Preparation of nano-scale Cr3C2 particles dispersed on alumina particles by MOCVD in fluidized reactor and carbothermal treatment

Article abstract of DOI:10.1016/j.jallcom.2005.07.071

Nano-scale Cr₃C₂ powder with an average particle size of 50-80 nm, uniformly coated on alumina particles, has been prepared by metal organic chemical vapor deposition (MOCVD) in the fluidized chamber, using the pyrolysis of Cr(CO)₆ precursor, and carbothermal treatment in the CH₄ and H₂ gas mixtures. The Cr(CO)₆ precursor was characterized by differential scanning calorimetry (DSC) to understand the decomposition temperature of the precursor, which was determined to be 163 °C. The precursor decomposed and formed the mixture of CrC_{1 - x}, Cr₂O₃ and free carbon on the surface of the Al₂O₃ particles when pyrolyzed in fluidized bed at 300 and 400 °C. The resultant powders were examined by X-ray diffractometer (XRD) and transmission electron microscopy (TEM) equipped with energy dispersion spectrometer (EDS), for investigating phase composition and microstructures. The as-deposited particles fabricated at 300 and 400 °C in fluidized chamber were amorphous and crystalline, respectively. The thermal behaviour investigated by TG/DTA indicates that the amorphous powder had a exothermic peak in the DTA curve at ～400 °C, which crystallized at this temperature. In order to prepare the nano-particle reinforced composites, the powder fabricated in the fluidized bed was carbonized in the mixed atmosphere of CH₄ and H₂. Both CrC_{1 - x} and Cr₂O₃ phase were transferred into Cr₃C₂ phase after the carbothermal treatment.

Full text of DOI:10.1016/j.jallcom.2005.07.071

Journal of Alloys and Compounds 417 (2006) 214–219
Preparation of nano-scale Cr C particles dispersed on alumina particles
3
2
by MOCVD in fluidized reactor and carbothermal treatment
a
a,∗
b
a
Hao-Tung Lin , Jow-Lay Huang , Sheng-Chang Wang , Chen-Fu Lin
a
Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan, ROC
Department of Mechanical Engineering, Southern Taiwan University of Technology, Tainan 710, Taiwan, ROC
b
Received 25 March 2005; accepted 29 July 2005
Available online 9 November 2005
Abstract
Nano-scale Cr
3
C
2
powder with an average particle size of 50–80 nm, uniformly coated on alumina particles, has been prepared by metal organic
chemical vapor deposition (MOCVD) in the fluidized chamber, using the pyrolysis of Cr(CO) precursor, and carbothermal treatment in the CH
6
4
and H
2
gas mixtures. The Cr(CO)
6
precursor was characterized by differential scanning calorimetry (DSC) to understand the decomposition
◦
temperature of the precursor, which was determined to be 163 C. The precursor decomposed and formed the mixture of CrC1 − x, Cr
carbon on the surface of the Al
2
O
3
and free
◦
2
O
3
particles when pyrolyzed in fluidized bed at 300 and 400 C. The resultant powders were examined by X-ray
diffractometer (XRD) and transmission electron microscopy (TEM) equipped with energy dispersion spectrometer (EDS), for investigating phase
◦
composition and microstructures. The as-deposited particles fabricated at 300 and 400 C in fluidized chamber were amorphous and crystalline,
respectively. The thermal behaviour investigated by TG/DTA indicates that the amorphous powder had a exothermic peak in the DTA curve at
◦
∼
400 C, which crystallized at this temperature. In order to prepare the nano-particle reinforced composites, the powder fabricated in the fluidized
bed was carbonized in the mixed atmosphere of CH
treatment.
4
and H
2
2 3 3 2
. Both CrC1 − x and Cr O phase were transferred into Cr C phase after the carbothermal
©
2005 Elsevier B.V. All rights reserved.
Keywords: Nano-scale; Pyrolysis; Metal organic chemical vapor deposition (MOCVD); Chromium carbide
1
. Introduction
and high temperature erosion resistance [4]. Quite promising
mechanical properties and high temperature oxidation resistance
of chromium carbide/alumina composites were reported in the
literatures [4,8–10].
Alumina has been broadly used because it possesses excel-
lent mechanical properties, and good chemical stability. How-
ever, because of covalent bonding, it has inherently low fracture
toughness and plastic deformation, which limited the applica-
tion. Therefore, to improve the toughening of alumina ceramics
is an important and challenging area. Incorporation of secondary
phases (e.g. particulates, fibers, or platelets) has been proven
to be an easy, safe and economical toughening technique for
alumina ceramics [1–3]. Among the reported toughening meth-
ods [4–7], the incorporation of hard particulate reinforcement is
the best effective technique for toughening of alumina ceram-
ics. Chromium carbide (Cr3C2) successfully incorporated into
Al2O3 for toughening purposes owing to its properties such
as high hardness, high melting point, high Young’s modulus
Niihara reported the advantages of nano-meter inclusions
could achieve several benefits [11], such as the reduction of grain
size of matrix grains, strengthening and toughening. For prepar-
ing particle-reinforced composite, nano-scale reinforcing parti-
cles are difficult to uniformly disperse on the micro-scale matrix
particles by using traditional mixing technique. This problem is
ascribed to that the nano-sized particles are easy to agglomerate
due to the high attractive force energy between the particles [12].
The agglomerate will promote the generation of voids during the
densification and microstructual non-homogeneity [13].
There are many advantages for preparing nano-composite in
a fluidized bed reactor. An even temperature can be achieved
because the alumina particles are in constant motion in the reac-
tion vessel and the large common area of the gas and solid
particles ensures that the heat transfer rate is fast. Therefore, the
fluidized bed reactor could supply an environment with uniform
∗
Corresponding author. Tel.: +886 6 234 8188; fax: +886 6 276 3586.
E-mail address: JLH888@mail.ncku.edu.tw (J.-L. Huang).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.07.071

H.-T. Lin et al. / Journal of Alloys and Compounds 417 (2006) 214–219
215
temperature and concentration of the coating precursor that can
provide the possibility of good dispersion of the nano-particles
on the matrices [14].
It has been found that the combination of conventional flu-
idized bed technology with chemical vapor deposition (CVD) is
an effective method to deposited particles. Morooka et al. [15]
fluidized fine Al2O3 particles and coated them uniformly with
TiO2 crystallites formed by the oxidation of TiCl4 and TiN par-
ticles were deposited on the surface of fluidized Si3N4 particles.
Tsugeki et al. [16] introduced Al2O3 crystallites were coated
with TiN crystallites formed by the reaction of TiCl4 with NH3
in a fluidized bed reactor. Wood et al. [17] coated the aluminum,
titanium, and titanium nitride on mica, and alumina on nickel.
Hua and Li [18] reported the nano-crystalline SnO2 films on
Fig. 1. Differential scanning calorimetry (DSC) curve of the precursor
Cr(CO)6.
Al2O3 particles. In this work, the Cr(CO) was used as a precur-
sor, which coated on Al2O3 matrices by MOCVD in fluidized
6
◦
pyrolyed at 400 C had higher deposition rate, therefore, in order
to compare decomposition of Cr(CO) at different temperature,
the carbonyl gas molecules were vaporized as gas stream, which
flowed into the fluidized chamber with temperature 300 and
bed. The decomposition of the precursor Cr(CO) , and the as-
6
6
deposited particles were analyzed. Furthermore, in order to form
the reinforced nano-chromium carbide particles in the alumina
matrix, carbothermal treatment was performed and characteri-
zations of treated powder were discussed in this paper.
◦
4
00 C.
Firstly, in this study, only the precursor was heated and alu-
mina powder was not added as matrix in the fluidized bed, the
powder attached on the chamber wall was collected to do analy-
sis. The TEM micrograph of the as-deposited particles prepared
2
. Experimental
◦
Chromium hexacarbonyl (Cr(CO)6, 99%, Strem Chemicals Co., USA) was
at 300 C for 30 min was shown in Fig. 2. It is clearly observed
used as precursor of chromium oxide in the MOCVD process. Aluminum oxide
powder (A16SG, Alcoa, USA) was used as matrix powder, the average size was
about 0.7 ␮m. The vapor of precursor was carried by He gas (99.9% pure) that
was introduced into the fluidized bed reactor for MOCVD process. Based on
Lander’s and Germer’s results [19], the precursor container was kept at 75 C in
vacuum (10 Torr) in present experiment. Cr(CO)6 vapor was decomposed in the
fluidized chamber, at 300 and 400 C, respectively, and it would deposit on the
fluidizing alumina particles in the chamber. A detail description of the process
was shown in our previous paper [20].
The precursor was characterized by DSC (differential scanning calorimetry,
TA Instrument Model 2920, USA). The deposited particles were analyzed by
XRD (Rigaku D/Max III, Japan), DTA (NERZSCH STA 409 PC), and TEM
that the particles were in the shape of sphere with a size of
about 10–40 nm. The XPS spectra of C 1s and Cr 2p regions
◦
of the as-deposited powder prepared at 300 C are displayed in
◦
Figs. 3 and 4. The XPS spectra of the C 1s regions in Fig. 3 pro-
vide the evidence for which at least two forms of carbon exist
in the as-deposited powder. One is carbon bonded to chromium
atoms (C Cr) at 283.5 eV and the other is free carbon (C C)
at 284.6 eV. Fig. 4 indicates two peaks corresponding to the
spin-orbit splitting 2p1/2 and 2p3/2 of Cr, which has bonding
energy of 586.3 and 576.6 eV, respectively. The band shift of
these two peaks is 9.7 eV, which is consistent with previously
◦
(
Hitachi FE-2000 field Emission Transmission Electron Microscopy) equipped
with energy dispersive X-ray spectroscopy (EDS) to identify the phase com-
position, morphology. XPS (X-ray photoelectron spectroscopy, VG Scientific
2
10, England) was used to characterize the coating phases by binding energy.
BET instrument (Micromeritics Gemini 2360, USA) was used to measure the
specific surface area by nitrogen adsorption, and C/O analyzer (LECO CS-244,
USA) was used to measure the carbon content of coated powder by a combus-
tion method. The thermal behavior of the particles prepared in fluidized bed
was investigated by difference thermal analysis/thermogravimetry (DTA/TG,
GravitNERZSCH STA 409 PC, Germany). The as-deposited particles were car-
bothermally treated in CH4 and H2 gas mixture.
3
. Results and discussion
.1. Analysis of Cr(CO) precursor
3
6
Fig. 1 is the DSC analysis of the Cr(CO) precursor, where
6
◦
the heating rate was 10 C in the N2 atmosphere which shows
the dissociation of this metallic carbonyls began at 150 C with
an endothermal peak at 163 C. This temperature is very close to
◦
◦
the observed decomposition temperature reported by Lide [21].
Again, Lo and Wei [22] reported that the deposited film had
bettermechanicalpropertieswhenthepyrolysistemperaturewas
◦
◦
◦
Fig. 2. TEM micrograph of nano-particles prepared at 300 C.
from 300 C up to 400 C. And Chen and Wei [23] indicated that

2
16
H.-T. Lin et al. / Journal of Alloys and Compounds 417 (2006) 214–219
Fig. 3. X-ray photoelectron spectra of the C 1s regions of fluidized composite
◦
powder prepared at 300 C in fluidized bed.
◦
◦
Fig. 5. XRD patterns of the fluidized powders prepared at (a) 300 C; (b) 400 C.
reported data [24], and confirms the existence of Cr2O3 particles
in the as-deposited powder. After the pyrolysis, the CO ligands
preted that small C atoms inserted in the octahedral interstitial
sites of the cubic-close Cr packings in this structure. From the
report of Bewilogua et al. [29], this NaCl (Bl) structure was also
found in Cr C films prepared by ion-plating. The broadening
of diffraction peaks and the visible fluctuated background imply
that the particles were nano-scale [30] and some nano-particles
were not fully crystallized.
in Cr(CO) disappeared [25] and the produced species include
6
Cr, and CO. However, Shinn and Madey [26] reported that the
CO molecules were unstable and had a tendency to chemisorb
onto the surface of transit metals and the bond between car-
bon and oxygen would break when the chemisorption occurred.
The possible reaction species were Cr, Cr C, C and O in the
fluidized chamber. Therefore, XPS result shows products of as-
deposited particles are Cr2O3, carbide (Cr C) and free carbon
are reasonable.
Fig. 6 shows the TEM electron diffraction pattern of the
◦
nano-particle prepared at 300 and 400 C. Fig. 6(a) confirmed
◦
that Cr2O3 synthesized at 300 C was amorphous, and Fig. 6(b)
and (c) demonstrate two types of crystalline particles in the as-
deposited powder: Cr2O3 phase with hexagonal structure and
CrC1 − x phase with NaCl (Bl) structure, in agreement with the
results of XRD shown in Fig. 5.
Fig. 5 shows two XRD patterns of the resultant powders
◦
obtained at 300 and 400 C. Although both testing tempera-
tures lead to a pyrolysis effect, the Cr2O3 powders generated
◦
at 300 C were found to be amorphous as indicated by Fig. 5(a),
◦
Fig. 7 is the TG/DTA curves of the amorphous and crys-
whereas the product generated at 400 C were found to be crys-
◦
talline particles prepared at 300 and 400 C in fluidized bed,
talline Cr2O3 with hexagonal structure and CrC1 − x phase with
NaCl (Bl) structure [27], as marked in Fig. 5(b). Schuste and
Maury [28] reported that the CrC1 − x phase and free carbon
◦
respectively. The particles were heated from 30 to 900 C at a
◦
−1
heating rate of 10 C min in Ar atmosphere. Comparing the
DTA curves of amorphous and crystalline powder, it is found
that there is an exothermic peak in the curve of amorphous pow-
were fabricated in the coating when Cr(CO) was used as the
6
precursor of MOCVD process. Bouzy et al. [27] proposed that
non-stoichiometric CrC1 − x was a metastable phase, and inter-
◦
der appears at ∼400 C, which is because of the crystallization
of nano-scale amorphous particles at that temperature. The max-
◦
imum of the peak is appeared at 415 C. TG curve indicates that
the mass of powder loses with the temperature increased in both
amorphous and crystalline powder, owing to the volatilities of
CO and CO2 from the powder. There is a sharp mass loss around
the crystalline temperature, it implies that the carbon content in
the crystalline powder was obviously lower than in the amor-
phous.
3.2. Nano-particles deposited on the alumina after being
fluidized
Fig. 8 displays the TEM micrograph of the as-deposited
particles with size 20–30 nm well-dispersed on the surfaces
of alumina particles when composite prepared in the fluidized
◦
chamber at 300 C for 2 h. Table 1 shows the carbon content of
◦
the samples prepared at 300 and 400 C was 0.75 and 0.23%,
Fig. 4. X-ray photoelectron spectra of the Cr 2p regions of the fluidized com-
◦
posite powder prepared at 300 C in fluidized bed.
respectively. It indicates the as-deposited amorphous powder

H.-T. Lin et al. / Journal of Alloys and Compounds 417 (2006) 214–219
217
◦
◦
Fig. 6. TEM diffraction patterns of the coated particles (a) prepared at 300 C in fluidized bed, showing amorphous phase; (b) and (c) prepared at 400 C in fluidized
bed. (b) Cr2O3 phase with hexagonal structure; and (c) CrC1 − x phase with NaCl (Bl) structure, respectively.
Fig. 9 shows the EDS results of the particle “A” indicated
in the Fig. 8. It indicates that the particle “A” was composed
of element C, Cr and O, meanwhile, the element Cu from Cu
grid was also detected. From the EDS results, it proved that the
particles coated on alumina were formed through the deposition
of the pyrolysis of precursor.
Above result indicates that the vaporized Cr(CO) precursor
6
can be deposited on the surfaces of fluidizing alumina particles
Fig. 7. The DTA curves of the amorphous and crystalline fluidized powder at a
◦
−1
heating rate of 10 C min in Ar atmosphere.
◦
prepared at 300 C had more carbon content than the crystalline
◦
powders prepared at 400 C. This was probably due to the amor-
phous powders had larger specific surface area than crystalline
powders.
Table 1
The specific surface area and carbon content of samples
Sample 1
Sample 2
◦
◦
Fluidized condition
Carbon content (%)
BET surface area (m /g)
300 C/2 h
400 C/2 h
0.75
25.5
0.23
11.2
2
Fig. 8. TEM micrograph of nano-particles deposited on the surface of Al2O3
particles.

2
18
H.-T. Lin et al. / Journal of Alloys and Compounds 417 (2006) 214–219
Fig. 9. EDS results of the deposited particle A of Fig. 8.
◦
Fig. 10. XRD patterns of the carbonizing powder synthesized at 700 C for 2 h
◦
◦
in CH4 H2 gas mixtures for the fluidized powder of (a) 300 C; (b) 400 C.
through the pyrolysis effect by mean of a combination of chem-
ical vaporized deposition and fluidized bed technique.
3
.3. Carbothermal treatment of the as-deposited powder
From previous paper [20], after the thermal treatment at
◦
1
150 C in graphite chamber, the Cr2O3 particles in as-deposited
◦
powder prepared at 300 C transferred into monolithic Cr3C2
phase, while the powder prepared at 400 C transferred into
◦
mixed phases of Cr3C2 and Cr7C3. According to the Table 1, the
former had enough carbon content to form monolithic Cr3C2,
but the latter formed both chromium carbide, Cr3C2 and Cr7C3,
owning to the carbon content was not enough. The Eqs. (1) and
(
2) explain more content of carbon is needed for Cr2O3 to form
Cr3C2, otherwise the Cr7C3 is formed [31].
3
Cr2O3 + 13C → 2Cr3C2 + 3CO
(1)
(2)
7
Cr2O3(s) + 27C(s) → 2Cr7C3(s) + 21CO(s)
To reduce the carbothermaltemperatureof Cr-spicesparticles
and prevent the pre-sintering of the Al2O3 particles at elevated
temperature, CH4 and H2 gas mixture were adopted in the ther-
maltreatmentprocess. FromthereportofLoubtereetal. [32], the
chromium oxide was carbonized in the CH4 and H2 gas mixtures
and transferred into chromium carbide (Cr3C2). Fig. 10 displays
◦
the XRD patterns of carbonizing powder synthesized at 700 C
for 2 h in CH4 and H2 gas mixture for the as deposited powder
◦
of 300 and 400 C. The XRD patterns indicate the Cr-spices in
both crystalline and amorphous powders are transformed into
Cr3C2 owning to the enough carbon was prepared in the CH4
and H2 mixture atmosphere. Fig. 11(a) shows the TEM micro-
graphs of the nano-composite particles carbothermally treated at
◦
7
00 C for 2 h in the CH4 H2 gas mixtures. The Fig. 11(a) dis-
plays the nano-particles were 50–80 nm in the size. The TEM
diffraction pattern of the particle “B” indicated in Fig. 11(a)
is showed in Fig. 11(b), the Cr3C2 phase with orthorhombic
structure is observed. Form the results of XRD and TEM, not
only the chromium oxide was carbonized to chromium carbide
Fig. 11. (a) TEM micrographs of the nano-particles Cr3C2 deposited on the
surface of the Al2O3 particles after the carbothermal treatment in the CH4 H2
gas mixtures at 700 C for 2 h; (b) TEM diffraction patterns of the particle “B”
(
Cr3C2), but the CrC1 − x was also transferred into Cr3C2 dur-
ing the thermal treatment. This result coincided with the reports
27,29], which reported that an annealing treatment and higher
◦
indicated in (a), showing Cr3C2 phase with orthorhombic structure.
[

H.-T. Lin et al. / Journal of Alloys and Compounds 417 (2006) 214–219
219
content of carbon caused a transformation of the metastable car-
bide CrC1 − x into the stable carbide phase Cr3C2.
[6] W.J. Tseng, P.D. Funkenbusch, J. Am. Ceram. Soc. 75 (5) (1992)
171–1175.
1
[
[
[
7] Y.S. Chou, D.J. Green, J. Am. Ceram. Soc. 75 (12) (1992) 3346–3352.
8] C.T. Fu, A.K. Li, J.M. Wu, J. Mater. Sci. 28 (1993) 6285–6294.
9] K.M. Shu, C.T. Fu, D.M. Liu, J. Mater. Sci. Lett. 13 (1994) 1146–1148.
4
. Conclusions
[
10] C.T. Fu, A.K. Li, J.M. Wu, Br. Ceram. Trans. 93 (5) (1994) 178–182.
Nano-scale carbide Cr3C2/Al2O3 composites were prepared
[11] K. Niihara, J. Ceram. Soc. Jpn. 99 (10) (1991) 974–982.
[
[
[
[
[
[
12] G.Y. Onada Jr., L.L. Hench, Ceramic Processing before Firing, Wiley,
New York, 1978, pp. 357–376.
13] E.A. Pugar, P.E.D. Morgan, J. Am. Ceram. Soc. 69 (6) (1984)
C120–C123.
14] D. Kunii, O. Levenspiel, Fluidization Engineering, Huntington, N.Y,
1977, pp. 195–223.
15] S. Morooka, A. Kobata, K. Kusakabe, AICHE Symp. Ser. 87 (1991)
through pyrolysis of Cr(CO) precursor by means of MOCVD
6
combined with the fluidized technique and carbothermal treat-
ment in the mixture of CH4 and H2 gas. The precursor Cr(CO)6
was decomposed into Cr2O3, CrC1 − x, and free carbon, which
deposited on the alumina surface in the fluidized chamber at 300
◦
◦
and 400 C. The collected particles were amorphous at 300 C
pyrolysis temperature, however the crystallization of amorphous
32–37.
16] K. Tsugeki, T. Kato, Y. Koyanagi, K. Kusakabe, S. Morooka, J. Mater.
Sci. 28 (1993) 3168–3172.
17] B.J. Wood, A. Sanjurjo, G.T. Tong, S.E. Swider, Surf. Coat. Technol.
49 (1991) 228–232.
◦
particles starts at about 400 C, which was confirmed in the
TG/DTA curve. Both the amorphous and crystalline particles
◦
of Cr2O3 and CrC1 − x fabricated at 300 and 400 C were trans-
◦
[
[
18] B. Hua, C. Li, Mater. Chem. Phys. 59 (1999) 130–135.
19] J.J. Lander, L.H. Germer, Am. Inst. Miner. Metal. Eng. Technol. 14 (6)
formed into monolithic Cr3C2 phase at 700 C in CH4 H2 gas
mixtures.
(
1947) 1–4.
20] H.T. Lin, J.L. Huang, W.T. Lo, W.C.J. Wei, J. Mater. Res. 20 (8) (2005)
154–2160.
21] D.R. Lide, Handbook of Chemistry and Physics, 72nd ed., CRC press,
Boston, MA, 1991–92, pp. 4–76.
[
[
Acknowledgment
2
The authors would like to thank National Science Council of
the Republic of China for its financial support under the Contract
No. NSC-92-2216-E-006-011.
[
[
[
22] M.H. Lo, W.C.J. Wei, J. Am. Ceram. Soc. 80 (4) (1997) 886–892.
23] C.L. Chen, W.C. Wei, J. Eur. Ceram. Soc. 22 (2002) 2883–2892.
24] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, Handbook of X-ray
Photoelectron Spectroscopy, Perkin-Elmer, Minnesota, 1979, 77 pp.
25] A. Kazusaka, R.F. Howe, J. Mol. Catal. 9 (1980) 199–207.
26] N.D. Shinn, T.E. Madey, J. Chem. Phys. 83 (11) (1985) 5928–5935.
References
[
[
[
[
[
1] S. Lio, M. Watanabe, M. Matsubara, Y. Matsuo, J. Am. Ceram. Soc. 72
10) (1989) 1880–1884.
2] W.J. Tseng, P.D. Funkenbusch, J. Am. Ceram. Soc. 75 (5) (1992)
171–1175.
3] Y.S. Chou, D.J. Green, J. Am. Ceram. Soc. 75 (12) (1992) 3346–
352.
[27] E. Bouzy, E. Bauer-Grosse, G. Le Caer, Philos. Mag. B. 68 (5) (1993)
619–638.
[28] F. Schuste, F. Maury, Sur. Coat. Technol. 43 (1990) 185–198.
[29] K. Bewilogua, H.-J. Heinitz, B. Rau, S. Schulze, Thin Solid Films 167
(1988) 233–243.
[30] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22–31.
[31] W.F. Chu, A. Rahmel, Oxid. Met. 15 (1981) 331–337.
[32] S. Loubtere, Ch. Laurent, J.P. Bonino, A. Rousset, J. Alloy. Compd. 243
(1996) 59–66.
(
1
3
[
[
4] C.T. Fu, J.M. Wu, A.K. Li, J. Mater. Sci. 29 (1994) 2671–2677.
5] S. Lio, M. Watanabe, M. Matsubara, Y. Matsuo, J. Am. Ceram. Soc. 72
(
10) (1989) 1880–1884.