解説

先進セラミックス研究センター年報（2019）. Vol. 8, 13‑22

Pulsed Electric Current Sintering
and Its Employment for Transparent Polycrystalline Al2O3
Nguyen Huu Hien* , Makoto Nanko** , Yuzin Xin* , Takashi Shirai****Advanced Ceramics Research Center, Nagoya Institute of Technology
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, JAPAN**Department of Mechanical Engineering, Nagaoka University of Technology
1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, JAPAN***Department of Life Science and Applied Chemistry, Nagoya Institute of Technology
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, JAPAN
Pulsed electric current sintering (PECS) has become more and more important in material sciences as an advanced method formaterial fabrication. The remarkable features of PECS in comparison with almost other sintering techniques are the direct heating
method and the assistance of pressure. Therefore, PECS has fast heating rate and this sintering technique can densify samples fast
without a severe grain growth. The high density and fine microstructure of materials sintered by PECS help a lot in improving
properties of materials, especially mechanical properties. Recently, transparent polycrystalline Al2O3 has attracted much attention
among ceramic materials. PECS is a superior sintering method for fabrication of transparent polycrystalline Al2O3. This article
briefly summarizes the features of PECS and its recent advances in fabrication of transparent polycrystalline Al2O3.
Keyword: Pulsed electric current sintering, two-step PECS, transparent polycrystalline Al2O3, microstructural heterogeneity.

Sintering is one of the excellent methods forshaping of materials. Conventionally, the powder isshaped by pressing in mold in order to form a green
body. After that, the green body is sintered by anelectrical furnace at high temperature. Conventionalsintering showed low ability of densification due to thelack of assistance of pressure. Currently, powdermaterials can be densified by numerous pressureassisted sintering techniques such as hot pressing (HP),
hot isostatic pressing (HIP) or pulsed electric current
sintering (PECS). Among those procedures, HP and HIP
methods, which use an indirect heating structure likeconventional sintering, often need a long heating time ofseveral hours in order to lead to full densification [1]. In
contrast, PECS, the latest developed pressure-assistedsintering process, is a rapid densification process. Thisfeature of PECS not only derives from the support of theuniaxial pressure but also comes from its direct heating
method by electrical current. Therefore, it has a big
potential to fully densify the powder compact withminimizing grain growth [2].
The schematical design of PECS is showed in Fig.
1. It simultaneously applies an electric pulse along witha uniaxial mechanical pressure in order to sinter the
powder compact in a mold set. The applied electric pulse
and mechanical pressure can be controled by a preset

program by controlling the electric current or the in-situ
measured temperature.

Fig. 1: Schematic diagram of pulsed electric current
sintering.

In typical PECS methods, powders are poured intoa die/punch set and heated by an electric current flowing
through the die/punch and powder compact system.
Either the powder or the die/punch set or both of themshould be electrical conductive in order to guarantee theflow of the electrical current as a close circuit. For thesame reason, all of the blocks, including electrodes,
spacers, plungers and punches inserted in the circuit (ifnecessary), should be made of electrical conducting
m aterial (stainless steel, graphite, carbides, etc).

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Pulsed Electric Current Sintering and Its Employment for Transparent Polycrystalline Al2O3

Conducting powders are heated both by Joule effectfrom the current within the powder compact and by heattransfer from the container and electrodes. Otherwise,
the non-conductive powders are heated only through thesecond route. The use of graphite containers, which aremostly applied in the current PECS processes, limits themechanical pressure levels to low values, generally
100 MPa. In spite of that limitation, the uniaxial pressureof 100 MPa is still high enough to densify in most cases.
Therefore, the graphite dies and punches are the mostwidely used in PECS because of its simple machining.
When the ultra high density is required, other materials
with stronger mechanical properties can be applied fordies and punches, such as WC or SiC. Grasso et al. useda special design for the mold set of PECS, in which asmaller SiC mold was inserted inside a bigger graphite
mold [3], as shown in Fig. 2. With the same design, they
could sinter with PECS at a pressure of 500 MPa. Using
the same design, Sokol et al. applied 400 MPa or Ratzker
et al. applied 800 MPa in their PECS processes [4, 5].

Fig. 2: Schematic of the high-pressure SPS device.
Reprinted with permission from J. Am. Ceram. Soc., 93

of the heat input, particularly when electricallyinsulating container is used and the electric current isapplied for extremely short duration.
The temperature of PECS process is in-situ
measured by a radiation thermometer focused on thesurface of the die or by a thermocouple inserted into ahole on the outer surface of the die. The measured
temperature at the surface of the die is usually lower
than the actual temperature of the sample inside it.
Depending on a number of factors such as thermal
conductivity of the die and the sample, the heating rate,
the applied pressure, the thermal insulation of the diefrom surrounding, etc, this temperature difference may
become quite large. One of the problems of PECS
processes is to achieve homogenous temperature
distribution in order to achieve a homogeneous sample.
In fact, current distribution and consequent temperature
distributions within the sample are very sensitive to thehomogeneity of the sample after sintering [6-8]. The
inhomogeneity of temperature in PECS can beconsidered as its weakness in comparison with othersintering techniques. However, PECS is still used widely
to prepare many kinds of materials such as advanced
alloy materials, functional gradient materials, advanced
ceramics, thermoelectric materials, nano-composites
because of its advantages - rapid densification withminimized grain growth.
Aluminum oxide (Al2O3), commonly referred toalumina, is one of the most widely used engineering
ceramic materials, especially its most stable crystalline
structure, α-Al2O3 (or corundum/sapphire). Table 1 lists
some of the properties of α-Al2O3, which was reported
by Doremus [9].

(2010) 2460-2462. Copyright: @ John Wiley and Sons.
Table 1: Physical and mechanical properties of α-Al2O3.

The biggest difference of PECS to other pressureassisted sintering techniques is the application of theelectric current through a power supply as the heating
source, which leads to a very rapid and efficient heating.
The heating rate during the PECS process depends onthe geometry of the container and the sample ensemble,
its thermal and electrical properties, as well as theelectric power supply. Heating rates as high as 100 up to
600°
C/min were achieved. In contrast, the typicalheating rate of HP or HIP processes is often limited at afew °C/min. As a result, the processing duration of PECS
is shortened in compared with HP or HIP processes,
which suppresses grain growth in sintering. Hence, themost advantage of PECS processes is the efficient usage

Properties

Typical values

Density

3.99 g/cm3

Flexural Strength379 MPa

Compressive Strength

2600 MPa

Hardness (HV)

1440 GPa

Fracture Toughness KIC

3 MPam1/2

Melting point
Thermal Conductivity
Coefficient of Thermal Expansion

2054°C
35 W/mK
8.4×10–6 K-1

With high melting point and chemically inertness,
Al2O3 is widely used as high-temperature components,
catalyst substrates or biomedical implants. Al 2O 3 is

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Nguyen Huu Hien, Makoto Nanko, Yuzin Xin, Takashi Shirai

useful for bearings and cutting tools because of its
excellent hardness, strength and good abrasion
resistance. Electrical insulators and components are also
applications for Al 2 O 3 based on high electricalresistance. With good optical transparency, Al2O3 is usedas transparent substrates for many optical parts. With theadditions of other materials such as graphite, even higher
temperatures, harsh environments, and severe
applications are envisaged, such as pouring spouts andsliding gate valves at high temperature. For laser hostapplications, by using ceramic materials, such as Al2O3,
significantly higher temperatures are potentiallyattainable.
Pure Al 2 O 3 is colorless, while the addition oftransition metal ions to Al2O3 creates spectacular colors,
gem stones, and practical applications such as rubylasers. Cr 2O 3-doped Al 2O 3, known as ruby, shows abeautiful red color with about 0.1 mol% of Cr2O3 [10].
The red color of ruby results from the transitions ofelectrons between energy levels in its crystal structure.
Doping other transition metal ions into Al2O3 crystals
may produce other colors within the range of visible
light. For example, the deep blue color is resulted by theaddition of a few hundred ppm of Fe 2+ and Ti 4+
impurities [11]. A wide variety of other colors are found innatural and synthetic Al2O3 crystals. Fig. 3 shows thevarious colors of transparent Al 2 O 3 and MgAl 2 O 4
ceramics, fabricated by adding an amount of transition
metal ions and sintering by PECS.

Fig. 3: Appearance of various transparent Al2O3 and MgAl2O4

added with different dopants. Reprinted with permission.
Copyright: © 2015 Nanko M, Dang KQ. Published in [Pulsed
electric current sintering of transparent alumina ceramics, in“Sintering techniques of materials”, InTech] under CC BY 3.0

license. Available from: http://dx.doi.org/10.5772/59170.

In preparation of transparent polycrystalline Al2O3,

the two most important factors are density and grain
size [3, 12-40]. Due to the negative influences of poreso n t h e o p t i c a l t r a n s p a r e n c y, t r a n s p a r e n tpolycrystalline materials requires extremely low
porosity (<0.01 vol%) [18, 20, 21, 37]. It is also believed thatpolycrystalline materials with nano-scaled grain size
would provide better transparency than materials with agrain size in the micrometric range. Moreover, the nanoscaled grain size of the polycrystalline matrix also
accomodates with significant improvement in themechanical properties. Traditional transparent
polycrystalline Al 2 O 3 are prepared by sintering inhydrogen gas at temperature generally above
1700°
C [13, 41]. That high sintering temperature produces
polycrystalline Al 2O 3 with low porosity but causes
extensive grain growth, which dramatically reduces themechanical strength, hardness and optical transparencyof the material. As a result, the optical in-line
transmission of polycrystalline Al 2O 3 produced byconventional sintering is typically below 10%, and thematerial appears translucent rather than transparent [14].
This matter obviously indicates that conventionalsintering is not appropriate to produce transparent
polycrystalline Al2O3.
Currently, fine-grained transparent Al2O3 have beensuccessfully prepared by HP, HIP or PECS at a lowtemperature range, from 1150 to 1400°
C [15, 18-20]. By HP
or HIP techniques, the sample is heated by the heattransfer from the external surface of the container to thepowder compacts. The consequent heating rate is thentypically slow, as lower as 10°
C/min and the process
normally lasts for hours. The long process at hightemperature causes serious grain growth, which isunfavorable for fabrication of transparent Al2O3. The
large grain size of Al2O3 bulks leads to the reduction of
the optical inline transmittance and the mechanicalproperties, such as hardness, fracture toughness orabrasion resistance [21, 22, 26]. The common strategy forsintering of transparent polycrystalline Al 2O 3 is toimprove the densification without significant grain
growth. The fine-grained microstructure (< 1 μm)
provides polycrystalline Al 2 O 3 with a significant
improvement in both mechanical strength and opticaltransparency. It is reported that bending strength of thefine-grained transparent Al2O3 is up to 400-600 MPa
with a high in-line transmission up to 60% for visible
light [14, 26]. The addition of MgO is well-known toannihilate normal and abnormal grain growth during
sintering of polycrystalline Al2O3 [16, 25]. As the result,― 15 ―

Pulsed Electric Current Sintering and Its Employment for Transparent Polycrystalline Al2O3

Route 1

Route 2

Route 3Temperaturepolycrystalline Al2O3 has finer microstructure in grain
size with higher final density.
Recently, PECS has become an alternative method
to fabricate transparent polycrystalline Al2O3 [3, 15-20, 37-41].
It is reported by Kim et al. [19] and Dang et al. [38, 39] that aslow heating rate, such as 2°
C/min, was the critical
PECS parameter for densification of transparent Al2O3
prepared by PECS. Grasso et al. reported that a highpressure of 500 MPa promoted densified and transparent
Al2O3 in PECS at low temperature of 1000°
C [3]. Langer
et al. reported about comparison between HP and PECS
processing for the Al2O3 powder (TM-DAR). By fixing
the sample geometry, the heating program, the appliedpressure and the atmosphere for both sintering processes,
at a given constant time, PECSed samples achieved
higher relative density than HPed ones [42].
In consideration of the most impressive advantage
of PECS as the high heating rate, Nanko et al.
established a new technique with PECS to prepare
polycrystalline Al2O3 with high transparency: two-step
PECS (TS-PECS) [37]. Fig. 4 shows the comparison ofsome common temperature programs of the PECS
processes. A normal one-step PECS process with highheating rate, indicated as the line no. 1, was usedcommonly but resulted in an opaque or translucent
polycrystalline Al2O3 samples. To obtain transparent
polycrystalline Al 2O 3, the one-step PECS program,which is indicated as line no. 2, employed a muchslower heating rate, such as 2°
C/min, leading to highdensity with small grain size [19] . TS-PECS wasestablished with two stages of temperature holding.

Time

Fig. 4: PECS temperature programs: (1) Normal PECS
with high heating rate, (2) Normal PECS with low heating
rate, (3) TS-PECS with high heating rate.

Instead of slowly heating up to the final sintering
temperature, a holding stage with lower temperature wasapplied for partial densification without serious grain
growth. After that, the temperature was risen to the final
sintering temperature to complete densification withpreserving the grain size. In both two steps of heatingup, the heating rate was 100°
C/min [37]. By using TSPECS, polycrystalline Al2O3 with comparatively hightransparency was fabricated in a shorter total time thanby using a normal PECS process with low heating rate.
Otherwise, Grasso et al. used PECS with two-steppressure pattern to improve the transparency ofpolycrystalline Al 2O 3 [43]. Fig. 5 shows their Al 2O 3
samples densified with two PECS pattern: constantpressure and two-step pressure. Based on theconsideration that the heterogeneous densification wasFig. 5: Photograph of alumina ceramics disks sintered by SPS at 11501C with a heating rate of 100 C/min. The sample shown infigure (a) was sintered for 30 min under 80 MPa constant pressure. The sample (b) and (c) were sintered with pressure two steps
pressure application for 30 and 60 min. The samples are 3 cm in diameter and are on top of the text. Reprinted with permission from J.
Am. Ceram. Soc., 94 [5] (2011), 1405–1409. Copyright: @ John Wiley and Sons.
― 16 ―

Nguyen Huu Hien, Makoto Nanko, Yuzin Xin, Takashi Shirai

the reason for the heterogeneous transparency, they
applied low pressure (35 MPa) before the dwelling stage
to slow down the densification of the sampleʼs border atthe beginning, consequently leading to a more uniform
densification at the final stage with higher pressure (80MPa).
As transparent Al2O3 has been a remarkable material
in industries, there are a huge amount of publications ontransparent polycrystalline Al2O3 fabricated by PECS.
However, almost of the polycrystalline Al2O3 with hightransparency fabricated by PECS were sintered at small
and thin size, such as 15 mm in diameter or evensmaller. The disadvantage of PECS is that there isalways a heterogeneity distribution of temperature andpressure inside the powder compact. At small size, thisheterogeneity should be not severe but it may be more
serious at larger sizes. Among many ceramic materials,
transparent polycrystalline Al 2O 3 is a temperaturesensitive material. If the temperature increases just alittle, the grain growth may occur seriously and reduce
the optical transparency. Because of this characteristic of
the PECS process and the strict requirements oftransparent polycrystalline Al2O3, this material fabricatedby PECS is often limited on sintering size. For larger
sizes, there have been many reports on the heterogeneityof the transparent polycrystalline Al 2 O 3 samples
produced by PECS [3, 12, 15, 17-20, 37, 40, 43-45]. Roussel et al.
showed their polycrystalline Al 2O 3 samples with adiameter of 20 mm [44]. Their samples were transparent
in the center but translucent at the edge and the opticaltransmittance was improved by using La dopant, asshown in Fig. 6. However, there was no discussion onthat heterogeneity of polycrystalline Al2O 3 samples.
Grasso et al. reported a reverse situation about
heterogeneity of sintered Al2O3 samples with a diameterof 30 mm. The border of sample was denser, had smaller
grain size and was transparent while the center was
opaque [43]. Wang et al. reported the heterogeneity ofgrain size of polycrystalline Al2O3 samples with various
diameters from 12 to 50 mm [46]. Regardless of thedifferences in grain size at different positions along theradial axis of the samples, the grain size atcorresponding positions between various samples was
totally different, even the sintering conditions were thesame for all of Al2O3 samples with various diameters.
This heterogeneity of the polycrystalline Al2O3 samples
derived from the heterogeneous distribution of thesample temperature and pressure during PECS
processes. The difficulty of controlling the distributionof temperature and pressure during sintering processes
by PECS causes the difficulty of sintering large-sized
specimens. Some studies on simulation of PECS
processes by finite element method (FEM) reported theheterogeneous distribution of the temperature during
PECS processes [46-48]. In almost cases, the distribution oftemperature was reported more heterogeneously with thelarger sintering size. Therefore, sintering of large-sizedtransparent polycrystalline Al 2O 3 by PECS is still achallenge.

Fig. 6: Dependence of sintering temperature on the real
inline transmittance of nondoped and La 3+-doped sinteredα- A l 2 O 3 , s h o w i n g a l a r g e r r a n g e o f d e n s i f i c a t i o ntemperatures with La 3+ dopant. Reprinted with permission
from J. Am. Ceram. Soc., 96 [4] (2013), 1039–1042.
Copyright: @John Wiley and Sons.

In our studies of TS-PECS, the transparent
polycrystalline Al2O3 samples with a diameter of 30 mm
were successfully fabricated by the combination of two
strategies: the optimization of sintering parameters and theusage of dopants. The sintering temperature of TS-PECS
program was 930°
C for the first soaking step and 1130°
C
for the second one. In order to decrease the heterogeneityof grain size of polycrystalline Al2O3 samples, the slow
heating rate, such as 10°
C/min, was applied.
The commercial high-purity Al2O3 powder (Taimei
Chemicals, TM-DAR) was sintered by TS-PECS withthree temperature patterns at the same heating rate of100°
C/min. The heterogeneity of grain size of those
three samples was shown in Fig. 7a. The Al2O3 sample
sintered at 900-1100°
C had the smallest and most
homogeneous grain size but low relative density, at
99.6%. Due to its low transparency, that sample was notfurther investigated. On the other hand, Al2O3 sample
sintered at 950-1150°
C obtained higher density, at
99.8%, but was opaque because of the large grain size.

― 17 ―

Pulsed Electric Current Sintering and Its Employment for Transparent Polycrystalline Al2O3

Therefore, the sintering temperature pattern of 9301130°
C was chosen to conduct further investigation of
the heating rate. In Fig. 7b, the heating rate was reduced
from 100 to 40 and 10°
C/min and the relative density of
those three samples were 99.8, 99.9 and 99.9%,
correspondingly. With decreasing of the heating rate, thegrain size of polycrystalline Al2O3 sintered by TS-PECS
method became smaller and more homogeneous. The
apparent optical transmittance of those three samples isshown in Fig. 8, where the optical transmittance washighest with the lowest heating rate, at 10°
C/min.
However, the optical transmittance of that sample
reduced at the edge of the sample. The appearance of thethree samples also showed the consistence with theresults of optical transmittance. The grain size at thecenter was larger than near the edge, which typically led
to a lower optical transmittance at the center of thesample. In this result, the lower optical transmittance atthe edge of the sample was supposed to indicate aheterogeneity of density or porosity of the sample.
Unfortunately, the relative density of the samples wasalmost 99.9-100% and the common Archimedes ʼ smethod is not accurate enough to detect the difference inrelative density at the level of 0.01%. The reasons of theheterogeneous transparency in this case were still
unknown and need further studying in the future.

Fig. 7: Distribution along the radial axis of grain size ofpolycrystalline Al 2 O 3 samples with different sinteringparameters.

Fig. 8: Appearant optical transmittance of polycrystalline
Al2O3 samples sintered by TS-PECS with different heating
rate.

With the optimization of the temperature andheating rate of TS-PECS program, the large-sizedtransparent polycrystalline Al2O3 was fabricated withgood transparency and significantly homogeneous grain
size. However, there was still a heterogeneity of the
optical transmittance along the radial axis of thetransparent bulk samples.
The other routes to control the fine andhomogeneous microstructure of the polycrystalline
Al2O3 are using dopants. Some studies reported that theusage of dopants could slow down the grain growth
behavior of the polycrystalline Al2O3 during sintering
process, so that the fine microstructure could bepreserved after sintering [17, 44, 47, 49-53]. One of the mostcommon dopants to reduce grain growth rate ofpolycrystalline Al2O3 was MgO. The influences of MgO
dopant in controlling microstructure of polycrystalline
Al2O3 were reported by Wang et al. [47] and Stuer et al. [17].
Besides MgO, Stuer also used Y and La as the dopants
and they reported that the triple doping of Mg-Y-La gave
the best results in decreasing the grain size andconsequently increasing the optical transmittance ofpolycrystalline Al2O3. Voytovych et al. reported thatyttrium doping inhibited both densification andcoarsening of sintered α-Al 2 O 3 at 1450°
C [52] . The
influences in densification behavior were suggested
relating to the transition from grain boundary diffusion
to lattice diffusion controlled densification withincreasing temperature. Bojarski et al. also reported asimilar influence of Y and La co-doping on the grain
growth behavior of Al2O3 [51]. Yoshida et al. investigatedthe densification of Ti-doped Al2O3 and showed that thedensification and grain growth of Al 2O 3 sintered in
N2+H2 gas atmosphere was depressed by Ti dopant [54].

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Nguyen Huu Hien, Makoto Nanko, Yuzin Xin, Takashi Shirai

Table 2: Sintering temperature of TS-PECS of undoped and doped polycrystalline Al2O3 samples, and relative density ofsintered bodies.
DopantSintering temperature
Annotation
1st – 2nd step

Relative
densityNotes-

930-1130°C Undoped

TiO2(0.1 mol%)

930-1130°C Ti1130

98.1% Opaque

1000-1200°C Ti1200

99.9% Transparent - Dark

930-1130°C Y1130

80.6% Opaque

1000-1200°C Y1200

92.8% Opaque

1100-1300°C Y1300

99.8% Translucent

1150-1350°C Y1350

99.9% Transparent - White

Y2O3(0.1 mol%)Trunec et al. also successfully reduced the grain size aswell as improved the optical transparency ofpolycrystalline Al2O3 by using zirconia-spinel co-dopingwith a combination of PECS and HIP [49].
I n o u r o w n s t u d y, t h e m i c r o s t r u c t u r e o fpolycrystalline Al 2O 3 was controlled by using two
dopants, TiO 2 and Y 2 O 3 . With both dopants, thedensification and grain growth of polycrystalline Al2O3
were depressed. However, the influences of Y2O3 dopant
was much stronger than TiO2. The influences of Ti andY dopants in densification of polycrystalline Al2O3 canbe seen obviously in Table 2. While the undoped Al2O3
was fully densified at the sintering temperature of 9301130°
C for TS-PECS, the Ti-doped and Y-doped ones
required 1000-1200°
C and 1150-1350°
C, respectively,
for fully densification.
Fig. 9 shows the distribution of grain size oftransparent samples along the radial axis. Even at muchhigher sintering temperature, 1150 and 1350°
C for thetwo steps, the grain size of Y2O3-doped Al2O3 sample
was still equivalent with that of the undoped ones. The
grain size of TiO2-doped Al2O3 was a bit smaller than the
undoped one, although the sintering temperature was a
bit higher. Both results indicated that the dopants
dramatically depressed the grain growth ofpolycrystalline Al 2O 3. Fig. 10 shows the appearent
optical transmittance of the three transparent samples,
measured at different positions along the radial axis.
Although the detailed understanding of the heterogeneityof optical transmittance has been still not clear yet, theresults showed obviously that the dopant of Y2O3 could
improve optical transparency of polycrystalline Al2O3 aswell as its homogeneity. It indicated that the additive of
Y2O3 dopant is excellent to control the microstructure ofpolycrystalline Al 2O 3. Although TiO 2 dopant could
depress the grain growth of polycrystalline Al2O3 andimprove the homogeneity of grain size, it caused a dark

99.9% Transparent - White

color for polycrystalline Al2O3 samples, which led to asignificantly lower optical transmittance. Therefore, theTiO2 dopant is not good for transparent polycrystalline
Al2O3.

Fig. 9: Distribution along the radial axis of grain size ofpolycrystalline Al2O3 samples with different dopants.

Fig. 10: Appearant optical transmittance of polycrystalline
Al2O3 samples sintered by TS-PECS with different dopants
and corresponding sintering temperature.

In summary, PECS has been used widely tofabricate transparent polycrystalline Al2O3. However, theoptical transparency of polycrystalline Al2O3 is still not― 19 ―

Pulsed Electric Current Sintering and Its Employment for Transparent Polycrystalline Al2O3

as good as single crystal sapphire, which has been
applied in many industries for many years. The
necessary improvements for transparent polycrystalline
Al2O3 includes the optical transparency, the homogeneityof the sintered bodies as well as the sintering size. Alongwith the developments of PECS and its related sintering
methods, polycrystalline Al2O3 is expected to substitutesingle crystal one in variety of industrial applications.

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