F. REACTIVE ELECTRODE SUBMERGED ARC

Hydrothermal Synthesis of Ceramic Oxide Powders



15



Teflon propeller



Heater



Heater



Autoclave



Balls

Starting materials

Teflon beaker



FIGURE 1.14 Experimental apparatus for hydrothermal mechanochemical reactions.



ambient water pressure. The starting solutions with the precipitate and stainless

steel balls (5 mm diameter) were placed in Teflon beakers. A Teflon propeller

was rotated in the beaker under 200°C and 2 MPa. The speed of the propeller

was from 0 to 107 rpm. The number of stainless steel balls was 200, 500, and

700. X-ray diffraction profiles are shown in Figure 1.15.36



BaO . 6Fe2O3

BaO . Fe2O3



(c)



(b)

(a)



25



30



35



40



45



50



20 Cuka



FIGURE 1.15 X-ray diffraction profiles of (a) starting materials, (b) material fabricated

at 200°C under 2 MPa for 4 h without rotation, and (c) material fabricated at 200°C for

4 h using 200 balls at 37 rpm.



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16



Chemical Processing of Ceramics, Second Edition



FIGURE 1.16 Microwave-assisted reaction system (MARS 5).



H. MICROWAVE HYDROTHERMAL PROCESS

Microwave-assisted hydrothermal synthesis is a novel powder processing technology for the production of a variety of ceramic oxides and metal powders under

closed-system conditions. Komarneni et al. developed this hydrothermal process

into which microwaves are introduced.37–48 This closed-system technology not

Cover



Stem



Locking nut

Thermowell

Thermowell

Vent fitting

Locking nut



Safety

membrane



Cover



Stem

Liner



Sleeve



FIGURE 1.17 Components of reaction vessel used in the MARS-5 unit.

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Hydrothermal Synthesis of Ceramic Oxide Powders



17



only prevents pollution during the synthesis of lead-based materials, but also

saves energy, and thus could substantially reduce the cost of producing many

ceramic powders. Hydrothermal microwave treatment of 0.5 M TiCl4 was done

in 1 M HCl to form rutile. The system (Figure 1.16) operated at a 2.45 GHz. The

vessel is lined with Teflon (Figure 1.17) and the system is able to operate up to

250°C. The parameters used are temperature, pressure, time, concentration of the

metal solution, pH, etc. The key result is crystallization reactions, which lead to

faster kinetics by one or two orders of magnitude compared to conventional

hydrothermal processing. The use of microwaves in both solid and liquid states

is gaining in popularity for many reasons, but especially because of the potential

energy savings. The use of microwaves under hydrothermal conditions can accelerate the synthesis of anhydrous ceramic oxides such as titania, hematite, barium

titanate, lead zirconate titanate, lead titanate, potassium niobate, and metal powders such as nickel, cobalt, platinum, palladium, gold, silver, etc., and this is

expected to lead to energy savings. The term “microwave-hydrothermal” processing was first coined by us for reactions taking place in solutions that are heated

to temperatures greater than 100°C in the presence of microwaves. The value of

this technique has been demonstrated in rapid heating to the temperature of

treatment, which can save energy; increasing the reaction kinetics by one to two

orders of magnitude; forming novel phases; and eliminating metastable phases.

Figure 1.18 shows a nanophase powder of hematite.



FIGURE 1.18 Hematite synthesized from 0.02 M ferric nitrate at 100°C under microwave-hydrothermal conditions.

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I.



Chemical Processing of Ceramics, Second Edition



HYDROTHERMAL SONOCHEMICAL METHOD



Ultrasonic waves are often used in analytical chemistry for dissolving powder

into solution.49 The hydrothermal sonochemical method is a new method for

synthesizing materials.50



III. IDEAL POWDERS AND REAL POWDERS

The characteristics of ideal powders and real powders produced by hydrothermal

processing are shown in Table 1.6 and Table 1.7. Hydrothermal powders are close

to ideal powders.



TABLE 1.6

Characteristics of an Ideal Powder

Fine powder less than 1 µm

Soft or no agglomeration

Narrow particle size distribution

Morphology: sphere

Chemical composition controllable

Microstructure controllable

Uniformity

Free flowing

Fewer defects, dense particle

Less stress

Reactivity, sinterability

Crystallinity

Reproducibility

Process controllable



TABLE 1.7

Characteristics of Hydrothermal Powders

Fine powder less than 1 µm

No or weak agglomeration

Single crystal in general; depends on preparation temperature

Flow ability: forming is good

Good homogeneity

Good sinterability

No pores in grain

Narrow particle size distribution

Ability to synthesize low-temperature form and/or metastable form

Ability to make composites such as organic and inorganic mixtures

Ability to make a material that has a very high vapor pressure



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Hydrothermal Synthesis of Ceramic Oxide Powders



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