Hydrothermal Synthesis of Transition Metal Oxides


Advantages of Hydrothermal Synthesis



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1.5 Advantages of Hydrothermal Synthesis

There are several advantages of hydrothermal method over conventional solid state. For example, compounds that have elements with an unusual oxidation state can be synthesized such as the formation of ferromagnetic chromium (IV) oxides [1].


Cr2O3 + CrO3 350oC, 440 bar 3CrO2 H2O
The hydrothermal method is also useful for synthesis of low temperature phases and metastable compounds by simply using quartz ampoules.

Although hydrothermal synthesis is considered as a high temperature technique, in reality these temperatures are low compared to most traditional melt techniques. There are many important advantages of low temperature crystal growth. For example, it allows the growth of low temperature polymorphs that are often difficult or impossible to prepare by other synthetic methods. The most well known example is -quartz. Alfa quartz is the most intensively studied material grown hydrothermally in industry for its piezoelectric properties and application in electronic devices. Piezoelectric -quartz is only stable below 580oC and must be grown below this temperature. This is a problem for conventional melt or flux crystal growth method.

Another important advantage is that reactions do not require much time compared to conventional methods. For instance, although a solid-state reaction can be performed in a few weeks hydrothermal reaction can be done in a few days.

Hydrothermal synthesis also has the advantage of rapid growth rates because of the rapid diffusion processes. The hydrothermal method is a growth technique from liquid solution, but the viscosity of the liquid is lower. Solubility of a liquid should be high for conventional methods or there will be a rate problem. Under hydrothermal conditions, diffusion is not a problem because of a low viscosity in comparison to viscosity at ambient temperature. The down side of this method is the high-pressure requirement and also difficulty on gathering data. Because of fast diffusion under hydrothermal conditions, super saturation occurs and results in dendritic growth that increases the chance of impurities and can reduce crystal quality. The diffusion is fast even for materials that have low solubility at ambient temperature. For example, quartz cannot be grown at low temperature, however under hydrothermal conditions; it can be grown at relatively high rates [25].

Hydrothermal synthesis is a technique that involves the growth of materials from aqueous solutions at elevated temperature and pressures. Temperature, pressure, and mineralizers are used in this process to increase precursor solubility and to change solution conditions to favor formation of the desired phase. Mineralizers are complexing agents that act to increase the solubility of the starting precursors by forming soluble complexes. This route has several advantages including simplicity, low processing temperatures, low cost, high product purity, and the ability to control the particle size [28].

1.6 Industrial Applications of Hydrothermal Method

After World War II, large single crystals of quartz were formed leading to the first major commercial application for hydrothermal synthesis. At approximately the same time, synthetic zeolitic materials such as zeolite A and X were grown hydrothermally [28].

The successful commercialization of quartz synthesis encouraged several groups to examine the growth of other compounds under hydrothermal conditions, and several other types of crystals were subsequently prepared commercially using hydrothermal techniques, including AlPO4, KTiOPO4 and emeralds [18].

In the past, investigations in this field were conducted by geologist studying the formation of rocks and minerals formed in the earth’s crust at high temperature and pressures [14]. Recently, researchers have utilized this process towards the synthesis of commercially important materials such as quartz [29, 30], zeolites [31].

A variety of materials have been synthesized by the hydrothermal method such as KTiOPO4 [32], tungstates [33], Tl- superconductors [34], layered compounds [35], artificial gems [36, 37], intercalation compounds [38], and zeolites [31].

The use of hydrothermal synthesis is proven to be a useful and relatively flexible tool for the production of a wide range of advanced materials. Some of these have already been commercialized; others are still in development or in the scale-up stage [39, 40]. Researchers engaged in synthesizing materials have a great interest for hydrothermal synthesis, and they are carrying out their studies efficiently, as illustrated in Table 1.1 [40]


Table1.1 Summary of high performance material applications of hydrothermal

synthesis.




Fuction

Material

Application

Electrical Insulator

Al2O3

IC circuit substrate

Ferroelectirc

BaTiO3, SrTiO3

Ceramic capacitor

Piezoelectric

Pb(Zr, Ti)O3,

-SiO2


Sensors, transducers, actuators

Semiconductor

BaTiO3, ZnO-Bi2O3, transition metal oxides

Thermistors and varistors

Chemical

ZnO, Fe2O3, ZrO2, TiO2, zeolites

Chemical sensor, catalyst, catalyst substrate, desiccant, gas adsorption/storage

Structural

ZrO2(TZP), cordierite, Al2TiO5, mullite, xonotlite

Automotive, heat exchangers, metal filters, light modulator

Biological

Hydroxyapatite

Artifical bone

Colorant

Fe2O3, Cr2O3, TiO2-(Ni,Sb), ZnFe2O4, aluminates, chromites, cobaltites

Ceramic pigments, paints, plastic colorants

Eletronic conductor

Precious metals and alloys, indium tin oxide

Electrode layers, transparent conductive films


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