Aluminum Phosphate Ceramics

Porcelain dental cements were developed by Steenbock [6] who produced silico-phosphate dental material using 50 wt% concentrated phosphoric acid solution and an aluminosilicate glass. Wilson et al. [7] showed that various brands of commercial cements consist of powdered alumina-lime-silica glass mixed with phosphoric acid, which form a hard and translucent product. The phosphoric acid used in these cements is partially neutralized by aluminum oxide. 

Unlike magnesium phosphate ceramics, phosphate bonded alumina ceramics consist of particles, whose surfaces are coated with berlinite (AIPO4), a crystalline orthophosphate. The bonding phase AIPO4 [8] is formed by the chemical reaction between the phosphoric 

One may recall from Chapter 9 that newberyite has also been found in ceramics formed by reaction of MgO and A1(H2P04)3. Detailed study by Finch and Sharp [18] showed that, when the ratio of MgO/Al H2P04)3 is 4 1, newberyite content is maximum in the matrix. In addition, they have also found AIPO4 and suspect phases of Al-Mg-P04 within the 

Unlike divalent oxides, the solubility of alumina is low and hence some warm temperature treatment is required. In addition, rather than using lower solubility phosphate solutions such as ammonium and potassium phosphate solutions, phosphoric acid solution is directly used. Wagh et al. [ 17] employed a thermodynamic analysis to study the elfect of the temperature on the solubility of individual phases of alumina on the formation of its phosphate phases during heat treatment where solubility of hydrated aluminum oxide, viz., hydrargillite (A1203-3H20) is enhanced, and that contributes further to the formation of berlinite phase. They confirmed this by differential thermal analysis (DTA) and X-ray diffraction (XRD) analysis on samples heated beyond 118°C. 

Production of alumina aquosols from alumina has been well researched in sol-gel science [19]. Yoldas [20] was the first one to show that monolithic alumina gels could be formed by hydrolysis and condensation of aluminum alkoxide. As discussed in Chapter 5, formation of aquosols and their gel is an intermediate step in the formation of chemically bonded phosphate ceramics. Condensation of the hydrated alumina sols by reaction with phosphoric acid to form A1(H2P04)3-H20 gel is the first step toward synthesis of a berlinite-bonded alumina ceramic. When this gel is heated to 150°C, this gel reacts with additional alumina and releases water, and crystalline berlinite is produced. This Chapter 

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Details of such reaction and formation of aluminum phosphate ceramics are discussed in Chapter 11. Unfortunately, this acid phosphate also has limited use in other applications, and hence its commercial availability is limited. 

The literature review in Chapter 2 reveals that divalent metal oxides such as oxides of calcium, magnesium, and zinc (CaO, MgO, and ZnO) are the major candidates for forming phosphate ceramics. These oxides are sparsely soluble in acidic solution, and as we shall see in Chapter 4, they are the most suitable ones to form ceramics. In addition, following the methods discussed in subsequent chapters in this book, aluminum oxide (alumina, AI2O3) and iron oxide (Fe203), which are abundant in earth s crust have excellent potential to form low cost CBPCs. For this reason, we have provided relevant information on these oxides. Table 3.2 gives some details. 

Fluoride emissions into the atmosphere are derived mainly from modern-day anthropogenic sources, particularly industrial sources. They include steel industry, phosphate fertilizer industry, aluminum industry, ceramics... 

Calcium phosphate A family of calcium phosphate ceramics including aluminum calcium phosphate, ferric calcium phosphate, hydroxyapatite and tricalcium phosphate (TCP), and zinc calcium phosphate which are used to substitute or augment bony structures and deliver drugs. Glass-ceramics A glass crystallized by heat treatment. Some of those have the ability to form chemical bonds with hard and soft tissues. Bioglass and Ceravital are well known examples. 

Ti02 powder (P-25, Degussa) was used as a photoeatalyst. The Al-sol contained 20wt% alumina (Nyaeol Al-20), Si-Sol containing 15 wt% silica was fiom Ludox, Aluminum phosphate was used for ceramic fiber honeycomb coating. 

In many ways the phosphates provide the richest field for CBC s (chemically bonded ceramics) because of the P—O bond strengths and the coordination demands of pentavalent phosphorus. Among silicates, room temperature hydration reactions seem to be possible only with Ca-compounds the magnesium and aluminum anhydrous silicates phases simply do not react. This book documents for the reader the wide application of these separate chemistries as true chemically bonded ceramics. 

Chapter 8 provides a review of the phosphate mineralogy that facilitates discussion on the applications discussed in the latter part of the book. The next five chapters discuss individual phosphate (magnesium, zinc, aluminum, iron, and calcium) ceramics that have potential for use in current and future applications. The approach provided in this book should guide researchers to many more formulations for specific needs in the future. 

Aluminum trimetaphosphate, A1(P03)3, is obtained from a melt of AI2O3 and HPO3. The stmctme is composed of chains of PO4 tetrahedra that are connected by AlOe octahedra. A1(P03)3 is a constituent of glasses, coatings, binders, and ceramics as well as a catalyst for esterifications and oxidation of alkenes (see Phosphates Solid-state Chemistry). 

ACGIH TLV TWA 2 mg(Al)/m3 SAFETY PROFILE Corrosive to the eyes, skin, and mucous membranes. When heated to decomposition it emits toxic fumes of POx. Used as an antacid and as a cement component, flux for ceramics, dental cement, glass, and gels. See also ALUMINUM COMPOUNDS and PHOSPHATES. 

Although Plaster of Paris was used inl892asabone substitute [Peltier, 1961], the concept of using synthetic resorbable ceramics as bone substitutes was introduced in 1969 [Hentrich et al., 1969 Graves et al., 1972]. Resorbable ceramics, as the name implies, degrade upon implantation in the host. The resorbed material is replaced by endogenous tissues. The rate of degradation varies from material to material. Almost all bioresorbable ceramics except Biocoral and Plaster of Paris (calcium sulfate dihydrate) are variations of calcium phosphate (Table 39.8). Examples of resorbable ceramics are aluminum calcium phosphate, coralline. Plaster of Paris, hydroxyapatite, and tricalcium phosphate (Table 39.8). 

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