EPR studies on defects in sol-gel derived alumina films

Article abstract of DOI:10.1016/S0022-3697(02)00377-3

We have studied the defect structures and mechanism for the luminescence of non-doped, Tb³⁺-doped and Tb³⁺-Zn²⁺ codoped alumina films treated at various temperatures from 300 to 800°C. The electron paramagnetic resonance (EPR) spectra observed before UV-irradiation are attributable to the non-bridging oxygen radicals; Al-O-O at treatment temperatures of 300-500°C and Al-O at 800°C. The UV-light irradiation of the non-doped alumina films gives the EPR spectra with the ²⁷Al hyperfine splitting. There exists a good correlation between the treatment temperature dependence on the light-induced EPR signal intensity of the trapped electron and the NMR peak intensity due to the five-coordinated aluminum ion. We concluded that the electron is captured at the oxygen vacancy connected with the five-coordinated aluminum. The broad EPR spectra due to the ground state of the Tb³⁺ ion were observed in the Tb³⁺-doped and Tb³⁺-Zn²⁺ codoped alumina films. The Tb³⁺ concentration dependence of the EPR signal intensity due to the non-bridging oxygens suggests that Tb³⁺ prefers to form Al-O-Tb bonds and prevent to form a cluster of rare earth ions itself. The light-induced EPR signal of the electron trapped at oxygen vacancy next the five-coordinated aluminum was also observed for the Tb³⁺-Zn²⁺ codoped alumina film, which produced a long-lasting luminescence. The photogenerated trapped electron remained for several minutes under the room temperature. These experimental facts indicate that the five-coordinated aluminum ions act as the trapping sites to produce the long-lasting luminescence.

Full text of DOI:10.1016/S0022-3697(02)00377-3

Journal of Physics and Chemistry of Solids 64 (2003) 801–806
www.elsevier.com/locate/jpcs
EPR studies on defects in sol–gel derived alumina films
a
T. Ishizaka , S. Tero-Kubota , Y. Kurokawa , T. Ikoma
b,
a
b
*
a
Department of Material Chemistry, Graduate School of Engineering, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8579, Japan
b
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8579, Japan
Received 23 April 2002; revised 27 June 2002; accepted 19 September 2002
Abstract
2þ
We have studied the defect structures and mechanism for the luminescence of non-doped, Tb -doped and Tb –Zn
3þ
3þ
codoped alumina films treated at various temperatures from 300 to 800 8C. The electron paramagnetic resonance (EPR) spectra
z
observed before UV-irradiation are attributable to the non-bridging oxygen radicals; Al–O–O at treatment temperatures of
z 27
00–500 8C and Al–O at 800 8C. The UV-light irradiation of the non-doped alumina films gives the EPR spectra with the Al
3
hyperfine splitting. There exists a good correlation between the treatment temperature dependence on the light-induced EPR
signal intensity of the trapped electron and the NMR peak intensity due to the five-coordinated aluminum ion. We concluded
that the electron is captured at the oxygen vacancy connected with the five-coordinated aluminum. The broad EPR spectra due
3
þ
3þ
3þ
2þ
3þ
to the ground state of the Tb ion were observed in the Tb -doped and Tb –Zn codoped alumina films. The Tb
þ
3
concentration dependence of the EPR signal intensity due to the non-bridging oxygens suggests that Tb prefers to form Al–
O–Tb bonds and prevent to form a cluster of rare earth ions itself. The light-induced EPR signal of the electron trapped at
3
þ
2þ
oxygen vacancy next the five-coordinated aluminum was also observed for the Tb –Zn codoped alumina film, which
produced a long-lasting luminescence. The photogenerated trapped electron remained for several minutes under the room
temperature. These experimental facts indicate that the five-coordinated aluminum ions act as the trapping sites to produce the
long-lasting luminescence.
q 2002 Published by Elsevier Science Ltd.
Keywords: A. Glasses; B. Sol–gel growth; D. Defects; D. Electron paramagnetic resonance (EPR); D. Luminescence
1
. Introduction
amorphous nature and defective structure of the produced
alumina. To further investigate this possibility, we here
examined the defect structures of alumina films using
electron paramagnetic resonance (EPR) and nuclear mag-
netic resonance (NMR).
Long lasting luminescent materials have attracted
attention [1–7] since their properties of accumulation of
photon energy can be utilized in various fields such as
battery-free illumination and information storage nanode-
vices [8–10]. Recently we reported that non-doped alumina
films show a luminescence with lifetime of a few 10 ms
2. Experiment
[
11], that rare earth ions can be highly doped into alumina
films prepared by using aqueous sol derived from AlCl ·6-
All reagents were of reagent grade and were obtained
commercially from Wako Pure Chemicals (Osaka, Japan).
The preparation of alumina film by the aqueous sol–gel
method was essentially identical to that previously
described [11–13]. Hydrous aluminum oxide was prepared
3
3
H O [12,13], and that Tb –Zn codoped alumina films
þ 2þ
2
show a long-lasting luminescence [11]. Based on the fact
that glasses prepared by the sol–gel method exhibit defects
even without X-ray, g-ray and neutron irradiation [14,15],
we hypothesized that these properties were due to the highly
through the reaction of aqueous AlCl
6
3
solution with aqueous
M NH solution. The precipitate hydroxide was aged for
3
*
Corresponding author. Tel./fax: þ81-22-217-5612.
12 h, then filtered and washed with pure water to remove
unreacted chloride. It was then made into a clear sol by
E-mail address: tero@tagen.tohoku.ac.jp (S. Tero-Kubota).
0022-3697/03/$ - see front matter q 2002 Published by Elsevier Science Ltd.
PII: S0022-3697(02)00377-3

802
T. Ishizaka et al. / Journal of Physics and Chemistry of Solids 64 (2003) 801–806
peptizing with acetic acid under reflux at 80 8C. Metal ion-
doped films were prepared as follows. The viscous sol was
mixed with rare earth chloride solution to give a
concentration of 1–15 mol% of rare earth ions per Al O .
2
3
The sol was transferred into a plastic Petri dish. Gelation
was performed in a glove box by the dehydration of aqueous
sol under room temperature. The resultant films were heat-
treated in air at different temperatures (200–800 8C) for 2 h.
2
7
Al MAS NMR measurements were performed at room
temperature on an FT-NMR spectrometer (JEOL JNM-
GX270). EPR measurements were carried out at 5 K using
an X-band spectrometer (Bruker ESP380E). Light
irradiation for measurement of EPR spectra was produced
using a high pressure Hg lamp (Ushio 500W).
3
. Results and discussion
The resultant alumina films were optically transparent up
to a treatment temperature of 900 8C because of the almost
amorphous structure. In fact, the X-ray diffraction patterns
(
XRD) show only a few weak and broad bands that were
assigned to a pseudo-boehmite structure at 25–400 8C, to a
g alumina (spinel) structure at 500–800 8C, and to d and u
alumina at 900–1000 8C. Above 1000 8C, the alumina film
became gradually opaque with increasing the heat-treatment
temperature since the crystallization with a alumina
2
7
Fig. 1. Al MAS NMR spectra for the alumina films heat-treated at:
a) 300 8C; (b) 400 8C; (c) 500 8C; (d) 600 8C; and (e) 800 8C. The
asterisk indicates a spinning side band.
(
corundum) structure was accelerated.
We detected the five-coordinated alumina, which have a
(
2
7
27
defect of oxygen, by Al MAS NMR. Fig. 1 shows the Al
MAS NMR spectra of the alumina films heat-treated at
various temperatures. The broadness of all the signals
indicates that the aluminum ions are present in various
environments, as reflected in the amorphousness of our
alumina. There are mainly six-coordinated aluminum ions
electron or is located near the position of the electron spin,
the EPR signal must be split into multi-lines because the
2
7
nuclear spin of Al, of which natural abundance is 100%, is
5/2. No apparent hyperfine structure of the EPR spectra for
the 300 and 500 8C-treated alumina films was observed,
suggesting that the paramagnetic site is different from those
of an Al center [16,17] or aluminum–oxygen hole center
(Al–OHC) [18–22]. The simple line with the g value larger
(
octahedra) in the films heat-treated at relatively low
temperatures. Four-coordinated aluminum ions (tetrahedra)
increase and the alumina develops a spinel structure with an
increase of heat-treating temperature. Signals of five-
coordinated aluminum ions were also observed. The five-
coordinated alumina is an alumina lacking oxygen, and it
exists in g alumina and other transition alumina. The signal
increases with heat-treating temperature, reaches a maxi-
mum at about 550 8C, and then decreases at higher
temperature. The signals of five-coordinated aluminum
ions were scarcely observed for the alkoxide-derived
alumina. Our alumina films derived from inorganic salt
are more amorphous and oxygen-defective than the alumina
derived from alkoxide.
than g (2.0023) for free electron is consistent with the
e
oxygen hole center (Oxy) [18–20]. The Oxy defect is
considered to have a structure of peroxy radicals, i.e. Al–
z
O–O in which the distance between aluminum atom and the
electron spin is long enough to neglect the hyperfine
interaction with Al. When the film was annealed at 800 8C,
z
the signal of the Oxy defect of Al–O–O disappeared and a
weak broad signal appeared. The broad signal can be
z
assigned to Al–OHC such as Al–O rather than the Oxy
defect, because the large hyperfine splitting due to Al and
large g-anisotropy were clearly observed. The observed
spectrum could be simulated with the EPR parameters listed
in Table 1.
Fig. 2 shows the EPR spectra of the non-doped alumina
films heat-treated at various temperatures observed before
UV-light irradiation. A weak narrow EPR signal was
detected around g ¼ 2.004 for the alumina films treated at
When the alumina films were irradiated with UV-light,
new EPR signals appeared as shown in Fig. 3. The EPR
signal intensity drastically decreased as the treatment
temperature rose above 500 8C. The non-doped alumina
3
00 8C. The signal increases in intensity for the 500 8C-
treated alumina film. If the Al atom holds the unpaired

T. Ishizaka et al. / Journal of Physics and Chemistry of Solids 64 (2003) 801–806
803
Fig. 2. EPR spectra for the non-doped alumina films heat-treated at:
a) 300 8C; (b) 500 8C; and (c) 800 8C. The spectra were measured
before irradiation at 5 K by using a field modulation system of
.5 mT amplitude and 100 kHz frequency. The dotted line is a
Fig. 3. Light-induced EPR spectra for the non-doped alumina films
heat-treated at: (a) 300 8C; (b) 500 8C; and (c) 800 8C. The spectra
were measured during UV-irradiation at 5 K by using a field
modulation system of 0.5 mT amplitude and 100 kHz frequency.
The dotted line is a powder pattern spectrum calculated with the
spin Hamiltonian parameters of the trapped electron (Table 1) in
assuming a Gaussian (G ¼ 0.6 mT) for a line shape.
(
0
powder pattern spectrum calculated with the spin Hamiltonian
z
parameters of the Al–O defect (Table 1) in assuming a Gaussian for
a line shape. The full-width-of-half maximums (G) of the Gaussian
function are 1.5 mT, 7.0 mT and 11.0 mT for the three principle
directions of the g-tensor, respectively.
^
^
^
A
(
m^ B^ ·g·S) interaction and hyperfine interaction ðS·A ·IÞ
B
of Al.
O
heat-treated at 300–500 8C showed luminescence, while
that heat-treated at 800 8C showed almost no luminescence
^
^
^
^
^
Al
H ¼ m B ·g·S þ g B ·I þ S·A ·I
ð1Þ
and gAl are, respectively, Bohr magneton and
S
B
0
Al
0
where m
B
[
11]. The good correspondence of the temperature depen-
2
7
magnetogyric ratio of Al nuclei. B
0
indicates magnetic
dences between the luminescence and EPR intensities
indicates that the defect giving the multi-lines is responsible
for the luminescent center in the alumina films.
ˆ
^
field. S and I stand for spin operators for unpaired electron
and Al nuclear spins. When not only the isotropic
hyperfine interaction but also anisotropic hyperfine
interaction was taken into account, the observed multi-
lines could be reproduced well (Fig. 3). The obtained EPR
parameters are listed in Table 1. Around the central field,
however, the calculated spectrum does not agree with
observed one. The disagreement indicates the presence of
minor defect that is not associated with Al atom. The
We have simulated the multi-lines by the spin
^
Hamiltonian ðH Þ comprised from the anisotropic Zeeman
S
Table 1
Spin Hamiltonian parameters for the defects in the alumina films
detected by EPR
iso
isotropic hyperfine term, A ð¼ ðA þ A þ A Þ=3Þ; was
1
2
3
Defect
g
1
g
2
g
3
A
1
A
2
A
3
determined to be 1.08 mT. This value is substantially
small compared with the Fermi-contact interaction for Al
(
mT)
(mT)
(mT)
(
suggesting that about 1% of the electron spin exists on
98.3 mT) calculated with aluminum 3s orbital [23],
z
Al–O–O 2.000
2.002
2.085
2.010
2.220
–
–
–
z
Al–O
2.066
,0.7
0.90
,1.7
15.0
1.44
iso
the Al atom. The anisotropic terms ðA 2 A Þ are
n
Trapped
electron
2.0021 2.0021 2.0021
0.90
an
an
2
an
3
estimated to be A1 ¼ 20:18 mT; A ¼ 20:18 mT; A
¼
þ0:36 mT: Such an axial anisotropy can be explained

804
T. Ishizaka et al. / Journal of Physics and Chemistry of Solids 64 (2003) 801–806
within a point dipole approximation expressed as follows
2
m
3 cos u 2 1
r3
0
Aan ¼ 4 gegAl
ð2Þ
p
where g
e
is magnetogyric ratios of electron, m
permeability of vacuum, u is the angle between the
0
is
external magnetic field (B ) and the direction connecting
0
the unpaired electron and the nearest Al atom. r indicates
an
the distance between the electron and Al atom. A3
corresponds to the hyperfine interaction when u is zero.
an
an
A1 and A2 are the value of u ¼ p=2: From these
anisotropic terms, hence, r for the light-induced defect
was calculated to be 0.16–0.17 nm. The distance almost
coincides with the bond distance of Al–O calculated from
3
þ
22
the ionic radii for Al (0.051 nm) and O (0.132 nm).
The estimated distance suggests that the light-induced
defect seems to be the Al–OHC, but the g anisotropies are
different from each other. While the g-values for the light-
e
induced defect are isotropic and smaller than g , those for
the Al–OHC are anisotropic and lager than ge [22].
Therefore, the paramagnetic center in which an electron is
captured in the nearest neighboring oxygen vacancy to Al
atom is a reasonable model for the light-induced defect in
3þ
Fig. 4. EPR spectra of: (a) non-doped; (b) Tb (3 mol%)-doped;
3þ
and (c) Tb (15 mol%)-doped alumina films heat-treated at 600 8C.
The spectra were measured before irradiation at 5 K by using a field
modulation system of 0.5 mT amplitude and 100 kHz frequency.
þ
the alumina films. This model is similar to the F -like
2 3
center detected in reduced CaO–Al O glasses [24].
2
7
The Al MAS NMR measurements suggest the presence
of the five-coordinated alumina lacking oxygen, which
decreases as the treatment temperature goes up to more than
3
þ
3þ
Tb in the alumina films. UV-irradiation of the Tb doped
film gave a weak and single EPR signal at g , 2.0. The
multiple hyperfine structures due to Al were not observed.
5
50 8C. Since the temperature behavior of the five-
2
7
coordinated alumina corresponds to the decrease of the
light-induced EPR spectrum at more than 500 8C, the light-
induced defect is ascribable to an electron trapped at oxygen
vacancy associated with the five-coordinated alumina. The
amorphous property of the alumina films in this study has a
large number of non-bridging oxygens and five-coordinated
alumina ions. These defective structures may work as the
trapping sites for electrons. Consequently, it is concluded
that the trapped electrons at oxygen vacancy play an
important role as long-lived luminescent centers.
2
We also examined the Zn ion codoping effect on the
þ
2þ
2þ
EPR spectrum, because the introduction of Zn (or Ca )
3þ
into the Tb -doped alumina films resulted in the long-
3þ
lasting luminescence from the excited state of Tb [25].
3þ
Although the lifetime of emission for the Tb -doped film is
2þ
3þ
ca. 2 ms, the luminescence of Zn –Tb -codoped alumina
films can be observed by naked eyes in more than 30 min
after UV-light irradiation. Based on these drastic phenom-
ena, we have proposed an electron trapping mechanism for
the long-lasting luminescence [11].
3þ
Fig. 4 shows the Tb doping effect on the EPR spectra
for the alumina films heat-treated at 600 8C. By doping of
During the light irradiation
3
þ
z
the Tb ions, the decrease of the signals due to Al–O was
accompanied by the concomitant increase of the very broad
EPR spectrum in the low field regions. The broad signal is
3þ
3þ þ
p
Tb þ UV light ! (Tb
)
þ e
p
þ
e þ oxygen defect center ! F -like center.
3
þ
assigned to the Tb ion with the electronic configuration of
4
usually show complicated resonance lines in the low
8
f , because the rare earth ions with several 4f-electrons
After the irradiation
3þ
þ
p
p
magnetic field. The doping concentration of Tb did not
3
significantly affect the EPR spectral shape of Tb
suggesting no drastic change of the local structure
F -like center þ phonon ! oxygen defect center þ e
þ
3þ þ
3þ
,
e þ (Tb
)
! Tb þ luminescence.
3þ
3þ
3þ
surrounding Tb . We previously reported that Tb and
other rare earth ion-doped alumina films showed no
significant luminescence quenching due to clustering of
During the light irradiation, Tb is photo-oxidized to
3
(Tb
þ þ
p
and generates an electron (e ). The electron is
)
trapped at the neighboring oxygen defect site. The trap
depth of the oxygen defect center is shallow and broadly
distributed by the structural variation owing to the codoped
3
þ
Tb [11,25]. These facts are interpreted in terms of the
greater preference for the bond formation between the
3
defective Al–O sites and Tb and for the dispersion of
þ
2þ
Zn
ions. Therefore, the electrons can be gradually

T. Ishizaka et al. / Journal of Physics and Chemistry of Solids 64 (2003) 801–806
805
released at room temperature, and the long-lasting lumines-
cence is realized.
the long-lasting luminescence described above. The oxygen
defect center is assigned to the oxygen vacancy close to the
five-coordinated aluminum ion of which the energy depth is
tunable by the codopant ions.
3þ
Fig. 5 shows the EPR spectra of the Tb (5 mol%)–
2
Zn (30 mol%) codoped alumina films treated at 800 8C
þ
before and after the UV-light irradiation. Before light
irradiation only a weak signal of paramagnetic defect was
observed, but an intense signal was detected after the light
irradiation at very low temperatures. The multi-lines of the
intense EPR spectrum were basically simulated by using the
same spin Hamiltonian parameters as the trapped electron at
oxygen vacancy next the five-coordinated alumina, which is
observed for the non-doped alumina film during the light
irradiation. The discrepancy between the calculated and
observed EPR spectra around the central field implies the
existence of another type of paramagnetic centers. While the
4. Conclusion
z
z
The EPR spectra due to the Al–O–O and Al–O radicals
were observed before light-irradiation of the non-doped
alumina films. The UV-photolysis generated the EPR
signals split by the hyperfine interaction with Al. The EPR
spectrum is attributed to the trapped electron at oxygen
vacancy next the five-coordinated aluminum, which is the
þ
F -like center. The EPR signals of the oxygen centered
3þ
3
þ
2þ
multiplet EPR signal for the Tb –Zn codoped alumina
film remained somewhat by keeping the film for a few
radicals decreased with doping of the Tb ion. It was
3
þ
suggested that Tb prefers the formation of Tb–O–Al
bonds and prevents the cluster formation in the present
1
0 min even at room temperature, the signal disappeared by
þ
amorphous films. The long-lived F -like center was also
heating for 10 min at 80 8C after the light irradiation. This
result would indicate that the trapped electrons are thermally
released from the oxygen vacancy site. The observation of
the trapped electron at the oxygen vacancy by irradiation,
3þ
2þ
observed for the Tb –Zn codoped alumina film, which
shows a long-lasting luminescence, by the UV-light
irradiation. It is thus shown that the five-coordinated
aluminum ions act as the trap center of the photo-ionized
electrons.
þ
which corresponds to F -like center and is thermally
recombined, verifies the electron trapping mechanism for
Acknowledgements
T.I. thanks for the financial supports from the REIMEI
Research Resources of Japan Atomic Energy Research
Institute and Izumi Science and Technology Foundation.
References
[
[
1] M. Yamazaki, Y. Yamamoto, S. Nagahama, N. Sawanobori,
M. Mizuguchi, H. Hosono, J. Non-Cryst. Sol. 241 (1998) 71.
2] H. Hosono, T. Kinoshita, H. Kawazoe, M. Yamazaki, Y.
Yamamoto, N. Sawanobori, J. Phys. Condens. Matter. 10
(
1998) 9541.
[
3] J. Qui, M. Kawasaki, K. Tanaka, Y. Shimizugawa, K. Hirao,
J. Phys. Chem. Sol. 59 (1998) 1521.
[
[
4] T. Kinoshita, H. Hosono, J. Non-Cryst. Sol. 274 (2000) 257.
5] T. Nakamura, T. Matsuzawa, C.C. Rowlands, V. Beltr a´ n-
L o´ pez, G.M. Smith, P.C. Riedi, J. Chem. Soc., Faraday Trans.
9
4 (1998) 3009.
[
[
6] K. Kaiya, N. Takahashi, T. Nakamura, T. Matsuzawa, G.M.
Smith, P.C. Riedi, J. Lumin. 87–9 (2000) 1073.
7] T. Nakamura, K. Kaiya, N. Takahashi, T. Matsuzawa, C.C.
Rowlands, V. Beltr a´ n-L o´ pez, G.M. Smith, P.C. Riedi, Phys.
Chem. Chem. Phys. 1 (1999) 4011.
3
þ
2þ
Fig. 5. EPR spectra of Tb (5 mol%)–Zn (30 mol%) codoped
alumina films heat-treated at 800 8C: (a) before; and (b) after UV-
light irradiation. The spectra were measured at 5 K by using a field
modulation system of 0.5 mT amplitude and 100 kHz frequency.
The dotted line is a calculated spectrum of the trapped electron
using a Gaussian of G ¼ 1.2 mT.
[8] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama,
J. Electrochem. Soc. 143 (1996) 2670.
[9] M. Sonoda, M. Takano, J. Miyahara, H. Kato, Radiology 148
(1983) 833.
[10] K. Miura, J. Qiu, S. Fujiwara, S. Sakaguchi, K. Fujita, K.
Hirao, Appl. Phys. Lett. 80 (2002) 2263.

806
T. Ishizaka et al. / Journal of Physics and Chemistry of Solids 64 (2003) 801–806
[
[
11] T. Ishizaka, Y. Kurokawa, J. Appl. Phys. 90 (2001) 2257.
12] Y. Kurokawa, T. Ishizaka, T. Ikoma, S. Tero-Kubota, Chem.
Phys. Lett. 287 (1998) 737.
[19] H. Hosono, K. Yamazaki, Y. Abe, J. Am. Ceram. Soc. 70
(1987) 870.
[20] H. Hosono, Y. Abe, J. Non-Cryst. Sol. 95–96 (1987) 717.
[21] M. Nofz, R. Stsser, F.G. Wihsmann, Phys. Chem. Classes 31
(1990) 57.
[
13] T. Ishizaka, R. Nozaki, Y. Kurokawa, J. Phys. Chem. Sol. 63
(2001) 613.
[
[
[
[
14] B.E. Yoldas, J. Non-Cryst. Sol. 147–148 (1992) 614.
15] B.E. Yoldas, J. Mater. Res. 5 (1990) 1157.
[22] D.A. Dutt, P.L. Higby, D.L. Griscom, J. Non-Cryst. Sol. 130
(1991) 41.
16] R. Schnadt, A. R a¨ uber, Solid State Commun. 9 (1971) 159.
17] D.L. Griscom, in: S.T. Pantelides (Ed.), The Physics of
[23] B.A. Goodman, J.B. Raynor, in: H.J. Emelius, A.G. Sharped
(Eds.), Electron Spin Resonance of Transition Metal Com-
plexes, Advances in Inorganic Chemistry and Radio Chem-
istry, vol. 13, Academic Press, New York, 1970.
[24] H. Hosono, N. Asada, Y. Abe, J. Appl. Phys. 67 (1990) 2840.
[25] T. Ishizaka, Y. Kurokawa, J. Appl. Phys. 90 (2001) 243.
2
SiO and its Interfaces, Pergamon Press, New York, 1978,
p. 232.
[18] H. Hosono, K. Yamazaki, Y. Abe, J. Am. Ceram. Soc. 70
(1987) 867.