Sparsely soluble oxide

The various CBPC products discussed in the last chapter reveal that CBPC powder consists of one or more sparsely soluble oxides and an acid phosphate. When this mixture is stirred in water, the acid phosphate dissolves first and makes the solution acidic, in which the sparsely soluble alkaline oxides dissolve and an acid-base reaction is initiated. This reaction produces slurry that subsequently hardens and a ceramic hard product is formed. If the acid phosphate is phosphoric acid solution, the setting reaction is too rapid. Such a process becomes impractical for production of large ceramic objects because the rapid acid-base reaction is exothermic and that boils the reaction slurry. Therefore, less acidic acid phosphates (such as chhydrogen phosphates) are preferred for fabrication of practical ceramics. 

The solubility of such solids is only a fraction of that of the acid phosphates that we discussed in the last chapter. A sparsely soluble oxide (or its hydroxide) dissolves in acidic solution in two steps. The first step is ionization or dissociation. When stirred in water, the oxide decomposes into its cations and anions. This decomposition occurs because of collisions between the oxide molecules and the polar molecules of water. Second is a screening step in which the two charged ions resulting from this dissociation are kept separate by the water molecules. These steps are described in detail below. 

In forming CBPCs, this dissociation is essential. The cations formed by dissociation react with phosphate anions that are present in the aqueous solution and form phosphate salt molecules. These salt molecules connect to each other and form a network and consolidate into a crystalline phosphate ceramic. Thus, success in forming CBPCs lies mainly in successfully dissociating sparsely soluble oxides in acidic solutions and precipitating salt in crystalline form. We wiU discuss the fundamentals of this dissociation in the next several chapters and present methods of dissociating various oxides in phosphate solutions to form ceramics. 

In Chapter 4, the ionization constant (i.e., the reaction constant of dissolution) for weak acids and acid phosphates was defined. The concept of the ionization constant is very general and useful while discussing dissolution of sparsely soluble oxides in acid-base reactions. We assign the symbol K for this constant. 

Below we will discuss this constant when a sparsely soluble oxide is dissolved in a phosphoric acid solution. The same discussion may then be generalized to other phosphate solutions. 

As evident from Fig. 16.1, the dissolution behavior of Cr203 is similar to that of other divalent oxides. Its solubility is high in the acidic region, drops almost linearly as pH increases, has a minimum at almost pH = 7, and then increases with pH. Its overall behavior is that of a sparsely soluble oxide. As a result, Cr203 will react with acid phosphates and form insoluble hydrophosphates or phosphates. 

Arsenic has two oxidation states, 3 - - and 5 - -. Figure 16.1 shows the dissolution characteristics of As for its 3 - - state. Compared to other sparsely soluble oxides, this oxide is soluble, and up to pH 9 its solubility is constant and then increases with pH. Thus, As will dissolve easily in the CBPC solution and can be converted to its phosphate form. Unlike the chromates, however, arsenates are insoluble in water but soluble or sparingly... 

These observations imply that, forming a phosphate ceramic requires either diluted phosphoric acid or a partially neutralized phosphate solution as a source of anions, and a sparsely soluble (slightly soluble) oxide or a mineral to provide cations. All ceramics are formed in an aqueous solution. In general, the following scheme seems to work best. 

Phosphoric acid may be diluted with water. This step provides the water fraction needed to form the ceramic. Monovalent alkali metal oxides, with their high aqueous solubility, may be used for partial neutralization of the acid, while sparsely soluble divalent oxides are good candidates for providing the cations. In particular, oxides of Mg, Ca, and Zn are preferred because they are inexpensive compared to similar oxides, and unlike oxides of Pb, Cr, Cd, Hg, and Ni, they are not environmentally hazardous (see Chapter 16). 

Aluminum oxide is the only trivalent oxide that has been used to form a ceramic some heat treatment is needed. Kingery claims to have observed a setting reaction between trivalent iron oxide and phosphoric acid, but this reaction may have been caused by traces of magnetite in the trivalent oxide. Pure trivalent iron oxide such as hematite (Fe203) does not react with phosphoric acid. Overall, trivalent metal oxides have a solubility that is only marginal and falls below that of even sparsely soluble divalent oxides, while the solubility of oxides of most quadrivalent metals (zirconium is an exception) is too low to form a ceramic. 

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. 

Because calcium oxide is a fairly reactive powder, it forms calcium hydroxide when in contact with water. This reaction is exothermic and hence heats water during formation of the hydroxide. Because of this excess heat, it cannot directly be used to form phosphate ceramics by reacting it with an acid phosphate solution and must be used in a less soluble form as sparsely soluble silicate or hydrophosphate. In spite of this difficulty, because human bones contain calcium phosphate, there have been sufficient efforts in developing methods of forming biocompatible CBPCs of calcium phosphate by using partially soluble phosphates of calcium rather than using oxide itself. A similar approach may also be taken if one uses partially soluble silicate or aluminate of calcium. These routes are discussed in Chapter 13. 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

Other isotopes (I, Co, etc.) or their oxides and soluble salts can also be present. Stabilization of such contaminants is handled in a manner similar to the stabilization of hazardous metals, as discussed in Chapter 16, because most of them are sparsely soluble compounds. 

The chemical oxidation of EDOS to PEDOS could be performed with iron(lll) chloride in acetonitrile. Dark blue, soluble PEDOS with an absorption band at = 594 nm was observed. From H-NMR spectroscopy a reasonably high molecular weight was concluded, because protons of selenophene moieties at about 6.82 ppm, as visible in the monomer, could not be found, indicating the lack of noteworthy amounts of end groups. These results were debated later. Obviously, a nondoped (or sparsely doped) material was obtained by Aqad, Lakshmikantham, and Cava. ... 

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