintering grade of zirconia was concerned.
AKNOWLEDGEMENT The author wishes to express his gratitude to Prof. Arthur H. Heuer, Case Western Reserve University, Cleveland, Ohio, for his helpful discussion in the preparation of this manuscript.
of sintered body for 6 mole % FIGURE 5 - Microstructure CaG - stabllized Zr@ prepared from acetate-based powder. Calcined at 1200% for 4 h and sintersd at 1800°C for 4 h. Unpolished. unetched. d - 69f.tm. 300x
REFERENCES and W.W. RHODES, Science 1. F.J. SCHNETTLER, F.R. MONFORTE of Ceramics, edited by G.H. Stewart, published by the British Ceramic Society, vol. 4 (1968) 79. Powder Met. Intern. 8 (1976) 2. P. REYNEN and H. BASTIUS. 91. M. FAIZULLAH and H.V. KAMPTZ, 3. P. REYNEN, H. BASTIUS. Ber. Dtsch. Keram. Ges. 54 (19771 63. 4. R.C. GARWE. J. Phys. Chem. 69 (19651 1238. and V.S. STUBICAN, J. Amer. Ceram. Sot. 5. D. VIECHNICKI 46 (1965) 292. STUBICAN and S.P. RAY. J. Amer. Ceram. Sot. 66 6. V.S. (1977) 534. P.S. NICHOLSON and W.W. SMELTZER, 7. J.F. SHACKELFORD, Amer. Ceram. Sot. Bull. 53 (1974) 865.
Received
May
29, 1978;
V pore * P0pm =
??
V .PnxPIC by H9.
,
revised
where
V,,,,
copy
is
received
the
pore
November
volume
28, 1978.
displaced