《固体化学》课程教学课件（英文讲稿）Chapter 2 Methods

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Preparative methods

Preparativemethods1) Ceramic methodsIn its simplest form, ceramic method consistsof heating together two solids which react to formthe required product.This method is used widely both industriallyand in the laboratory; the first high temperaturewere, for example, madebysuperconductorsceramic method

Preparative methods 1) Ceramic methods In its simplest form, ceramic method consists of heating together two solids which react to form the required product. This method is used widely both industrially and in the laboratory; the first high temperature superconductors were, for example, made by ceramic method

Consists of heating two non-volatile solids whichreact to form the required product.The solid-state method can be used to prepare awhole range of materials including mixed metaloxides, sulfides, nitrides, aluminosilicates, ect.heat to 1300°0ZrO, (s) + SiO2(s) > ZrSiO4 (s)

Consists of heating two non-volatile solids which react to form the required product. The solid-state method can be used to prepare a whole range of materials including mixed metal oxides, sulfides, nitrides, aluminosilicates, ect

BATamman's Rule suggestsa temperature of abouttwo-thirds of the meltingOriginal interfacepoint (K) of the lowermelting reactant is neededicBAto have reaction to occurKin a reasonable time.Product LayerPieces half the original size.Piecesonequartertheoriginal size.Piecesoneeighththeoriginal sizeTwicethesurlaceareaFourtimesthesurfaceareaEighttimesthesurfaceareaA=24cm2A=6cm2A=12cm2A=48cm2

Tamman’s Rule suggests a temperature of about two-thirds of the melting point (K) of the lower melting reactant is needed to have reaction to occur in a reasonable time

The sample might be ground, then heated in atantalumtube forhightemperature reactions (to2300K)

The sample might be ground, then heated in a tantalum tube for high temperature reactions (to 2300K)

Synthesis of some compounds must be carried outunder a special atmosphere.: an noble gas, argon, may be used too preventoxidation to higher oxidation state.an oxidizing gas, oxygen, might be used to form ahigh oxidation state.: a reducing gas, hydrogen, might be used to form alow oxidation stateEXHAUSTGAS02

Synthesis of some compounds must be carried out under a special atmosphere. • an noble gas, argon, may be used too prevent oxidation to higher oxidation state. • an oxidizing gas, oxygen, might be used to form a high oxidation state. • a reducing gas, hydrogen, might be used to form a low oxidation state

PreparativemethodsDisadvantages of ceramic methodDespite its widespread use, the simple ceramicmethod has several disadvantages.Firstly, high temperatures are generally required(2300K in the example we gave). This requires a largeinput of energy. In addition, the phase or compounddesired may be unstable or decompose at such hightemperatures

Preparative methods Disadvantages of ceramic method Despite its widespread use, the simple ceramic method has several disadvantages. Firstly, high temperatures are generally required (2300K in the example we gave). This requires a large input of energy. In addition, the phase or compound desired may be unstable or decompose at such high temperatures

PreparativemethodsDisadvantages of ceramic methodA second disadvantage is the slowness of solicstate reactions. In general, the solids are not raisedto their melting temperatures, and so the reactionoccurs in the solid stateRaising the temperature enables reaction at theinterface and diffusion through the solid to proceedfaster than they do at room temperatures

A second disadvantage is the slowness of solid state reactions. In general, the solids are not raised to their melting temperatures, and so the reaction occurs in the solid state. Raising the temperature enables reaction at the interface and diffusion through the solid to proceed faster than they do at room temperatures. Preparative methods Disadvantages of ceramic method

Preparative methodsDisadvantages of ceramic methodEven so, diffusion is often the limiting stepBecause of this, it is important that the startingmaterials are ground to give a small particle sizeand very well mixed to maximize the surfacecontact area and minimize the distance that thereactants have to diffuseA third disadvantage is that the product is oftennothomogeneous in composition

Preparative methods Disadvantages of ceramic method Even so, diffusion is often the limiting step. Because of this, it is important that the starting materials are ground to give a small particle size, and very well mixed to maximize the surface contact area and minimize the distance that the reactants have to diffuse. A third disadvantage is that the product is often not homogeneous in composition

PreparativemethodsMicrowave synthesisTo use microwave heating in solid state synthesisat least one component of the reaction mixture mustabsorbmicrowaveradiationThe speed of the reaction process is thenincreased by both increasing the rate of the solidstate reaction and by increasing the rate of diffusion.which as we mentioned earlier is often the rate-limiting step

Preparative methods Microwave synthesis To use microwave heating in solid state synthesis at least one component of the reaction mixture must absorb microwave radiation. The speed of the reaction process is then increased by both increasing the rate of the solid state reaction and by increasing the rate of diffusion, which as we mentioned earlier is often the rate￾limiting step