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.
2.3 Intelligent Control of Phase
ceramics, contains a signifi cant number of species and indepen- dent chemical reactions. Determining the equilibrium concen- trations for each of the species requires the use of automated computer-based solution algorithms. Stability diagrams con- cisely represent the thermodynamic state of multicomponent,
multiphase aqueous systems in wide ranges of temperature and reagent (precursor) concentrations (Figure 2.3). Yield diagrams specify the synthesis conditions suitable for quantitative precipi- tation of the phase of interest (Figures 2.3b and 2.4a). Calculations of stability and yield diagrams are based on a thermodynamic
−2
−1.6
−1.2
−0.8
−0.4 0
0
0
−8
−6.4
−4.8
−3.2
−1.6 0
−8
−6.4
−4.8
−3.2
−1.6 0
2 4 6 8
pH
10 12 14
0 2 4 6 8 10 12 14
(a) 2
(b)
−2
−1.6
−1.2
−0.8
−0.4
0 0 2
Nb2O5 Nb2O5 + Pb3Nb4O13
T= 200⬚C (473 K)
T= 160⬚C (433 K) Yield = 0.99
BaCO3{s}
BaTiO3{s}
Aqueous species Pb3Nb4O13
stable
KNbO3+ Mg(OH)2 + Pb3Nb4O13
KNbO3 stable Mg(OH)2 stable
Pb3Nb4O13 stable
Mg(OH)2 stable
Mg(OH)2
log10 [m(Pb(acet)2)=1.5m(NbCl5)=3m(Mg(NO3)2)]
log10 [m(TiO2)=m(Ba(OH2)=10m(CO2)]
All aqueous
species
4 6 8 10 12
4 6 8
m (KOH)
10 12 14
FIGURE 2.3 (a) Calculated stability diagram for the Pb–Mg–Nb–K–H2O system at 200°C where input precursor concentration is plotted as a function of mineralizer (KOH) concentration. Th e symbols denote experimentally obtained phase assemblages corresponding to the reaction conditions specifi ed by the equilibrium diagram: Mg(OH)2 + Pb3Nb4O13, KNbO3 + Mg(OH)2. (b) Calculated stability/yield diagram for the Ba–Ti–CO2–H2O system at 160°C using Ba(OH)2 and TiO2 (rutile) when the amount of CO2 in precursors is 0.1 times the amount of TiO2 and the Ba/Ti ratio is equal to 1. Th e solid lines denote the incipient precipitation boundaries for BaTiO3 and BaCO3. Th e shaded area corresponds to the BaTiO3 yield >99%.
model that combines the Helgeson–Kirkham–Flowers–Tanger (HKFT) equation of state for standard-state properties of aque- ous species with a nonideal solution model based on the activity coeffi cient expressions developed by Bromley and Pitzer, and modifi ed by Zemaitis et al. For solid species, standard-state properties are used in conjunction with basic thermodynamic relationships. Fugacities of components in the gas phase are cal- culated from the Redlich–Kwong–Soave equation of state. A more detailed description of the thermodynamic model as well as citations that cover more of the fundamentals can be found elsewhere [1,13].
2.3.2 Methodology for Generating Stability and Yield Diagrams
First, the desired product and components of the hydrothermal system have to be defi ned. Th us, the identities of the precursors, mineralizers, and other additives needed for the synthesis of a
required solid phase need to be specifi ed. Th is information is used as input data, along with the range of reagent concentra- tions, temperature, and pressure specifi ed by the user. Th erefore, it is important that there is a data bank that is relevant for all the components in the system, which contains standard-state ther- mochemical properties and independent reaction sets for all species, HKFT equation of state parameters for aqueous species, and Redlich–Kwong–Soave equation of state parameters for gas- eous species. Th e OLI soft ware also stores binary parameters for ion–ion, ion–neutral, and neutral–neutral species interactions.
It should be noted, however, that the standard-state properties and parameters are frequently obtained by regressing numerous kinds of thermodynamic data. Th ese data include vapor pres- sures, osmotic coeffi cients, activity coeffi cients, enthalpies, and heat capacities of solutions, and solubilities and heat capacities of solids. When data are not available in the OLI data bank for the specifi ed hydrothermal system, a private data bank must be constructed. Th e literature can be consulted for data, as well as 0
-0.8
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−2.4
−3.2
0 2 4
(a)
6 8
[KOH] (mol/k g H2O)
log (Ti)
10 12 14
No PZT PbO + PZT
0% < Yield < 99%
PZT 70/30, yield > 99%
• •
PbO
All aqueous species
0 1 2 3 4 5 6 7 8 9 10 11 12
TiO2 precursor
6m KOH, 0.33m PZT, 150⬚C, 24h ZTO precursor
1m TMAH, 0.11m PZT, 150⬚C, 24h
0 200 400 600 800 1000 1200 1400 1600 1800 0
1 2 3 4 5 6 7 8 9 10 11 12
Stirring speed (rpm)
Average particle size (μm)
(d) 5 µm (c) 5 µm
(b)
FIGURE 2.4 Example of intelligent control of chemical composition, size, and morphology of PZT crystallites using thermodynamic and non- thermodynamic variables. (a) Calculated stability/yield diagram in Pb–Ti–Zr–K–H2O system at 150°C showing stability fi eld of the PZT phase with over 99% yield. (b) Control of crystallite size between 250 nm and 10 μm using simple agitation speed during hydrothermal synthesis and type of the precursor used. (c) and (d) Control of morphology during hydrothermal synthesis at 150°C using concentration of the TMAH mineralizer, which was (c) 0.5 m and (d) 1.0 m.
several methods for estimating thermochemical data. Th e chem- istry model generation step creates the species and reactions that are possible with the given components of the system. All possi- ble combinations of ions, neutral complexes, and solids are con- sidered in this step. Th e chemical speciation model is a set of equations, which contains chemical equilibrium equations, phase equilibrium equations, mass balance, and electroneutrality equations. Once the thermodynamic conditions are specifi ed, the chemical speciation model is solved. Equilibrium concentra- tions of all species are calculated as a function of variables such as mineralizer concentration. Th is gives the equilibrium compo- sition of a specifi c set of reaction conditions, but to understand the overall behavior of the system computations must be performed over a wide range of conditions. For this purpose, we specify the processing variables of interest. Th e OLI soft ware off ers a fl exible choice of independent variables as x- and y-axes of stability and yield diagrams, which include precursor and mineralizer concentrations (Figures 2.3a and 2.4a), solution pH (Figure 2.3b) and temperature, in addition to the electrochemical potential, the latter being used for the simulation of corrosion.
In the case of yield diagrams, the yield value (e.g., 99%, 99.9%, and 99.95%) of the desired material must be chosen. Th e upper temperature limit of the OLI soft ware for creation of both sta bility and yield diagrams is around 300°C, which covers mild hydrothermal synthesis conditions for most ceramic materials.
2.3.3 Utilization of Thermodynamic Modeling
Calculated phase diagrams can perform many functions during the course of intelligent synthesis of ceramics. In addition to precisely defi ning the concentration–temperature–pressure pro- cessing variable space over which the phases of interest are stable, many diff erent types of precursor systems and additives can be compared, and experiments can be designed to make materials that have never been previously prepared in hydrothermal solu- tion [1,13]. Moreover, the thermodynamic modeling enables to compute chemical supersaturation in a given system. Th us, one can evaluate how the processing variables on the phase diagram infl uence supersaturation within a phase stability region. Once the supersaturation is known, by using conventional crystal growth models, one can identify when the process is dominated by nucleation and when by the growth rate, which can be utilized in morphology and size control, as demonstrated in the following section.