RF PLASMA MODIFICATION OF ZIRCONIA POWDER
G. Paskalov1, S. Maginnis1, Ildar Gafarov2, Ildar Abdullin3
1
Plasma Microsystem LLC, Los Angeles, CA, USA
2
3
RENARISORB Ltd., Moscow, Russia
National Researches Technological University, Kazan, Russia
Abstract: Our interest is especially focused on zirconia nano-crystalline structure modification. It is
shown that RF plasma technology is capable of processing (sintering) partially and fully stabilized Zirconia
in flight without grain growth or binders. The densification and microstructural evolution of the samples
are studied for different plasma conditions. Plasma processed zirconia has nearly 100% theoretical density.
The process of producing of pure zirconia powder from Zircon (ZrSiO 4) also is presented in this paper.
Keywords: RF plasma, zirconia, sintering, production
1.
Introduction
Known methods of plasma processing, in particular those aimed at the thermal treatment of powdered
material use either arc or RF and microwave technique. Plasma process control is quite complicated and requires
different diagnostic tools, starting from accurate feeding rate and ending by product collection. It is known that
plasma (especially plasma jet) pulsation substantially changes the heat exchange efficiency between plasma and
particles. Time of flight and reaction zone dimension stability is important parameters for commercial plasma
applications. One of the most important factors to increase the heat exchange between particles and plasma is the
modulation of plasma parameters [1, 2]. The velocity of plasma gas is less inertia than the velocity of particles and
typically follows the modulation frequency. As is shown in [2] the coefficient of heat exchange, could reach 40 %
for Molybdenum and Tungsten powders, processed in RF plasma reactor. Last decade we were focusing on a
commercialization of plasma Zirconia powder process having a much smaller particle size, typically less than one
micron. This study focuses on RF-plasma processing to enhance the physical and chemical properties of Zirconia
powder.
2.
Experimental Set Up
The typical layout of a basic RF Plasma system is shown on Fig.1. RF generator (here shown as a Lepel T100) includes DC power supply and oscillator cabinet. The grid circuit of the oscillator was modified by adding
frequency modulator. Modulation frequency can be varied from 10 Hz to 10 kHz. The ICP torch (100 kW; ID = 70
mm Plasma Microsystem LLC) is connected to a stainless steel water-cooled reaction vessel via the powder feeding
flange. The quenching device is connected directly to powder feeding flange in a distance of 100 mm. All liquid
argon lines are thermally insulated. Liquid argon flow is controlled by differential pressure control valve. The
product is collected in three different containers, connected to the reactor, cyclone and filter, respectively. The
plasma gas system supplies the required gases or gas mixtures. Argon, oxygen and pure air have been used as a
plasma gases. The flow rates were controlled by MFC and varied from 5 to 10 m3/Hr. Initial material was
introduced into plasma jet by special probes. The powder feed rate was controlled by digital screw feeder and carrier
gas flow. Two locations for powder injection were employed: at the torch nozzle and opposite direction to the
plasma stream. Plasma velocity and temperature were measured by enthalpy probe without powder. We use the
same enthalpy probe for dynamic pressure measurement. Intensity of plasma radiation in absence and presence of
powder was measured by using portable spectrometer (model EPP2000Cs StellarNet Inc.). Exhaust gas was
measured and analyzed by industrial combustion gas & analyzer (model E4400, E-Instrument Inc.).
3.
Model
Particle velocities, coefficient of heat exchange, temperature and efficiency were calculated based on the
theoretical model as it applies to Zirconia powder. Heating a single particle in the plasma flow, melting and partially
evaporating were calculated based on non-linear differential equations described in [3]. In order to evaluate the
interactions between particles and heat exchange between plasma and solid particles, we use semi-experimental
equations and experimental plasma temperature profile and particles trajectories. The efficiency of the plasma
process ( m) was evaluated in accordance with the following formula [2]:
m
= 3km
mL/(2(1-km)dm mVplCpm)
where m is equal 4 for sphere; km – plasma stream loading ratio; L – plasma active zone; dm – particle diameter;
, Cpm – density and heat capacity of Zirconia, respectively; Vpl – plasma velocity. The plasma modulation is
performed by modulation of induction coil current. The theoretical model is similar to the same as presented in [2].
4.
m
Result and Discussion
The plasma process parameters and quality of product are very dependant on the raw material particle size. Typical
particle size distribution is shown on Fig. 2. This type of RF plasma system was designed for thermal treatment of
various powders, but mostly oxides (SiO2; MgO, Al2O3). A typical plasma gas is air, but for zirconia we use an airoxygen mixture (50:50) or pure oxygen. The zirconia powder was processed under atmospheric and low pressure RF
plasmas. The average plasma gas temperature at atmospheric pressure varies from 6000 K to 8200 K and is in an
equilibrium state. Low-pressure plasma is in non-equilibrium, i.e. gas temperature was 1200 K and much lower than
electron temperature, which was about 14,000 K.
Fig. 2 Zirconia particle size distribution
X-ray analysis of the plasma processed zirconia show two different crystal phases: tetragonal zirconium
oxide/yttrium oxide ( Zr0.92Y0.08O1.96) and monoclinic zirconium oxide (ZrO2). For most of the processed samples
tetragonal phase is typically dominant. Also in plasma treated zirconia we observed weak reflects of d= 6.95 A0,
which are not related to the above described phases. In plasma treated powder the concentration of the monoclinic
zirconium oxide decrease compare to the non-processed zirconium oxide by at least 2.5 times (based on observation
of the most powerful reflects 111 with d= 3.16 Ao). Plasma processed zirconia has narrow diffraction picks, which is
related to the tetragonal phase. Intensity of reflect 101 with d = 2.96 Ao is changed from 0.270 o2 in initial material
to 0.185 02 in treated material. This is due to better crystal structure after plasma treatment. Diffractograms of some
samples show high basic level, which could be related to the amorphous phase.
100
18
90
16
80
14
70
12
60
10
50
8
40
30
20
10
0
6
4
2
105 74 62
53 44 37
20 10
Fig. 4 Process efficiency vs
Particle Size
0
0.5
1
2.5
5
10
12.5
25
5
Fig. 3 Power consumption (kW) vs
Feed rate (kg)
Zirconia has better crystallization properties compare to other oxides, such as silica, alumina etc.. We
observed well-defined particles in lower temperature plasma at lower pressure, which is almost impossible for silica
or alumina. At lower pressure plasma the particles are “energize”, i.e. collect some positive charge, which keep
particles away of each other (repel each other). This is important effect for product collection. If we collect the
product by dry method [9] for example using an electrostatic filter, most of the particles are of equal size. If we use a
water based collection system, as in a wet scrubber, the product has a broad particle size distribution in the range of
20 nm to 100 nm. We did not observe this effect in atmospheric pressure discharge. The product has a very small
variability of grain size with an average grain size of about 10 to 20 nm. The process efficiency for different particle
sizes of zirconia is shown on Fig.4 at a plasma power of 7.3 kW and powder feed rate of 1 kg/Hr. As is shown on
Fig. 4 zirconia processing efficiency is much higher at smaller particle size up to 5 microns. The power consumption
for different powder feed rate is shown on Fig.3. Smaller particle sizes require special feeding system and have not
yet been fully tested. The process efficiency dramatically depends on the plasma system configuration. For example,
feed material could be introduced into plasma stream at different angles. Counter feed flow increased the quality of
product, but also increase the evaporation (up to 25% evaporated material), which is not applicable for this process.
Vapor phase was collected for future assay. Based on experimental and theoretical data a commercial plasma
system was designed.
5.
Conclusion
Experimental data allows us to observe a interesting and unusual effects during RF plasma processing of
zirconia. Plasma modulation is a powerful tool to increase the efficiency of the process. Feeding systems and
collection methods are optimized in order to commercially produce powdered zirconia in a wide range of particle
sizes.
6.
References
[1] Y Tanaka et al. A large amount synthesis of nano-powder using modulated induction thermal plasmas
synchronized with intermittent feeding of raw materials J. Phys.: Conf. Ser. 406 2012
[2] G. Paskalov Application of Modulated RF-Plasma for Powder Treatment 14th International Symposium on
Plasma Chemistry, August 2-6, 1999, Prague Czech Republic, p. 2265.
[3] S.V. Dresvin, S.G. Zverev Heat exchange in plasma, Sankt-Petersburg, 2008, 212 p.