Metal oxides, ceramic powders

Dogan and Hausner [5.1] presented a survey of the applications of freeze drying in ceramic powder processing, the three main objectivesof which have been pursued  

The metal solutions are sprayed into cold liquids for rapid freezing, after which the droplets are freeze dried and decomposed to metal oxides. Due to the homogeneous distribution of the components, the reactions in the solid state occur at lower temperatures compared with conventionally produced powders. 

The drying of the precipitates conventionally leads to hard agglomerates, which densify still more during the calcination to oxides. In the freeze dried precipitates only soft agglomerates are formed with fine pores, as shown in Fig. 5.1. During sintering of this product, a high relative density can be achieved at substantially lower temperatures (300-400 °C), as shown in Fig. 5.2. 

freeze dried after precipitation and washing 2, chamber drying at 120 °C after precipitation and washing 3, commercially available product (Fig. 1 from [5.1]). 

Reetz and Haase [5.2] used different freezing rates to freeze Zr02, and found slow freezing of this product to result in better technological qualities, e. g. free-flow and sinter ability than quick freezing. On the other hand, complex Zn- solutions can only be frozen quickly to arrive at a product homogeneous in chemical structure and grain size distribution. 

Freeze-Drying. SecondEdition. Georg Wilhelm Oetjen, Peter H aseley Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheiin ISBN 978-3-527-30620-6 

Ca (CH 3COO)2-(hydrolysed) PO (OCH 3) system 3, ZrOCI2-YCI3 system  

simultaneous freezing of droplets from 1 and 2 on the liquid nitrogen-cooled bottle 6, chamber (i.d. 95 mm, height 

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Two types of oxidation reactions are of interest in ceramics oxidation of metals and oxidation of sulfides. The oxidation of sulphides is a common extractive metallui cal process, generating an oxide ceramic powder. The oxide product is usually an intermediate product on the way to metal production but if sufficiently pure it can be used directly as a ceramic powder. A common example is the roasting of zinc sulphide to form zinc oxide,... 

The first six reactions form mixed oxide ceramic powders. The last three reactions are carbothermal reductions to produce different metal carbides. The most famous is the Atcheson process for synthesis of SiC from Si02 and carbon, where the carbon in the mixture of reactant powders is used as a resistive electrical conductor to heat the mixture to the reaction temperature. This reaction is performed industrially in a 10-20 m long bunker fixed with two end caps that contain the source and sink for the cLc current. The reactant mixture is piled to a height of 2 m in the bunker and a current is applied. The temperature rises to the reaction temperatures, and some of the excess C reacts to CO, providing further heat. The 10-20 m bunker is covered with a blue flame for most of the reaction period. The resulting SiC is loaded into grinding mills to produce the ceramic powders and abrasives of desired size distributions. 

This chapter focuses only on the syntheses of oxide ceramic powders. The precursors of these powders are metal-organic compounds, mainly metal alkox-ides. The different varieties of ceramic powders synthesized by mixed metal alkoxide precursors are the focal points of this chapter. Numerous references on the fundamental principles of sol-gel processing are available in the literature and within other chapters of this book [10-12]. 

The oxide ceramic powders produced through mixed-metal alkoxides have many advantages over powders prepared conventionally some of the advantages are lower temperature processes, higher purity, more homogeneous distribution of constituents, finer particle size, and easier compositional alteration. Most metal alkoxides exposed to moisture and/or heat cause decomposition of the alkoxide and thus provide forming methods for fine ceramic powders (notable examples for decomposition are thermal decomposition and hydrolytic decomposition). 

Small particles can be introduced into metals or ceramics in other ways. The most obvious is to mix a dispersoid (such as an oxide) into a powdered metal (aluminium and lead are both treated in this way), and then compact and sinter the mixed powders. 

Catalysts can be metals, oxides, sulfides, carbides, nitrides, acids, salts, virtually any type of material. Solid catalysts also come in a multitude of forms and can be loose particles, or small particles on a support. The support can be a porous powder, such as aluminium oxide particles, or a large monolithic structure, such as the ceramics used in the exhaust systems of cars. Clays and zeolites can also be solid catalysts. 

Strength, brittleness, and solvent permeability properties are limited because of lack of control of the ceramic composition on a macro- and microlevel. Even small particle sizes are large compared with the molecular level. There have been a number of attempts to produce uniform ceramic powders including the sol-gel synthesis in which processing involves a stable liquid medium, coprecipitation in which two or more ions are precipitated simultaneously. More recently, Carraher and Xu have used the thermal degradation of metal containing polymers to deposit metal atoms and oxides on a molecular level. 

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