Synthesis of nanosized ceramic oxide powders by microwave plasma reactions

N^NoSTRUCTURI:=D MATERIALS VoL. 1, PP.427-437, 1992 COPYRIGHT©1992 PERGAMONPRESSLTO. ALL RIGHTSRESERVED.

SYNTHESIS

0965-9773/92 $5.00 + .00 PRINTEDIN THE USA

OF NANOSIZED CERAMIC OXIDE POWDERS MICROWAVE PLASMA REACTIONS

BY

Dieter Vollath Kernforschungszentrum Karlsruhe P. O. Box 3640, W-7500 Karlsruhe, Germany Kurt E. Sickafus Los Alamos National Laboratory Los Alamos, N.M. 87545, USA (Accepted November 1992)

Abstract----This paper describes a novel process using a microwave plasma as a source of energy to synthesize ceramic oxide powders with mean particle size in the range of 5 to 30 nm. The process works without solvents by evaporation of chlorides of the elements used in ceramics. The process was demonstrated by the synthesis of alumina-, titania- and zirconia- based ceramic powders. Air or one of the noble gases mixed with any amount of oxygen and water was used as process gas. Electron microscopy revealed that through proper selection of synthesis conditions, it is possible to obtain nanocrystalline powders. Due to the extreme conditions during synthesis it is possible to prepare non-equilibrium phases or solid solutions in systems exhibiting no equilibrium solubility.

INTRODUCTION Nanocrystalline or 'nanosized' ceramic materials with particle sizes below 10 nm exhibit interesting physical properties (1). The standard method for synthesis of nanosized oxide powder developed by Gleiter et al. (1-3) is a two-step process: In the first step the metal is evaporated and collected as a fine condensate on a cold finger. In the second step the metal condensate is oxidized and than scraped off the cold finger. This process is highly energy consuming as it is necessary to first produce the metals and then to evaporate them. Another process developed by Kagawa et al. (4,5) involved spraying aqueous solutions into an inductively coupled plasma. This process works at temperatures above 5000°C and delivers in some cases amorphous products. Besides these two methods, many other attempts to obtain nanosized ceramic powders have been published, such as laser pyrolysis. It might be more energy efficient to start with a chemical compound, which is produced as an intermediary product in the preparation of the pure metal. Usually these intermediary products 427

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are the halides. The synthesis of fine oxide powders by hydrolyzing chloride vapors in a hydrogenoxygen flame has already been reported (6,7). Furthermore, some literature exists on the enhancement of chemical reactions in a plasma. The intent of this contribution is to combine these two procedures in a new process for the synthesis of nanophase oxide ceramic powders. A fh-st attempt in this direction was an rf glow discharge method, using gas phase reactions at low pressures (8). The silicon nitdde product obtained was amorphous and not fully reacted to the intended phase. In addition, working at low pressures (e.g., < 1 mbar) limits the production rate significantly. Therefore a process working at relatively high pressures had to be developed.

PROCESS TECHNOLOGY Microwave Plasma The energy Iransferred to a particle in an oscillating field is proportional to 1/(m.f2) (m...mass, f...frequency). For microwaves of high frequency, the amount of energy transferred to the ions is small, relative to the substantial amount of energy transferred to the electrons. Because of this, the "temperature" of the free electrons is much higher than the "temperature" of the ions. This leads to the fact that the "overall temperature" of a gas passing a microwave plasma is not as high as in a de or rf plasma, where temperatures between 5000 and 15000°C are obtained. In a microwave plasma the temperature can be adjusted in a range between 300 and 900°C by properly selecting field strength, gas pressure, and gas species. Gas pressure and temperature needed for the synthesis of significant amounts of ceramic powders can be adjusted in a range that is optimal for the chemistry going on. By selecting proper experimental conditions it is possible to obtain a plasma with virtually any gas or gas mixture. In addition, the plasma enhances the kinetics of the chemical reactions. The reason for this is the ionization of the reactive molecules and, at least to some extent, the dissociation of those molecules. The most important reaction in this respect is the dissociation of water: H20 --->H + + OH-. The presence of water in a plasma simultaneously increases the degree of ionization in the plasma (9-11). This also leads to higher temperatures in the flowing gas after the plasma zone.

Plasma Reactor The synthesis is performed in a 50 mm diameter reaction vessel made of quartz, placed in a cavity connected to a microwave generator. The microwave cavity was designed as a singlemode cavity using the TEl0 mode in a WR 975 waveguide. For the experiments a microwave system with a frequency of 0.915 GHz was used. A standing wave was adjusted using a sliding short circuit. Additional impedance matching using a tristub tuner was not necessary. The halide salts of the metals to form the oxide ceramics are vaporized outside the reaction vessel and introduced well below the entrance to the plasma gas. In some cases water is introduced into the plasma chamber by means of a two-phase nozzle. As described previously (12-15), the nozzle is located in a frit through which a carder gas flows. This arrangement has the advantage of avoiding vortices beside the nozzle.

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Chemistry To demonstrate the ability of a microwave plasma to act as a source of energy for powder synthesis, ceramic powders based on zirconia, alumina, and titania were produced. The vapors of the chlorides in question were produced outside the reaction chamber and introduced between the gas inlet and the plasma reaction zone. When water is added, it is important that the water be completely evaporated before it enters the reaction zone, where the chloride vapors are introduced. Otherwise, the droplets act as condensation seeds for the halides, leading to large particle sizes. One has also to ensure that the gas, containing the water vapor, is hot enough to avoid precipitation of the chlorides. This precipitation leads to large particle sizes. The nozzle used for introducing water delivers a mist of droplets with a mean diameter of about 7 lain (16). The gas mixture consisting of the carrier gas, halide vapor and water vapor passes through the plasma zone, where the nanosized oxide particles are formed. In the plasma the following reactions may take place: (1) Without addition of water: MeCln + m/202 --->MeOm + n/2C12 MeCln stands for A1C13, TiCi4 or ZrCI4. (2) With addition of water: In this case we have to consider that OH- radicals are formed in the plasma. Therefore, there is the possibility of two reaction routes: MeCln + m/202 + n/2H2 ~ MeOm + nHCI and probably MeCln + (m + x)OH-+ (m + x)H+ ---> MeOm + xHC10 + (n - x)HCI + (m + 2x - n)H2 x << n is assumed. The addition of water has two effects: by forming HCI, and possibly HC10, it increases the reaction enthalpy and acts as a catalyst in increasing dissociation (9-11). The temperature of the gas was monitored after passing the reaction (plasma) zone. Under identical conditions, the temperature detected was higher in the case of water addition. Powder samples were collected after measuring the temperature. In a typical run, a gas volume of about 5 to 10 m3/h passed the plasma zone. This gas carded the evaporated metal chloride equivalent to 0.2 to 5 g/min. The amount of water added was in the range of 0.2 to 5 ml/min. POWDER SYNTHESIS To demonstrate the ability of the microwave plasma method for powder preparation, results of the synthesis of nanophase pure zirconia, alumina, and titania are presented. Characterization

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of this material was done by high resolution electron microscopy, which revealed mean particle sizes in the range of about 5 to about 30 nm. The particlesare usually monocrystalline and form loosely connected agglomerates. Experimental details are given in Table 1.

Figure 1. Electron micrograph of ZrO2 powder synthesized with the addition of water. (a) low magnification showing particle size dislribution, (b) high magnification lattice image showing that particles are single crystals.

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Synthesis of Zirconia To synthesize zirconia, experiments with and without the addition of water were performed. In both cases, the product powder was completely crystallized in the cubic structure. Amorphous

Figure 2. Electron micrograph of ZrO2 powder synthesized without the addition of water. (a) showing uniform particle size distribution, (b) showing that the particles are single crystals.

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particles were not found. In the case of water addition (example 1) the chloride was evaporated at a temperature of above 300°C, leading to an evaporation rate of about 5 g/min. The product obtained had, as shown in Figure l(a), particle sizes between 10 and 30 nm. A few particles with sizes up to 120 nm were also found. High-resolution electron microscopy revealed that even those large particles were monocrystalline with cubic lattice structure (Figure l(b)). It is assumed that these larger particles were present because a small fraction of ZrCI4 condensed in the carrier gas stream, cooled by the evaporation of the water added. The particle size is reduced significantly when water is not added. Figure 2(a) exhibits a product obtained in a run (example 2) without the addition of water. In this case the temperature for evaporation was reduced to 220°C. The product obtained had particle sizes between 3 and 8 nm. Again, the particles are monocrystalline with cubic structure (Figure 2(b)). As the process of particle formation is stochastic, one may believe that the formation of a few larger particles is unavoidable. Two particles with sizes up to 30 nm were observed. As particles with sizes between about 10 nm and about 30 nm are absent, it seems that these larger particles must have become temporarily stuck on the wall of the reaction vessel, leading to an increase in size as a result of their prolonged residence time in the reaction zone. In the case of water addition to the plasma the temperature after the plasma zone increased about 20°C, whereas in the water-free experiments a decrease of about 50°C was observed.

Synthesis of Alumina Crystallization behavior of alumina synthesized in a microwave plasma is entirely different. The most striking difference is that it is possible to obtain an amorphous or poorly crystallized product if the reaction is carried out without the addition of water (example 3), Figures 3(a) and (b). The evaporation temperature of the chloride was 140°C. Even when electron diffraction (Figure 3(b)) gave a faint indication of a diffraction pattern and micrographs obtained in dark field (Figure 3(c)) showed speckles that indicate at least the onset of crystallization, lattice contrast fringes were not observed in high resolution electron microscopy. The electron diffraction pattern can not be exactly attributed to any of the known alumina phases. The pattern, close to the pattern of alpha alumina, probably describes some short range order. The product obtained with this experiment probably consists of amorphous-like particles with sizes between 4 and 8 nm. Larger particles were not found. As in the synthesis of zirconia, the temperature decreased when the chloride vapor was introduced.

Synthesis of Alumina-Zirconia Alumina and zirconia exhibit a very small mutual solubility (I 5). However, it was found that it is possible to obtain a nonequilibrium solid solution between these two oxides crystallized in the cubic fluorite structure (16,17). Synthesizing a mixture of alumina with about 10% zirconia without adding water (example 4) to the plasma gas yielded a product similar to pure alumina (Figure 4(a)), with particles sizes between 4 and 9 nm. Larger particles were not observed. Electron diffraction (Figure 4(b)) gave a faint indication of a more pronounced diffraction pattem, as in the case of pure alumina. Some particles exhibiting lattice fringes were observed. Again, it

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Figure 3. Electron micrograph of A1203 powder, synthesized without the addition of water. (a) showing the uniform distribution of particle sizes, (b)showing distinct diffraction rings, (c) showing bright speckles that indicate crystallized particles.

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Figure 4. (a) Electron micrograph of A1203 10% ZrO2 powder, synthesized without the addition of water, showing uniform particle size distribution, (b) electron diffraction pattern showing some degree of crystallization.

NANOSIZEDCERAMICOXIDEPOWDERS

Figure 5. (a) Eleclron diffraction pattern of TiO2 powder, synthesized with the addition of water, showing essentially ruffle structure, (b) showing that the particles are single crystals.

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TABLE 1 Experimental Data used for the Synthesis of Nanosized Ceramic Powders in a Microwave Plasma. Frequency of the microwaves used is 0.915 GHz. Example No. Starting Material

Gas Pressure mbar Temperature °C after plasma zone Plasma Gas in vol% Gas Flow m3/h STP Water Addition g/min

1

2

3

4

5

A1CI3

AICI3 ZrCl4

TIC14

ZrCI4

ZrCi4

100

77

80

93

85

630

600

630

600

670

Ar 20% 02

Ar 20% 02

Ar 20% 02

air

Ar 20% 02

8

6

7

7

8

5

0

0

0

0,2

was not possible to correlate exactly the diffraction pattern observed with any of the known phases of alumina or zirconia.

Synthesis of Titania Titania was synthesized with the addition of small amounts of water (example 5). The product was crystallized in the ruffle structure, but contained one strong diffraction line corresponding to the anatas crystal structure (Figure 5(a)). As shown in Figure 5(b), the particle size in this case was between 10 and 30 nm. This is about the same particle size range as in the case of pure zirconia synthesized with water addition. As in the case of zirconia, the particles are single crystals. DISCUSSION OF THE

RESULTS

The experimental results point out three important features: - - Zirconia and titania are in their high-temperature phase in accordance with Ostwald's rule. In the case of titania, the transformation to the low-temperature phases anatas or brookite is never observed. This is different in the case of zirconia, where transformation to the tetragonal and

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monoclinic low-temperature phases is normally observed. In this example, the cubic structure is stabilized by the compressive stresses resulting from surface tension introduced by the extremely small particle size. This was already observed in the case of zirconia based ceramic powders synthesized by microwave plasma pyrolysis ( 18,19). A similar phenomenon was reported by Hahn et al. (20) in the case of nanocrystalline yttria. - Independent of the material synthesized, it was observed that the addition of water leads to significantly larger particle sizes, in the range of about 20 nm, compared to about 5 nm obtained without water addition. As in the case of the reaction without water addition, the powder particles were single crystals. This is certainly not an effect of the slightly higher reaction temperature, as in the case of air as plasma gas particles of that size were not observed. Obviously, water addition can be used as a means of adjusting particle sizes. - The plasma reaction and the stability of the process for the synthesis of nanocrystaUine oxide powders do not depend on the use of a noble gas. The process works equally well with air as the plasma gas. This is of importance to industrial application.

R E F E R E N C E S

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

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