Hydrothermal Synthesis Study

Advances in Science and Technology Vol. 45 (2006) pp. 184-193 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland

Hydrothermal Synthesis of Advanced Ceramic Powders

Wojciech L. Suchanek1, a and Richard E. Riman2, b 1Sawyer Technical Materials, LLC, 35400 Lakeland Boulevard, Eastlake, OH 44095, USA 2Rutgers University, Department of Materials Science and Engineering, 607 Taylor Road, Piscataway, NJ 08855, USA [email protected], [email protected]

Keywords: Hydrothermal synthesis, review powder synthesis, morphology control, hydroxyapatite, PZT, alumina, zinc oxide, carbon nanotubes.

Abstract. This paper briefly reviews hydrothermal synthesis of ceramic powders and shows how understanding the underlying physico-chemical processes occurring in the aqueous solution can be used for engineering hydrothermal crystallization processes. Our overview covers the current status of hydrothermal technology for inorganic powders with respect to types of materials prepared, ability to control the process, and use in commercial manufacturing. General discussion is supported with specific examples derived from our own research (hydroxyapatite, PZT, Į-Al2O3, ZnO, carbon nanotubes). Hydrothermal crystallization processes afford excellent control of morphology (e.g., spherical, cubic, fibrous, and plate-like) size (from a couple of nanometers to tens of microns), and degree of agglomeration. These characteristics can be controlled in wide ranges using thermodynamic variables, such as reaction temperature, types and concentrations of the reactants, in addition to non-thermodynamic (kinetic) variables, such as stirring speed. Moreover, the chemical composition of the powders can be easily controlled from the perspective of stoichiometry and formation of solid solutions. Finally, hydrothermal technology affords the ability to achieve cost effective scale-up and commercial production.

Introduction Hydrothermal research was initiated in the middle of the 19th century by geologists and was aimed at laboratory simulations of natural hydrothermal phenomena. In the 20th century, hydrothermal synthesis was clearly identified as an important technology for materials synthesis, predominantly in the fields of hydrometallurgy and single crystal growth [1]. However, the severe (supercritical) reaction conditions required particularly for growing single crystals have discouraged extensive research and commercialization for many materials. In recent years, commercial interest in hydrothermal synthesis has been revived in part because a steadily increasing large family of materials, primarily ceramic powders, has emerged that can be prepared under mild conditions (T<350ºC, P<100 MPa). The growing number of scientific papers on hydrothermal synthesis of ceramic powders, which almost quadrupled between 2000 and 2004 (Fig. 1a), illustrates the rising interest in this area, with China, Japan, and USA publishing most extensively (Fig. 1b).

Hydrothermal Synthesis as a Materials Synthesis Technology Process Definition. Hydrothermal synthesis is a process that utilizes single or heterogeneous phase reactions in aqueous media at elevated temperature (T>25ºC) and pressure (P>100 kPa) to crystallize ceramic materials directly from solution. However, researchers also use this term to describe processes conducted at ambient conditions. Syntheses are usually conducted at autogeneous pressure, which corresponds to the saturated vapor pressure of the solution at the specified temperature and composition of the hydrothermal solution. Upper limits of hydrothermal synthesis extend to over 1000ºC and 500 MPa pressure [2]. However, mild conditions are preferred for commercial processes where temperatures are less than 350ºC and pressures less than

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 12.45.174.100-24/10/06,17:51:02) 185 11th International Ceramics Congress

600 All papers on hydrothermal technology Italy 2% (b) Papers on hydrothermal synthesis of powders only 500 Taiwan 2% Others Germany 2% 11% 400 (a) India 3% France 3% China 300 UK 3% 44% USSR/Russia 200 4% USA 100 11% Japan

Number of Scientific Papers 15% 0 1990 1992 1994 1996 1998 2000 2002 2004 2006 Publication Year Fig. 1. (a) Number of papers on hydrothermal processing of ceramic powders published between 1989-2005 vs. all materials science-related papers on hydrothermal technology [4]; (b) geographical distribution of the research on hydrothermal processing of ceramic powders (1989-2005). Based on information available in Science Citation Index, Chemical Abstracts, and INSPEC databases.

approximately 50 MPa. The transition from mild to severe conditions is determined mostly by corrosion and strength limits of the materials of construction that comprise the hydrothermal reaction vessels. Intensive research has led to a better understanding of hydrothermal chemistry, which has significantly reduced the reaction time, temperature, and pressure for hydrothermal crystallization of materials (T<200ºC, P<1.5 MPa) [1-5]. This breakthrough has made hydrothermal synthesis more economical since processes can be engineered using cost-effective and proven pressure reactor technology and methodologies already established by the chemical process industry. Merits of Hydrothermal Synthesis. Hydrothermal synthesis offers many advantages over conventional and non-conventional ceramic synthetic methods. All forms of ceramics can be prepared with hydrothermal synthesis, namely powders, fibers, and single crystals, monolithic ceramic bodies, and coatings on metals, polymers, and ceramics. From the standpoint of ceramic powder production, there are far fewer time- and energy-consuming processing steps since high- temperature calcination, mixing, and milling steps are either not necessary or minimized. Moreover, the ability to precipitate already crystallized powders directly from solution regulates the rate and uniformity of nucleation, growth and aging, which results in improved control of size and morphology of crystallites and significantly reduced aggregation levels, that is not possible with many other synthesis processes [6]. The elimination/reduction of aggregates combined with narrow particle size distributions in the starting powders leads to optimized and reproducible properties of ceramics because of better microstructure control. Precise control of powder morphology can also be significant. For instance, powders with crystallites having well-developed shapes corresponding to particular crystallographic directions, such as whiskers, plates, or cubes, can be oriented to form materials with single crystal-like properties, such as polymer-ceramic composites or textured ceramic-ceramic composites with anisotropic properties. Another important advantage of the hydrothermal synthesis is that the purity of hydrothermally synthesized powders significantly exceeds the purity of the starting materials. It is because the hydrothermal crystallization is a self-purifying process, during which the growing crystals/crystallites tend to reject impurities present in the growth environment. The impurities are subsequently removed from the system together with the crystallizing solution, which does not take place during other synthesis routes, such as high- temperature calcination. Hydrothermal processing can take place in a wide variety of combinations of aqueous and solvent mixture-based systems. In general, processing with liquids allows for automation of a wide range of unit operations such as charging, transportation, mixing and product separation. Moreover, relative to solid state processes, liquids give a possibility for acceleration of diffusion, adsorption, reaction Advances in Science and Technology Vol. 45 186 rate and crystallization, especially under hydrothermal conditions [4]. However, unlike many advanced methods that can prepare a large variety of forms and chemical compounds, such as chemical vapor-based methods, the respective costs for instrumentation, energy and precursors are far less for hydrothermal methods. Hydrothermal methods are more environmentally benign than many other synthesis methods, which can be attributed in part to energy conserving low processing temperatures, absence of milling, ability to recycle waste, and safe and convenient disposal of waste that cannot be recycled [4]. The low reaction temperatures also avoid other problems encountered with high temperature processes, for example poor stoichiometry control due to volatilization of components (e.g., Pb volatilization in Pb-based ceramics). Materials synthesized under hydrothermal conditions often exhibit differences in point defects when compared to materials prepared by high temperature synthesis methods. For instance, in barium titanate, hydroxyapatite, or Į-quartz, water-related lattice defects are among the most common impurities and their concentration determines essential properties of these materials. The problem of water incorporation can be overcome by either properly adjusting the synthesis conditions or by use of non-aqueous solvents (solvothermal processing). Another important technological advantage of the hydrothermal technique is its capability for continuous materials production, which can be particularly useful in continuous fabrication of ceramic powders [7]. Moreover, hydrothermal crystallization can be monitored in-situ using a range of techniques [8-9], which allow determination of the crystal growth mechanism and better control of the hydrothermal synthesis process. Ceramic Powder Compositions and Morphologies. A variety of ceramic powders have been synthesized by hydrothermal methods. Most common are oxide materials, both simple oxides, such as ZrO2, TiO2, SiO2, ZnO, Fe2O3, Al2O3, CeO2, SnO2, Sb2O5, Co3O4, HfO2, etc., and complex oxides, such as BaTiO3, SrTiO3, PZT, PbTiO3, KNbO3, KTaO3, LiNbO3, ferrites, apatites, tungstates, vanadates, molybdates, zeolites, etc., some of which are metastable compounds, which cannot be obtained using classical synthesis methods at high temperatures. Hydrothermal synthesis of a variety of oxide solid solutions and doped compositions is common. The hydrothermal technique is also well suited for non-oxides, such as pure elements (for example Si, Ge, Te, Ni, diamond, carbon nanotubes), selenides (CdSe, HgSe, CoSe2, NiSe2, CsCuSe4), tellurides (CdTe, Bi2Te3, CuxTey, AgxTey), sulfides (CuS, ZnS, CdS, PbS, PbSnS3), fluorides, nitrides (cubic BN, hexagonal BN), aresenides (InAs, GaAs), etc. Crystalline products can be in a variety of forms, such as equiaxed (for example cubes, spherical), elongated (fibers, whiskers, nanorods, nanotubes), plates, nanoribbons, nanobelts, etc. Core-shell particles (CdS/ZnO, CoAl2O4/ZnAl2O4) and composite powders consisting of a mix of at least two different powders (TiO2/HAp, TiO2/SiO2) can be also prepared. Quite often, the size and morphology can be engineered over a wide range and even adopt nonequilibrium morphologies.

Design of Hydrothermal Experiments A majority of the hydrothermal synthesis work that has been done in the past has used Edisonian trial and error methods for process development. This type of experimental approach suffers from its time-consuming nature and the inability to clearly discern between processes that are controlled by either thermodynamics or kinetics. Thermodynamic modeling can be used to design a process to be thermodynamically favored using fundamental principles instead of the Edisonian methods [10-11]. For a given precursor system, the effects of concentration, temperature and pressure can be modelled to define the processing variable space over which the phase(s) of interest are stable. Moreover, many different types of precursor systems can be compared and experiments can be designed to make materials that have never been previously prepared in hydrothermal solution. Such a design approach requires far fewer experiments than Edisonian methods. It can be accomplished using commercially available computer software, such as OLI software, which also contains a database of 187 11th International Ceramics Congress

thermodynamic properties for many common systems. The methodology for these calculations have been reviewed elsewhere [10-11]. With the thermodynamic variables processing space well defined for the material of interest, a range of conditions can be explored for control of reaction and crystallization kinetics for the purpose of developing a process suitable to produce the desired form of the ceramic powders (e.g, particular size, morphology, aggregation level). Thermodynamic processing variables, such as temperature, pH, concentrations of reactants and additives, determine not only the processing space for a given material but also influence both reaction and crystallization kinetics. The phenomena that underlie the size and morphology control using the thermodynamic variables are the overall nucleation and growth rates, which control crystal size, and the competitive growth rates along principal crystallographic directions that control morphology. Size of the crystals can be thus controlled by varying temperature and concentration. Crystal morphology and size can be additionally affected by surfactants, which can adsorb on specific crystallographic faces and solvents, which adsorb similarly as well as regulate solubility. Unfortunately, changing the thermodynamic synthesis variables is constrained by the phase boundaries in a specific phase diagram. Therefore, it may or may not be possible to exploit all sizes and morphologies for a certain material by modifying only the thermodynamic variables. However, non-thermodynamic variables are also extremely important when operating in thermodynamically limited processing variable space. For instance, changing the stirring speed during powder synthesis can change the particle size by orders of magnitude [12].

(a) (c)

5 ȝm

(b) (d)

5 ȝm

Fig. 2. (a) and (b) SEM photographs of submicron ZnO powders synthesized hydrothermally without stirring (magnification is the same in both cases). (c) and (d) SEM photographs of PZT crystals prepared hydrothermally at 150ºC for 24 h at moderate stirring. Total concentration of (Zr+Ti) was 0.1 m and the Pb:Zr:Ti ratio was 1.1:0.52:0.48. Concentration of the TMAH mineralizer was (c) 0.5 m, (d) 1.0 m. Advances in Science and Technology Vol. 45 188

Exploiting Synthesis Variables: Representative Systems Representative examples of hydrothermally synthesized oxide powders, such as hydroxyapatite, PZT, and ZnO, all derived from our own research, are discussed in this section. These examples demonstrate the ability of the hydrothermal technology to control chemical composition, aggregation level, size and morphology in wide range, resulting in powder characteristics that are difficult to achieve by classical powder synthesis methods. Thermodynamic Variables (ZnO, PZT, HAp). The technique of using thermodynamic variables, such as temperature, concentrations of reactant, various additives and/or solvents, is the most widely used method to control powder size and morphology under hydrothermal conditions. In our first example, ZnO powders were synthesized from aqueous solutions containing dissolved Zn salts in the presence of basic mineralizers at temperatures ranging between 150-300ºC under saturated vapor pressures without stirring. Concentrations of the zinc salts varied between 0.3 m and 6.0 m, concentration of the mineralizers were set between 0 and 12.0 m. The synthesized powders were in most cases pure-phase ZnO (zincite) with equiaxed morphology and low aggregation level. Size of the crystallites ranged between tens of nanometers and tens of microns. Fig. 2a,b shows two examples of powders with different crystallite sizes. Within the range of ZnO phase stability, with increasing temperature, and zinc concentration, size of the crystals generally increased. Changing reactants concentration can be used not only to control size of the crystallites but also to control their shape. Fibers or cubes of lead zirconate titanate (PZT, chemical formula Pb(ZrxTi1- x)O3, where x=0-1) can form during the hydrothermal synthesis at 150ºC from precursors derived from Zr and Ti alkoxides in the presence of TMAH mineralizer (Fig. 2c,d). TMAH concentration of 0.5 m results in the formation of PZT fibers while 1 m-TMAH yields PZT cubes [13]. The experiments with hydroxyapatite powders [HAp, chemical formula Ca10(PO4)6(OH)2] were (a) (c)

200 nm 10 ȝm

(b) Fig. 3. HAp nanosized crystals prepared hydrothermally at 200ºC for 24 h under moderate stirring in (a) 50 vol. % 2- propanol (aq); (b) in pure water with 1 wt% KCl; (c) HAp whiskers synthesized hydrothermally at 200ºC for 5 h under moderate stirring.

200 nm 189 11th International Ceramics Congress

focused on size and morphology control by adjusting chemical additives. Very fine, nearly nanosized HAp crystals have been synthesized at 200ºC for 24 h under autogenous pressure of about 2 MPa under moderate stirring, using the method described elsewhere [14]. HAp crystals synthesized in 50 vol.% 2-propanol(aq) had low aspect ratios ranging between 2-3 and diameters between 20- 40 nm (Fig. 3a). Conversely, uniform, nanosized needles (dimensions of about 20 nm by 100- 160 nm, aspect ratio of 5-8) were formed when 1 wt% KCl additive was used (Fig. 3b). HAp crystals prepared under similar conditions but without additives, were 20 nm by 50-100 nm in size, yielding aspect ratios between 3-5. In addition to fine HAp powders, the hydrothermal technique allows synthesis of larger HAp crystals, such as micron-sized HAp whiskers. The HAp whiskers can be prepared at 200ºC for 5h under the pressure of about 2 MPa, under moderate stirring, from aqueous solutions containing various concentrations of Ca(OH)2, H3PO4, and lactic acid [15]. In this experimental set-up, Ca ions are being chelated by the lactic acid, preventing precipitation of amorphous calcium phosphate at ambient temperatures. Precipitation at elevated temperatures yields large HAp crystals. Despite acidic pH in the starting solutions, the concentrations of the reactants were still within the stability field of HAp at 200ºC [14]. The diameters, lengths, and aspect ratios of the synthesized HAp whiskers were in the range of 1-10 ȝm, 30-60 ȝm, and 5-35, respectively (Fig. 3c). With increasing concentration of the lactic acid in the starting solution, whisker diameters increased and aspect ratios decreased while lengths remained almost unchanged. The Ca/P molar ratios of the HAp whiskers were in the range of 1.59-1.62, as compared with 1.67 for stoichiometric HAp. They increased slightly with increasing concentration of the lactic acid and were almost independent of pH and the Ca/P ratio in the starting solution. Non-thermodynamic Variables (PZT) Non-thermodynamic synthesis variables, such as stirring rate and method of precursor were found to be essential in morphology control of the PZT crystals. Hydrothermal synthesis of PZT cube-shaped crystallites was performed at 150ºC for 2-24 h in stirred autoclaves at a stirring speed ranging from 200-1700 rpm [12]. Concentration of the All aqueous species 0 PbO+PZT (a) PbO (b) PZT 70/30, yield > 99% -0.8 • •

-1.6 0% < Yield < 99% Log [Ti]

-2.4 No PZT

-3.2 0 2 4 6 8 10 12 14 5 mm

[KOH] (mol/kg H2O)

m) 14

Fig. 4. (a) Phase equilibria in the Pb-Zr-Ti- m TiO -based precursor K-H2O system at 150ºC as functions of 12 2 Alkoxide-derived precursor precursor and mineralizer concentrations. 10 The symbol, Ɣ, denotes experimental reaction conditions; (b) SEM photograph of 8 PZT particulates, synthesized at 150ºC for 6 (c)

24 h at 700 rpm stirring; (c) Average particle 4 size of PZT particulates as a function of stirring speed during hydrothermal synthesis 2 at 150ºC for 24 h. Error bars denote 0 Average Particle Size of PZT ( Average Particle 0 200 400 600 800 1000 1200 1400 1600 1800 standard deviation values. Stirring Speed (rpm) Advances in Science and Technology Vol. 45 190 chemicals used was well within the calculated stability field of the PZT phase, which is shown in Fig. 4a. With increasing stirring speed, size of the PZT crystals became smaller and they exhibited better-defined cube habit (Fig. 4b). This effect was more strongly pronounced in the case of using the TiO2-based precursor: increasing the stirring speed from 200 to 1700 rpm has reduced the particle size of PZT crystals almost 2 orders of magnitude, from over 10 ȝm to a submicron range (Fig. 4c). Generally speaking, stirring during crystal growth leads to an increase in the probability of spontaneous nucleation, a decrease in supersaturation inhomogenities, and an increase of the growth rate [10-11]. The final size of the crystals is then determined by a balance between the nucleation and growth rates. In this particular case, the nucleation rate seems to dominate over the growth rate, therefore the crystal size decreased with increasing stirring speed.

Hydrothermal Hybrid Techniques In order to additionally enhance the reaction kinetics or the ability to make new materials, a great amount of work has been done to hybridize the hydrothermal technique with microwaves (microwave-hydrothermal processing), electrochemistry (hydrothermal-electrochemical synthesis), ultrasound (hydrothermal-sonochemical synthesis), mechanochemistry (mechanochemical- hydrothermal synthesis), optical radiation (hydrothermal-photochemical synthesis), and hot-pressing (hydrothermal hot pressing), as reviewed elsewhere [1-4]. Below we show an example of a novel hydrothermal hybrid technique, which was developed by the authors [16-17]. Mechanochemical-Hydrothermal Synthesis of Nanosized Hydroxyapatite Powders with Controlled Chemical Composition. The mechanochemical-hydrothermal synthesis is a hybrid of the mechanochemical and hydrothermal synthesis methods and should be clearly distinguished from the mechanochemical synthesis, which involves only solid-state reactions. The mechanochemical- hydrothermal synthesis (sometimes called “wet” mechanochemical) takes advantage of the presence of an aqueous solution in the system, which can actively participate in the mechanochemical reaction by acceleration of dissolution, diffusion, adsorption, reaction rate, and crystallization [4]. The mechanochemical activation of slurries can generate local zones of high temperatures (up to 450- 700ºC) and high pressures due to friction effects and adiabatic heating of gas bubbles (if present in the slurry), while the overall temperature is close to the room temperature. The mechanochemical- hydrothermal route produces comparable amounts of powders as the hydrothermal processing but it requires lower temperature, i.e. room temperature, thus, there is no need for a pressure vessel and external heating, because the reaction is accelerated via comminuting methods. A variety of undoped, carbonate-, sodium-and-carbonate-, and Mg-substituted crystalline hydroxyapatite powders (denoted hereafter HAp, CO3HAp, NaCO3HAp, and Mg-HAp, respectively) were prepared at room temperature via heterogeneous reactions between resepectively Ca(OH)2, CaCO3, Na2CO3, and/or Mg(OH)2 powders and (NH4)2HPO4 aqueous solution using the mechanochemical-hydrothermal route described in detail elsewhere [16-17]. In the case of the HAp without substituting ions, the synthesis product was phase-pure, highly-crystalline HAp, which was thermally stable at 900ºC and had nearly stoichiometric composition. Room temperature products of carbonated HAp synthesis were phase-pure CO3HAp and NaCO3HAp containing 2.5-12 wt% of carbonate ions in the lattice [16]. The purified powders substituted with Mg were phase-pure Mg- HAp containing 0.24-28.4 wt% of Mg [17]. Surprisingly, concentration of Mg in the Mg-HAp powders was higher than in the starting slurries, indicating preferential incorporation of Mg over Ca, which has never been reported before and seems to be related to the unique mechanochemical- hydrothermal synthesis conditions only. All synthesized HAp, CO3HAp, NaCO3HAp, and Mg-HAp powders, independently of their chemical compositions, exhibited high surface area ranging between 82 and 269 m2/g, corresponding to crystalite size of 7 nm to 23 nm. The crystallites were rather heavily agglomerated, with median particle sizes between 100 nm and 1.6 mm. Additional control of crystallization pH and use of surfactants allowed reducing the agglomerate size to about 90 nm [14]. Our results emphasize importance of the aqueous solution, which actively participates in the 191 11th International Ceramics Congress

synthesis reaction by dissolving at least one of the reactants, in our case (NH4)2HPO4, which is not observed with conventional mechanochemical synthesis of HAp [16-17]. The mechanochemical- hydrothermal synthesis allows preparation of HAp powders with a variety of chemical compositions and with well defined stoichiometry levels but is not well suited to control aggregation level, size and morphology of the HAp crystallites.

Hydrothermal Synthesis of Non-Oxide Materials The hydrothermal technique has been mostly known for preparation of oxide materials, which is also the main focus of this article. However, non-oxide materials, including a wide range of metals, semiconductors, intermetallics, selenides, tellurides, sulfides, fluorides, nitrides, arsenides, etc., can also be synthesized from aqueous solutions in a variety of sizes and morphologies. Here we present an example of multiwalled carbon nanotubes, which in addition to other forms of carbon (amorphous carbon, diamond) can be synthesized under hydrothermal conditions from a variety of carbon sources, as demonstrated by our research [18]. Hydrothermal Synthesis of the Carbon Nanotubes. During the hydrothermal treatment of C60 at 700ºC (100 MPa) in the presence of 3 wt% of nickel, high-quality open-ended multi-walled carbon nanotubes were formed in the vicinity of the nickel particles [18]. These nanotubes had typically an outer diameter of 30-40 nm and wall thickness of 5 nm with a graphitization level similar to carbon nanotubes prepared by CVD techniques, as shown in Fig. 5b. In the follow-up works, cheaper carbon sources, such as polyethylene or amorphous carbon were used in hydrothermal synthesis of the carbon nanotubes primarily in the presence of the Ni catalysts. Currently, a variety of sizes of the multiwalled carbon nanotubes with yields approaching 100% and diameters ranging between 50 and 500 nm can be prepared by properly adjusting synthesis conditions of the hydrothermal treatment of very simple and cheap carbon sources. Nanotubes with diameters 50- 100 nm are shown in Fig. 5a. The hydrothermal technology gives promise for development of inexpensive mass-production of carbon nanotubes in the near future.

(a)

5 nm

2 mm

(b) 20 nm

Fig. 5. (a) High-yields of the multiwalled carbon nanotubes with diameters of 50-100 nm synthesized using polyethylene carbon source; (b) TEM micrographs of typical carbon nanotubes produced by hydrothermal treatment of fullerene C60 with 3% Ni, 30% water at 700ºC for 168 h under 100 MPa. Insert shows graphite fringes in the nanotube wall. Advances in Science and Technology Vol. 45 192

Industrial Production Several hydrothermal technologies, primarily for the production of single crystals, such as a-quartz for frequency control and optical applications, ZnO for UV- and blue light-emitting devices, and KTiOPO4 for nonlinear optical applications, have already been developed that demonstrate the commercial potential of the hydrothermal method. However, the largest potential growth area for commercialization is ceramic powder production. The widely used Bayer process uses hydrothermal methods to dissolve Bauxite and subsequently precipitate aluminum hydroxide, which is later heat- treated at high temperature to crystallize as a-alumina. In 1989, the worldwide production rate was about 43 million tons/year. The production of perovskite-based dielectrics and zirconia-based structural ceramics is a promising growth area for hydrothermal methods. Corporations such as Cabot Corporation, Sakai Chemical Company, Murata Industries, Ferro Corporation and others have established commercial hydrothermal production processes for making dielectric ceramic powder compositions for capacitors. Hydrothermal Production of Į-Al2O3 Powders. Sawyer Technical Materials, LLC recently developed hydrothermal production of high-purity Į-Al2O3 powders. Sawyer’s products line of the Į- Al2O3 powders called DiamoCor, comprises both undoped and Mn or Cr-doped compositions with the following grain sizes: 40 ȝm, 20-25 ȝm, 10 ȝm, 6 ȝm, 3 ȝm, 1 ȝm, and 100-250 nm. Phase purity of the powders is 100% of the Į-Al2O3 phase. Chemical purity ranges between 99.9% and >99.97%. The powders are being produced at Sawyer in hundreds of pounds batches, with very high reproducibility and a total production capacity of 100,000 lbs/month. Representative corundum powders with a 10 ȝm grain size are shown in Fig. 6a. The picture reveals typical as-synthesized powder, with uniform particle size distribution and well-defined crystal faces in each grain, which is a single crystal of the Į-Al2O3 phase. The morphology of the

(a) (b)

10 mm 500 nm

3 mm 10 mm Fig. 6. Į-Al2O3 DiamoCor powders produced (c) by Sawyer Technical Materials, LLC: (a) as- 100 160 nm synthesized powders with median particle size 80 of 10 ȝm; (b) as-synthesized powders with 1 mm 60 median particle size of about 100 nm (c) 25 mm typical particlesize distributions of various 40 powders after milling/dispersing, nominal median particle sizes are marked. 20 40 mm

Cummulative Weight Percent Weight Cummulative 0

0.1 1 10 100 Particle size (mm) 193 11th International Ceramics Congress

powders in addition to their phase and chemical composition make DiamoCor Į-Al2O3 a very aggressive and long-lasting abrasive material. The powders have been successfully used in industrial lapping and in thermal-spray applications, due to the combination of chemical and phase purity, morphology and tight particle size distribution, which result in good flowability. The morphology of submicron Į-Al2O3 powder is shown in Fig. 6b. The particle size distributions of selected powders are revealed in Fig. 6c as cumulative curves. The example of Į-Al2O3 DiamoCor indicates a commercial potential of the hydrothermal technique, which allows reproducible synthesis of large volumes of powders with a wide range of crystallite sizes and chemical compositions.

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