Hydrothermal Synthesis of Transition Metal Oxides

1.7 Crystal Growth

The crystal growth is important for producing suitable single crystals for X-ray diffraction measurements. It has been kept general because growing crystals is very much an art, and for any particular substance the method to be chosen and the variations in its use must usually be decided on the basis of exploratory experiments. While initial attempts to grow crystals may be disappointing, it should not be surprising that a number of compounds that at first seemed incapable of producing satisfactory crystals did yield them when the proper conditions were found.

The hydrothermal method of crystal growth has several advantages. It has critical importance for the technological efficiency in developing bigger, purer, and dislocation-free single crystals. The method has been widely accepted since 1960s and practically all inorganic species, starting from native elements to the most complex oxides, silicates, germanates, phosphates, chalcogenides, carbonates, and so on, have been obtained by this method. The technique is being employed on a large scale to prepare piezoelectric, magnetic, optic, ceramic and a host of other materials both as single crystals and polycrystalline materials.

Crystals may be grown from solid, liquid (melt), vapor and solution phases although, usually, only the vapor and liquid phases give crystals of sufficient size to be used in applications or for property measurements. Main categories of crystal growth methods are, growth from the solid: S-S process involving solid-solid phase transition, growth from the melt: L-S process involving liquid-solid phase transition, growth from the vapor: V-S process involving vapor-solid phase transition and growth from solution.

1.7.1 Solid Growth Techniques

Require atomic diffusion. At normal temperatures such diffusion is usually very slow (except in the case of superionic materials where the small cation is quite mobile). Annealing and sintering - hot pressing are two important solid growth techniques.

1.7.2 Vapor Phase Growth

This method depends on the existence of reversible equilibrium between reactant A, transporting agent B, and gaseous product AB.

A (s) + B (g) AB (g)

1.7.3 Solution Growth

In contrast to the other methods in which melts solidify to give crystals that have the same composition as the melt, precipitation from solution methods involve the growth of crystals from a solvent of different composition to the crystals. The major advantages of solution method are that it permits crystal growth at a temperature well below the melting point and isothermal conditions with slow growth rates give quality crystals of low defect concentration. The disadvantages of this method are its slow grow rate and contamination by the container or flux. The other types of this method are gel growth, flux growth, molten metal solution growth, organic solution growth, thermal freezing method (Bridgman-Stockbarger), zone melting method, flame fusion method (Verneuil method), Czochralski’s method, flux pulling (top seeded solution growth-TSSG) and the aqueous solution growth methods (hydrothermal method).Typically the temperature in a hydrothermal process falls between the boiling point of water and the critical temperature of 374^oC, while the pressure reaches 22.1 MPa.

In this thesis, our goal was to synthesize new metal oxides using hydrothermal method. Product was planned to be obtained as single crystals by using the ability of hydrothermal method on good quality single crystal growth.

CHAPTER 2

EXPERIMENTAL METHOD

2.1 Teflon-Lined Acid Digestion Parr Autoclave

The autoclave design used was a Parr acid digestion bomb purchased from Parr Instrument Company. These reaction vessels are lined with a removable Teflon insert and posses a maximum operating temperature and pressure of 250^oC and 1800 psi, respectively. The closure design consists of a spring-loaded, broad flanged closure that is sealed by tightening the bomb cap with a hook spanner wrench (Figure 2.1) [41]. Due to the larger coefficient of thermal expansion of Teflon (the liner) versus metal (the material in which the liner is enclosed), the Teflon will expand and contract much more upon heating and cooling cycles than its enclosure material. Therefore, a spring-loaded closure is used to maintain a constant pressure on the Teflon seal during both heating and cooling cycles.

These autoclaves have many important advantages over other reaction vessels such as quartz and steel. For instance, they are completely inert to aqueous base and fluorides, making them a workhorse for the zeolite industry. These are sold by a number of vendors and are inexpensive and easy to handle, and require no special auxiliary equipment. The main disadvantage of these autoclaves is that the fluoropolymer liner will begin to deform and weaken above about 240^oC, thus cannot contain fluids approaching true supercritical aqueous fluids. Therefore, they have become the primary vessel for hydrothermal work done below 200^oC.

The autoclave must be tested carefully prior to heating. Before using a new PTFE cup and cover, these parts should be heated in a bomb with a charge of pure water. This pre-treating will help to develop the required seals and it may prevent annoying leakage in subsequent procedures. The amount of water used in this pre-treatment should not exceed 40 percent of the capacity of the cup. Maximum charge for inorganic sample is 1.0 gram. These autoclaves can be heated up to 250^oC and 1800 psig pressure

A 23-mL Teflon-lined autoclave (Parr Instruments, model 4749) was used as reaction autoclave. Starting materials were added to Teflon cup. After adding, 9 mL of B(OH)₃ was added to Teflon cup to provide about 32% fill. Finally, pieces of autoclave are placed in order. These pieces of autoclave are shown in Figure 2.1. The disc order is so important. Firstly, Corrosion disc (thinner one) is placed and then rupture disc (thicker) is put. After closing the autoclave, it is placed into the Carbolite CWF 1100 (Figure 2.2) furnace for at least 1 day at 170-200^oC. After the reaction is completed, the autoclave is allowed to cool in the furnace.

(a)

(b)
Figure 2.1 (a) Schematic representation of an autoclave, (b) A Parr acid digestion bomb.

Figure 2.2 Picture of Carbolite CWF 1100 furnace.