Surface & Coatings Technology 208 (2012) 87–91
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology
journal homepage: www.elsevier.com/locate/surfcoat
Thermal diffusivity and conductivity of yttria stabilized zirconia coatings obtained by
suspension plasma spraying☆
Leszek Łatka a, b, Andrea Cattini a, c, Lech Pawłowski a,⁎, Stephan Valette a, Bernard Pateyron a,
Jean-Pierre Lecompte a, Rishi Kumar d, Alain Denoirjean a
a
SPCTS, University of Limoges, UMR CNRS 7315, F-87068 Limoges, France
Faculty of Mechanics, Wrocław University of Technology, Pl-50371 Wrocław, Poland
Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, I-41125 Modena, Italy
d
Indian Institute of Technology, Bombay, 400076 Mumbai, India
b
c
a r t i c l e
i n f o
Article history:
Received 22 June 2012
Accepted in revised form 8 August 2012
Available online 18 August 2012
Keywords:
Suspension plasma spraying
Thermal conductivity
Yttria stabilized zirconia coatings
a b s t r a c t
Plasma generated by SG-100 torch was applied to spray suspension formulated with the use of ZrO2 + 8 wt.%
Y2O3 (8YSZ) solid phase. The solids have the mean size of about 4.5 μm and were obtained by milling
commercial Metco 204NS powder. The suspension was formulated with the use of 20 wt.% of solid phase,
40 wt.% water and 40 wt.% of ethanol. The plasma spray parameters were optimized with the electric
power equal to 40 kW, working gases composition Ar—45 slpm and H2—5 slpm, spray distance varying
from 40 to 60 mm and torch linear speed with regard to substrate varying from 300 to 500 mm/s. The
coatings of the thickness ranging from 51 to 106 μm, were sprayed onto stainless steel substrates. The porosity of the samples was found, with the help of image analysis of metallographically prepared cross-sections of
the samples, to be in the range of 8–12%. Thermal diffusivity was measured with the use of the commercial
NanoFlash system in the temperature range from room temperature to 523 K. The measurements were
made with the use of the coatings sprayed on the substrate and the 2-layers numerical model was applied
to determine thermal diffusivity of the coatings. The diffusivity was in the range 0.196 × 10 −6 to
0.352 × 10 −6 m2/s in room temperature depending on the spray parameters. The obtained data were then
associated with the literature data on density, specific heat and experimental porosity to find thermal conductivity which was in the range of 0.47 to 0.86 W/(mK) at room temperature depending on the spray run.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Suspension thermal spraying is a new family of coating deposition
methods which use a slurry with fine (nanometric or submicrometric)
solids instead of coarse powder. The suspension plasma spraying
seems to have been the most studied by now [1,2]. During the deposition process, the slurry's liquid evaporates after injection to the plasma
jet and the solids may agglomerate, melt and impact the substrate to
form a coating. The method of slurry introduction to the torches with
radial feedstock injectors, being the most popular in research and
industrial applications, may be realized by: (i) an atomizer; or, (ii) a
nozzle (mechanical injector). The injection mode influences strongly
the sprayed coatings' microstructure. The fine grained microstructure
is characteristic for the coatings sprayed with the use of an atomizer
☆ A part of the paper was presented at ITSC 2012, Houston, Texas, USA, 21–24 May
2012.
⁎ Corresponding author. Tel.: +33 587 50 24 12.
E-mail address: lech.pawlowski@unilim.fr (L. Pawłowski).
0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.surfcoat.2012.08.014
[3]. On the other hand, the injection through a nozzle results in a
two-zone-microstructure which has been analyzed recently [4]. Partly
stabilized zirconia has been applied for 4 decades as a thermal barrier
coating (TBC) [5,6]. The progress in the technology of thermal barriers
concerns the choice of thermal insulating materials which includes
pyrochlores, hexaaluminates, and perovskites [7]. Thermal properties
of new materials such as lanthana doped yttria stabilized zirconia was
tested by Rauf et al. [8] and samarium zirconate having a pyrochloric
structure—by Yu et al. [9]. Another axis of TBC development is related
to new technology. In fact, solution plasma spraying of yttria stabilized
zirconia was tested by Jordan et al. [10] and suspension plasma spraying
of a similar material was tested by Bacciochini et al. [11] and Kozerski
et al. [12]. The present paper is devoted to the experimental characterization of thermal diffusivity of yttria stabilized zirconia sprayed using
varying operational parameters. Thermal diffusivity data are subsequently a basis for the determination of thermal conductivity verified
by the numerical simulations of the experiments. The experiments
were carried out to understand the influence of operational spray
parameters on thermal conductivity and the measurements of diffusivity were carried out in relatively low temperatures.
L. Łatka et al. / Surface & Coatings Technology 208 (2012) 87–91
88
Table 2
Results of thickness and porosity characterizations of suspension plasma sprayed YSZ
coatings.
10 µm
Fig. 1. SEM (secondary electrons) micrographs of 8YSZ powder used to formulate the
suspension.
Run
Thickness, mean value
[μm]
Thickness, standard
deviation, [μm]
Porosity
[%]
1
2
3
4
5
100
106
53.6
51.1
59.7
0.9
1.5
0.9
2.9
4.5
9
8
9
10
12
and analyzed using scanning electron microscope Philips 515 (Eindhoven,
Netherlands) with the use of secondary electron detectors.
X-ray diffraction patterns were obtained with a D8 Advance diffractometer equipped with a fast linear detector Lynxeye and using
monochromatic Cu Kα1 radiation. The typical recording conditions
are reported elsewhere [12]. The DIFFRAC+ suite of the program
was used for the preliminary crystal phase analysis and TOPAS V4.1
for a Rietveld method study of sprayed deposits.
2.4. Determination of coatings thickness and porosity
2. Experimental methods
The initial powder of 8YSZ was the commercially available, Metco
204NS. The powders were ball milled for 2 h and their initial mean
diameter of dvs = 38 μm was reduced to 4.5 μm as shown by the measurements of laser granulometry. The morphology of milled powder
is shown in Fig. 1. The suspensions used for the spray experiments
were formulated with the use of 20 wt.% of powder, 40 wt.% of
water and 40 wt.% of ethanol. The suspension feed rate was equal to
about 39 g/min in all experiments and the static pressure in a pneumatic container was about 0.03 MPa. The suspension was introduced
via internal injection mode using a continuous-stream injector (with
an internal diameter of 0.5 mm) installed inside the anode-nozzle of
the plasma torch.
2.2. Plasma spraying
The torch used for plasma spraying was the SG-100 (Praxair S.T.,
Indianapolis, IN, USA) DC single cathode torch animated by linear
movement in the face of substrates installed on a rotating swivel.
The working gases were a mixture of Ar + H2 (45 + 5 slpm) and
applied electric power was constant and equal to 40 kW. After each
scan of the torch along the swivel, the deposition was interrupted
and coatings were cooled down to temperatures (measured using a
pyrometer) of about 60–80 °C. The variable spray parameters and
other experimental details are given in Table 1. The coatings were
sprayed onto stainless steel cylindrical substrates and sand blasted
prior to deposition. The bond coatings were not applied.
2.3. Structure and microstructure characterization
The powder was analyzed using a field emission scanning electron microscope Jeol JSM‐7400. The coatings were prepared metallographically
Table 1
Variable spray parameters and coating temperatures.
Run
1
2
3
4
5
Torch linear speed
[mm/s]
300
500
400
300
500
Spray distance
[mm]
40
40
50
60
60
Maximum surface
temp. [°C]
570–690
490–650
460–530
410–570
380–460
Metallographically prepared samples were observed using an
optical microscope Nikon Eclipse LV100. The thicknesses of coatings
were found from 15 measurements in different regions of coatings.
The porosity was determined from the image analysis. The SEM
images having 10,000× magnifications were used to this end. The
software used for the analysis was the Image J public domain program
[13]. The results of thickness and porosity characterizations are collected in Table 2.
2.5. Thermal diffusivity measurements
The samples submitted to thermal diffusivity measurements were
coated with a thin graphite layer deposited onto zirconia coatings.
The measurements were made using LFA 447 NanoFlash® Flash
Lamp System. The setup enables the measurements in the temperature range from room temperature to 250 °C in air. The measurements were made by applying the software developed for 2-layer
samples. Thermal diffusivity can be calculated from the well-known
expression, developed for a single layer configuration [14]:
2
α¼
0:1388·d
t 0:5
ð1Þ
in which a is the thermal diffusivity, d is the sample thickness and
t0.5 is the half rise time. The value of t0.5 corresponds to 50% of the
Back face signal-sample YSZ-1
2.5
Electrical signal [V]
2.1. Suspension preparation
2
1.5
1
0.5
0
0
200
t 0.5
400
600
800
1000
Time [ms]
Fig. 2. Experimental evolution of the electrical signal collected by an infrared detector
at the back face of the sample sprayed in run 1 during the measurement of thermal
diffusivity by the flash method at room temperature.
L. Łatka et al. / Surface & Coatings Technology 208 (2012) 87–91
89
Table 3
Thermal dilatation and specific heat values used in calculations of thermal conductivity
of YSZ coatings [20,21].
Temperature
[K]
Specific heat
cp [J/(kg·K)]
ΔL
L [1/1]
298
323
373
423
473
523
438
453
480
501
518
531
4.2606 × 10−5
2.5646 × 10−4
6.8840 × 10−4
1.1262 × 10−3
1.5702 × 10−3
2.0206 × 10−3
Thermal dilatation
sample's back face temperature rise (see Fig. 2). The measurements of
thermal diffusivity were made for the YSZ coatings on stainless steel
substrates. In this case a “double layer” mode of the setup was chosen,
in which there is one unknown thermal diffusivity value (of the
tested coating). During the experiment the model of Cowan including
2 layers was used [15,16]. The necessary data input included properties such as density ρ, specific heat cp (for both types of materials) and
thermal diffusivity or substrate. The model of Cowan includes the
possibility of heat loss from both the front and back face of the sample. In addition, it was also assumed that the energy pulse was rectangular in shape and of relatively short duration (pulse time, tp b b t0.5).
In fact, in these experiments, the duration of the pulse tp is equal to
0.18 ms and t0.5 is longer than 300 ms (see Fig. 2).
Thermal conductivity was calculated from following equation:
ρ300
1 þ 3·ΔL
L ðT Þ
ð2Þ
in which λ is the thermal conductivity, a is the thermal diffusivity, cp
is the specific heat, ρ300 is the apparent density in room temperature,
and ΔL/L is the thermal dilatation. The density used in the calculations
was corrected for porosity as in following expression:
ρ300 ¼ ρ0 ð1−P Þ
Fig. 4. SEM (secondary electrons) of the cross-section of 8YSZ coating sprayed in run 3.
2.7. Verification of thermal conductivity
The thermal diffusivity experiments were simulated using
COMSOL Multiphysics 3.4 Client software. There were 2 layers, for
the substrate and coating and the virtual layer, which was the TCR
(thermal contact resistance) layer. The heat conduction equation
has the following form:
2.6. Thermal conductivity determination
λðT Þ ¼ α ðT Þ·cp ðT Þ·
10 µm
ð3Þ
in which ρ0 is the density of tetragonal phase YSZ, being the major phase
in the sprayed coatings [15], and is equal to 6050 kg/m 3 and P is the
porosity. The specific heat was calculated from the Kopp–Neumann
law by taking into account the chemical composition of the powder
used as a spray. Table 3 shows all the values of specific heat and thermal
dilatation in the temperature in which diffusivity was being measured.
Thermal diffusivity of used stainless steel substrates was measured
experimentally in the temperatures of interest and was found to
match well with the literature data.
ρcp
∂T
þ ∇ð−λ∇T Þ ¼ A
∂t
ð4Þ
where T is the temperature, cp is the specific heat, ρ is the density and
A is the heat production rate per unit time and unit volume. The heat
conduction equation was solved numerically for cylindrical coordinates. The temperature vs. time response was collected in point on
the top of the coating surface (Fig. 3). The boundary condition on
the side of the substrate included heat production by the Xenon
lamp pulse which is described by a heat flux q:
q ¼ q0 þ hðT ext −T Þ
ð5Þ
in which q0 is a flux generated by the lamp and Text is the external
temperature. Finally the thermal contact resistance (TCR) between
the substrate and coating was considered. The value was taken to
be in the range of TCR = 1.5 × 10 −4–2.5 × 10 −4 m 2K/W after [17].
The results of the simulations were the temperature evolution of
the rear side of irradiated samples. These evolutions were fitted to
the experimental curves (such as this shown in Fig. 2) with the use
of the least squares method [18] by changing thermal conductivity
until high correlation was reached.
Fig. 3. Axial symmetry model used in the simulation.
L. Łatka et al. / Surface & Coatings Technology 208 (2012) 87–91
Table 4
Calculation of the content of phases and lattice parameters using the Rietveld method of
YSZ coatings suspension plasma sprayed in different runs.
Sample
Tetragonal zirconia
Monoclinic zirconia
Content, wt.%
a, Å
c, Å
Content, wt.%
a, b, Å
c, Å and γ, °
1
89.2
3.6147
5.1650
10.8
2
90.4
3.6144
5.1657
9.6
3
91.6
3.6157
5.1634
8.4
4
92.6
3.6159
5.1630
7.4
5
92.9
3.6159
5.1629
7.1
5.172
5.206
5.171
5.204
5.167
5.207
5.168
5.207
5.170
5.204
5.319
99.12
5.318
99.11
5.318
99.15
5.318
99.16
5.318
99.13
YSZ-1
YSZ-2
300
350
YSZ-3
YSZ-4
YSZ-5
1.2
Thermal conductivity [W/m.K]
90
1
0.8
0.6
0.4
0.2
0
250
400
450
500
550
Temperature [K]
3. Results
Fig. 6. Thermal conductivity of 8YSZ coatings suspension plasma sprayed in different
experimental runs.
3.1. Microstructure
The suspension plasma coatings surface shows generally fine grains
having a spherical form and well molten large lamellae (Fig. 4). The
cross section shows relatively well molten lamellae and some porosity between them. It is possible to distinguish the small grains inside
the lamellae. Some of them have vertical cracks, which could have
been generated by the relaxation of thermal stress after deposition.
All the suspension plasma sprayed coatings were tetragonal and
89 to 93 wt.% and the residual phase was monoclinic zirconia (see
Table 4).
3.2. Thermal diffusivity measurements
Thermal diffusivity data are shown in Fig. 5. The samples sprayed
in run 2 have the greatest diffusivity and the lowest—the samples
sprayed in the run 4. Thermal diffusivity is greatest for the less porous
sample (see Table 2) and the porosity, in turn, decreases with the
decrease of the spray distance (see Table 1).
3.3. Thermal conductivity
Thermal conductivity was calculated with the use of Eq. (2) using
the specific heat and thermal expansion data collected in Table 3.
The results are shown in Fig. 6. The calculated values of thermal conductivity are in the range of 0.5 to 1.0 W/(mK) in the discussed temperatures. The thermal conductivity data were verified with the use of
simulation of thermal diffusivity experiments. The thermophysical
YSZ-2
YSZ-3
YSZ-4
YSZ-5
4. Discussion
The suspension plasma spray technique needs more careful optimization of processing parameters and, in particular, the deposition
rate is lower than that obtained at the conventional coarse powder
spraying. The suspension sprayed yttria stabilized zirconia has a
two-zone-microstructure characteristic for suspension sprayed coatings which was discussed in Ref. [4]. Such a microstructure is characterized by a presence of large, well-molten grains and by small, fine
particles, which are more or less sintered. The processing parameters
of suspension preparation were improved with regard to the previous
paper presented in Ref. [12] and the distribution of the initial fine
powder was narrower and monomodal. Consequently, the resulting
Back face temperature -sample YSZ -1
0.35
299.6
299.4
0.3
Temperature [K]
Thermal diffusivity [m2/S] x 10-6
YSZ-1
0.4
properties of the coating were the same as those used in the determination of thermal conductivity and the data for the substrate were
found in the database. The initial values of the thermal conductivity
of the coating were taken from the experiments. The simulations
enabled us to find the temperature of the back side after the lamp
pulse (see Fig. 7). Theses curves were compared with curves obtained
experimentally, such as those shown e.g. in Fig. 2. The comparison
started by setting the minimum and maximum values of the temperature and electric signal evolution as equal. Then, the simulated
temperature curve was adjusted to the experimental one by changing
thermal conductivity in the simulation until the shapes of curves
were identical. The fitting was made with the least squares method
to reach high correlation. The results of the fitting are shown in
Table 5. Thermal conductivity found in the fitting process corresponded
fairly well to the experimental data found in Eq. (2).
0.25
0.2
0.15
299.2
299
298.8
298.6
298.4
298.2
0.1
250
350
450
550
Temperature [K]
Fig. 5. Thermal diffusivity of 8YSZ coating suspension plasma sprayed in different
experimental runs. The curves show the experimental data and standard deviations.
298
0
200
400
600
800
1000
Time [ms]
Fig. 7. Simulation of temperature evolution of the back face of the sample sprayed in
run 1 after lamp pulse at room temperature.
L. Łatka et al. / Surface & Coatings Technology 208 (2012) 87–91
Table 5
Thermal conductivity of YSZ coatings obtained by fitting the simulated curves to experimental ones.
Temperature T [K]
298
R2
323
R2
373
R2
423
R2
473
R2
523
R2
Thermal conductivity [W/(m·K)] Correlation coefficient [1/1]
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
0.8
0.9987
0.8
0.9966
0.8
0.9953
0.8
0.9965
0.9
0.9979
0.9
0.9992
0.9
0.9952
0.9
0.9976
0.9
0.9982
0.9
0.9973
1.0
0.9946
1.0
0.9969
0.7
0.9972
0.6
0.9989
0.7
0.9944
0.7
0.9969
0.7
0.9981
0.9
0.9965
0.5
0.9959
0.5
0.9976
0.5
0.9972
0.5
0.9963
0.6
0.9975
0.6
0.9971
0.5
0.9979
0.5
0.9964
0.6
0.9988
0.6
0.9971
0.6
0.9958
0.7
0.9985
coatings were denser and more homogenous (see Fig. 4). The careful
analysis of the microstructure enables us however to distinguish the
small grains inside the larger lamellae and the micro-porosity. This
micro-porosity may explain low thermal conductivity such as that
observed for the sample sprayed in experimental run 4 (see Fig. 6).
This value is much lower than the conductivity λ = 0.69 W/(mK) observed for the 8YSZ samples of suspension plasma sprayed previously
which have had a porosity of 17% which is much greater than that of
10% found for the samples sprayed in run 4 (see Table 2). The influence of micro-porosity on thermal conductivity is confirmed by the
data presented by Raghavan et al. [22] for 8YSZ having cubic and tetragonal structures and a crystal size of 83 nm is about 2 W/(mK) in similar range of temperatures. These authors showed also that thermal
conductivity of conventional APS coatings used for thermal barriers
was about 1.2 W/(mK) for high density coatings and about 0.9 W/
(mK) for low density coatings. On the other side, thermal conductivity
for EB PVD zirconia coatings was above 1.5 W/(mK) in the same temperatures. In general, thermal diffusivity and thermal conductivity are
lower for the samples sprayed from a larger distance and having greater
porosity (see Figs. 5 and 6 and Table 2). Mechanical properties of the
coatings are discussed elsewhere [19].
5. Conclusions
8YSZ coatings were obtained using suspension plasma spraying.
The solid phase in the suspension with a mean size of 4.5 μm was
obtained by milling coarse Metco 204NS powder. The fine solids
were mixed with 40 wt.% water and 40 wt.% of ethanol to obtain a
suspension. The plasma spray parameters were optimized by keeping
constant the electric power and working gases composition and varying the spray distance and torch scan linear speed. The porosity of the
samples was found to be in the range of 8–12%. Thermal diffusivity
was measured with the use of the commercial NanoFlash system in
the temperature range from room temperature to 300 °C. The measurements were made with the use of the coatings sprayed on the substrate
and the 2-layer numerical model was developed to determine thermal
91
diffusivity of the coatings. The diffusivity was in the range of 0.196×
10−6 to 0.352 × 10 −6 m 2/s in room temperature depending on the
spray parameters. The obtained data were then associated with the
literature data on density, specific heat and experimental porosity
to find thermal conductivity which was in the range of 0.47 to
0.86 W/(mK) at room temperature depending on the spray run. The
obtained results were successfully verified by applying a numerical
simulation of the experiment of thermal diffusivity. The small values
of conductivity are explained by the micro-porosity of the coatings.
Further studies on thermophysical properties will be carried out in
higher temperatures of up to 1200 °C.
Acknowledgments
The part of the study was financed by the Ministry of Science and
Higher Education of Poland, Grant NN503 134138. The French Embassy
in Poland financed the stay of Mr. Łatka in France. The foundation
“Perenne” supported the stay of Mr. Rishi Kumar in France. The authors
are thankful to Dr. Geffroy for his help in milling the powders, to
Dr. Kozerski for the useful discussions, to Dr. Laval for carrying out the
X-ray studies and Prof. Kępinski for the SEM examinations.
References
[1] L. Pawłowski, Surf. Coat.Technol. 203 (2009) 2807.
[2] P. Fauchais, G. Montavon, J. Therm. Spray Technol. 19 (1–2) (2010) 226.
[3] R. Jaworski, L. Pawłowski, F. Roudet, S. Kozerski, A. Le Maguer, J. Therm. Spray
Technol. 71 (1) (2008) 73.
[4] L. Łatka, S.B. Goryachev, S. Kozerski, L. Pawłowski, Materials 3 (7) (2010) 845
(open access journal).
[5] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280.
[6] R.A. Miller, Surf. Coat.Technol. 30 (1987) 1.
[7] R. Vassen, M.O. Jarligo, T. Steinke, D.E. Mack, D. Stöver, Surf. Coat.Technol. 205
(2010) 938.
[8] A. Rauf, Q. Yu, L. Jin, C. Zhou, Scr. Mater. 66 (2012) 109.
[9] J. Yu, H. Zhao, S. Tao, X. Zhoua, C. Ding, J. Eur. Ceram. Soc. 30 (2010) 799.
[10] H. Jordan, L. Xie, X. Ma, M. Gell, E.H. Padture, B. Cetegen, A. Ozturk, J. Roth, T.D.
Xiao, P.E.C. Bryant, J. Therm. Spray Technol. 13 (1) (2004) 57.
[11] A. Bacciochini, F. Ben-Ettouil, E. Brousse, J. Ilavsky, G. Montavon, A. Denoirjean, S.
Valette, P. Fauchais, Surf. Coat.Technol. 202 (2010) 683.
[12] S. Kozerski, L. Łatka, L. Pawlowski, F. Cernuschi, F. Petit, C. Pierlot, H. Podlesak, J.-P.
Laval, J. Eur. Ceram. Soc. 31 (2011) 2089.
[13] http://en.wikipedia.org/wiki/ImageJ consulted on 21 June 2012.
[14] W.J. Parker, R.J. Jenkins, C.P. Butler, G.L. Abbott, J. Appl. Phys. 11 (9) (1961) 1679.
[15] R.D. Cowan, J. Appl. Phys. 32 (7) (1961) 1363.
[16] R.D. Cowan, J. Appl. Phys. 34 (4—part 1) (1963) 926.
[17] F. Gitzhofer, L. Pawłowski, D. Lombard, C. Martin, R. Kaczmarek, M. Boulos, High
Temp.-High Pressures 17 (1985) 563.
[18] L. Pawłowski, P. Fauchais, Rev. Phys. Appl. 21 (1986) 83.
[19] L. Łatka, D. Chicot, L. Pawłowski, A. Cattini, S. Kozerski, F. Petit, A. Denoirjean, in:
R.S. Lima, A. Agarwal, M.M. Hyland, Y.-C. Lau, C.-J. Li, A. McDonald, F.-L. Toma
(Eds.), Thermal Spray 2012, Proceedings from the International Thermal Spray
Conference and Exposition, May 21–24, 2012, Houston, Texas, USA, Ed. ASM
Int., Materials Park, USA, 2012, p. 809.
[20] L.B. Pankratz, in: U. S. Bureau of Mines Bulletin, 1982, p. 672.
[21] Y. Takeda, H.Y. Tu, H. Sakaki, N. Imanishi, O. Yamamoto, M.B. Philips, N.M.
Sommes, J. Electrochem. Soc. 144 (8) (1997) 2810.
[22] S. Raghavan, H. Wang, R.B. Dinwiddie, W.D. Porter, M.J. Mayo, Scr. Mater. 39 (8)
(1998) 1119.