2.2.1 Process Defi nition
Hydrothermal synthesis is a process that utilizes single or hetero- geneous phase reactions in aqueous media at elevated temperature (T > 25°C) and pressure (P > 100 kPa) to crystallize ceramic mate- rials directly from solution [1,6]. Reactants used in hydrother- mal synthesis are usually called precursors, which are administered in forms of solutions, gels, or suspensions. However, hydrothermal growth of single crystals requires in most cases use of solid nutrient, which recrystallizes during the growth process. Mineralizers are organic or inorganic additives that are used to control pH or enhance solubility. Other additives, also organic or inorganic, are used to serve other functions, such as controlling crystal morphology, chemical composition, particle dispersion, etc. Syntheses are usually conducted at autogeneous pressure, which corresponds to the saturated vapor pressure of the solution at the specifi ed temperature and composition of the hydrothermal solution. Higher pressures up to over 500 MPa and temperatures over 1000°C [5] may be necessary to facilitate reactant dissolution and growth of certain types of ceramic materials, and are usually applied in single crystal growth.
Nevertheless, mild conditions are preferred for commercial pro- cesses where temperatures are less than 350°C and pressures less than approximately 50 MPa. Intensive research has led to a better
2
Intelligent Synthesis of Smart Ceramic Materials
2.1 Introduction ...2-1 2.2 Hydrothermal Synthesisof Smart Ceramic Materials—An Overview ...2-1
Process Defi nition • Merits of Hydrothermal Synthesis of Ceramics • Chemical Compositions and Morphologies of Smart Ceramics • Hydrothermal Hybrid Techniques • Industrial Production of Smart Ceramic Materials
2.3 Intelligent Control of Phase Assemblage ...2-4 Construction of a Th ermodynamic Model • Methodology for Generating Stability
and Yield Diagrams • Utilization of Th ermodynamic Modeling
2.4 Intelligent Control of Crystal Size and Morphology ...2-7 Th ermodynamic Variables • Nonthermodynamic Variables
2.5 Summary ...2-8 Acknowledgments ...2-8 References ...2-8 Wojciech L. Suchanek
Sawyer Technical Materials
Richard E. Riman
Rutgers University
understanding of hydrothermal chemistry, which has signifi - cantly reduced the reaction time, temperature, and pressure for hydrothermal crystallization of ceramic materials, predomi- nantly powders and coatings (T < 200°C, P < 1.5 MPa) [2,4,5,7].
Th is breakthrough has made hydrothermal synthesis more economical since processes can be engineered using cost-eff ective and proven pressure reactor technology and methodologies already established by the chemical process industry.
2.2.2 Merits of Hydrothermal Synthesis of Ceramics
Hydrothermal synthesis off ers many advantages over conven- tional and nonconventional ceramic synthetic methods. All forms of ceramics can be prepared with hydrothermal synthesis,
such as powders, fi bers, and single crystals, monolithic ceramic bodies, and coatings on metals, polymers, and ceramics (Figure 2.1).
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 crystal- lites and signifi cantly reduced aggregation levels, that is not pos- sible with many other synthesis processes [8]. Figure 2.2 shows several examples of the varieties of morphologies and particle sizes possible with hydrothermal processing. Th e elimination or reduction of aggregates combined with narrow particle size distributions in the starting powders leads to optimized and
(a) 5 cm 10 nm
3 μm
1 μm 6 μm
5 μm
(b)
(c) (d)
(e) (f)
FIGURE 2.1 Examples of various forms of ceramic materials synthesized hydrothermally: (a) α-quartz single crystal, (b) carbon nanotube, (c) PZT powder, (d) carbon fi bers, (e) epitaxial KNbO3 fi lm on SrTiO3 wafer, and (f) epitaxial KNbO3 fi lm on LiTaO3 wafer.
reproducible properties of ceramics because of better micro- structure control. From the standpoint of thin fi lms (coatings), other methods such as physical vapor deposition, chemical vapor deposition, and sol–gel suff er from the disadvantage that they all require high-temperature processing to crystallize the ceramic phase. Th ermally induced defects result, such as cracking, peel- ing, undesired reactions between the substrate and coating, and decomposition of the substrate material. In contrast, hydrother- mal synthesis can be used to directly crystallize fi lms on to sub- strate surfaces at low temperatures, thereby enabling new combinations of materials such as ceramic coatings on polymer substrates.
Hydrothermal processing can take place in a wide variety of combinations of aqueous and solvent mixture-based systems.
Relative to solid-state processes, liquids accelerate diff usion,
adsorption, reaction rate, and crystallization, especially under hydrothermal conditions [3,7]. However, unlike many advanced methods that can prepare a large variety of forms and chemical compounds, such as chemical vapor-based methods, the respec- tive 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, ability to recycle waste, and safe and convenient disposal of waste that cannot be recycled [3]. Th e low reaction temperatures also avoid other problems encountered with high-temperature processes, particularly during single crystal growth, for example, poor stoichiometry control due to volatilization of components or crystal cracking due to phase transformations taking place during cooling.
300 nm
10 μm (f)
(a)
200 nm (b)
10 μm (e)
5 μm
(c) (d)
2 μm
FIGURE 2.2 Examples of various sizes and morphologies of ceramic powders synthesized hydrothermally: (a) equiaxed nanosized ZnO, (b) nanosized hydroxyapatite needles, (c) LiMnO3 platelets, (d) carbon nanotubes, 50–100 nm in diameter, several microns in length, (e) hydroxyapa tite whiskers, and (f) equiaxed α-Al2O3.
An important advantage of the hydrothermal synthesis is that the purity of hydrothermally synthesized materials signifi cantly exceeds the purity of the starting materials. Th is is because hydrothermal crystallization is a purifi cation process in itself, in which the growing crystals or crystallites reject impurities pres- ent in the growth environment. Materials synthesized under hydrothermal conditions oft en exhibit diff erences in point defects when compared to materials prepared by high-tempera- ture synthesis methods. For instance, in barium titanate, hydroxyapatite, or α-quartz, water-related lattice defects are among the most common impurities and their concentration determines essential properties of these materials. Th e problem of water incorporation can be overcome by properly adjusting the synthesis conditions, use of water-blocking additives, or even nonaqueous solvents (solvothermal processing). Another impor- tant technological advantage of the hydrothermal technique is its capability for continuous materials production, which can be par- ticularly useful in continuous fabrication of ceramic powders [9].
2.2.3 Chemical Compositions and Morphologies of Smart Ceramics
A great variety of ceramic materials 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, fer- rites, 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. Th e hydrothermal technique is also well suited for nonoxides, such as pure elements (for example Si, Ge, Te, Ni, diamond, carbon nanotubes), selenides (CdSe, HgSe, CoSe2, NiSe2, CsCuSe4), tellurides (CdTe, Bi2Te3, CuxTey, AgxTey), sulfi des (CuS, ZnS, CdS, PbS, PbSnS3), fl uorides, nitrides (cubic BN, hexagonal BN), aresenides (InAs, GaAs), etc. [2,4,10–12].
Crystalline products with a specifi c chemical or phase com- position can be usually synthesized hydrothermally in several diff erent forms, such as single crystals, coatings, ceramic mono- liths, or powders. Among them, the powders exhibit the largest variety of morphologies, such as equiaxed (for example cubes, spherical), elongated (fi bers, whiskers, nanorods, nanotubes), platelets, nanoribbons, nanobelts, etc., with sizes ranging from a few nanometers to tens of microns (Figure 2.2). Core–shell par- ticles and composite powders consisting of a mix of at least two diff erent powders can be also prepared in one synthesis step.
Some of the powders can even adopt nonequilibrium morpholo- gies (Figure 2.4c and d).
2.2.4 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), electro- chemistry (hydro thermal–electrochemical synthesis), ultrasound (hydrothermal–sonochemical synthesis), mechanochemistry (mechano chemical–hydrothermal synthesis), optical radiation (hydrothermal–photochemical synthesis), and hot-pressing (hydrothermal hot pressing), as reviewed elsewhere [1,3,5–7].
2.2.5 Industrial Production of Smart Ceramic Materials
Several hydrothermal technologies, primarily for the produc- tion of single crystals, such as α-quartz for frequency control and optical applications (Sawyer Technical Materials, Tokyo Denpa, NDK), ZnO for UV- and blue light-emitting devices (Tokyo Denpa), and KTiOPO4 for nonlinear optical applications (Northrop Grumman-Synoptics), have already been developed that demonstrate the commercial potential of the hydrothermal method. Th e volume of the hydrothermal production of α-quartz single crystals is estimated at 3000 tons/year [2]. However, the largest potential growth area for commercialization is ceramic powder production. Th e widely used Bayer process uses hydro- thermal methods to dissolve bauxite and subsequently precipi- tate aluminum hydroxide, which is later heat-treated at high temperature to crystallize as α-alumina. In 1989, the worldwide production rate was about 43 million tons/year. Th e production of perovskite-based dielectrics and zirconia-based structural ceramics is a promising growth area for hydrothermal methods [9]. Corporations such as Cabot Corporation, Sakai Chemical Company, Murata Industries, Ferro Corporation, Sawyer Technical Materials, and others have established commercial hydrothermal production processes for preparing ceramic powders.