材料的制备与加工(3).ppt

1、4.7 Fabrication Of Clay Products,The characteristics of clay Composites of clay products Fabrication of clay Products Drying and firing,The characteristics of clay,(1) Hydroplasticity（含水塑性） when water is added，they become very plastic，a condition termed hydroplasticity. (2) Fusion temperature range

2、clay fuses or melts over a range of temperatures；thus，a dense and strong ceramic piece may be produced during firing without complete melting such that the desired shape is maintained. This fusion temperature range，of course，depends on the composition of the clay.,（3）Composites Clays are aluminosili

3、cates（铝硅酸盐），being composed of alumina (Al2O3) and silica (SiO2)，that contain chemically bound water. They have a broad range of physical characteristics，chemical compositions，and structures； common impurities include compounds (usually oxides) of barium，calcium，sodium，potassium（钾），and iron， and also

4、 some organic matter.,(4) Structure Crystal structures for the clay minerals are relatively complicated； however，one prevailing characteristic is a layered structure. The most common clay minerals that are of interest have what is called the kaolinite（高岭石） structure. Kaolinite clay Al2(Si2O5)(OH)4 ,

5、When water is added，the water molecules fit in between these layered sheets and form a thin film around the clay particles. The particles are thus free to move over one another，which accounts for the resulting plasticity of the waterclay mixture.,Composites of clay products,(1) clay-plastic ingredie

6、nts (2) nonclay minerals quartz The quartz is used primarily as a filler material，being inexpensive，relatively hard，and chemically unreactive（惰性）. It experiences little change during high-temperature heat treatment because it has a high melting temperature；when melted， however，quartz has the ability

7、 to form a glass.,feldspar（长石） When mixed with clay，a flux（熔剂） forms a glass that has a relatively low melting point. The feldspars are some of the more common fluxing agents； they are a group of aluminosilicate materials that contain K，Na，and Ca2ions.,Conclusion As would be expected， the changes th

8、at take place during drying and firing processes， and also the characteristics of the finished piece， are influenced by the proportions of these three constituents: clay，quartz，and flux. A typical porcelain（瓷器）might contain approximately 50% clay， 25% quartz，and 25% feldspar.,Fabrication of Clay Pro

9、ducts,Fabrication of Clay Products techniques are utilized for forming clay-based compositions: hydroplastic forming（含水塑性成型） and slip casting（浇铸法） .,(1) Hydroplastic Forming（含水塑性成型） clay minerals， when mixed with water， become highly plastic and pliable （韧性）and may be molded without cracking. The mo

10、st common hydroplastic forming technique is extrusion（挤出），in which a stiff plastic ceramic mass is forced through a die （模具） orifice（针孔，管口） having the desired cross-sectional geometry; it is similar to the extrusion of metals. Brick， pipe， ceramic blocks， and tiles are all commonly fabricated using

11、hydroplastic forming.,（2）Slip Casting（浇铸法） A slip（浆料） is a suspension of clay and/or other nonplastic materials in water. When poured into a porous mold (commonly made of plaster灰泥，灰浆 of paris熟石膏)，water from the slip is absorbed into the mold， leaving behind a solid layer on the mold wall the thickn

12、ess of which depends on the time. This process may be continued until the entire mold cavity becomes solid (solid casting) as demonstrated in Figure a.,It may be terminated when the solid shell wall reaches the desired thickness， by inverting（翻转） the mold and pouring out the excess slip； this is ter

13、med drain casting (Figure b) As the cast piece dries and shrinks，it will pull away (or release) from the mold wall； at this time the mold may be disassembled（拆卸） and the cast piece removed.,Application The rather complex ceramic shapes that may be produced by means of slip casting include sanitary（卫

14、生）lavatory ware，art objects，and specialized scientific laboratory ware such as ceramic tubes.,Drying,several stages in the removal of water from between clay particles during process. (a) wet body (b)partially dry body(c)completely dry body.,As a clay-based ceramic body dries，it also experiences som

15、e shrinkage. In the early stages of drying the clay particles are virtually surrounded by and separated from one another by a thin film of water. As drying progresses and water is removed，the interparticle separation decreases，,Drying and firing,Firing,（1）After drying，a body is usually fired at a te

16、mperature between 900 and 1400 (1650 and 2550)； （2）The firing temperature depends on the composition and desired properties of the finished piece. （3）During the firing operation，the density is further increased (with an attendant decrease in porosity) and the mechanical strength is enhanced.,（4）Vitr

17、ification（玻璃化） When clay-based materials are heated to elevated temperatures，some rather complex and involved reactions occur. The gradual formation of a liquid glass that flows into and fills some of the pore volume. The degree of vitrification depends on firing temperature and time，as well as the

18、composition of the body.,The temperature at which the liquid phase forms is lowered by the addition of fluxing agents such as feldspar（长石）. This fused phase flows around the remaining unmelted particles and fills in the pores as a result of surface tension forces (or capillary action)； shrinkage als

19、o accompanies this process. Upon cooling，this fused phase forms a glassy matrix that results in a dense，strong body. Thus，the final microstructure consists of the vitrified（玻璃化） phase，any unreacted quartz particles， and some porosity.,Conclusion The degree of vitrification，of course，controls the roo

20、m-temperature properties of the ceramic ware； strength，durability，and density are all enhanced as it increases. The firing temperature determines the extent to which vitrification occurs；that is，vitrification increases as the firing temperature is raised. Building bricks are ordinarily fired around

21、900 (1650) and are relatively porous. On the other hand，firing of highly vitrified porcelain，which borders on being optically translucent（半透明的），takes place at much higher temperatures. Complete vitrification is avoided during firing， since a body becomes too soft and will collapse.,4.8 Powder of Pre

22、ssing,1.Characteristic Powder pressing，is used to fabricate both clay and nonclay compositions，including electronic and magnetic ceramics as well as some refractory brick products. In essence，a powdered mass，usually containing a small amount of water or other binder （粘结剂） ，is compacted into the desi

23、red shape by pressure. The degree of compaction is maximized and fraction of void space is minimized by using coarse and fine particles mixed in appropriate proportions. There is no plastic deformation of the particles during compaction，as there may be with metal powders. One function of the binder（

24、粘结剂） is to lubricate the powder particles as they move past one another in the compaction process.,2. Three basic powder pressing procedures There are three basic powder pressing procedures: uniaxial（单向加压），isostatic (or hydrostatic)（等静压）and hot pressing（热压）.,(1) uniaxial pressing The powder is compa

25、cted in a metal die by pressure that is applied in a single direction. The formed piece takes on the configuration of die and platens through which the pressure is applied. This method is confined to shapes that are relatively simple; however， production rates are high and the process is inexpensive

26、.,Schematic representation of the steps in uniaxial power pressing. (a)The die (模具) cavity is filled with power. (b)The power is compacted by means of pressure applied to the top die. (c)The compacted piece is ejected by rising action of the bottom punch（下冲模）. (d)The fill shoe pushes away the compac

27、ted piece, and the fill step is repeated.,(2) isostatic pressing For isostatic pressing，the powdered material is contained in a rubber envelope and the pressure is applied by a fluid，isostatically (i.e.，it has the same magnitude in all directions). More complicated shapes are possible than with unia

28、xial pressing; however，the isostatic technique is more time consuming and expensive.,sintering,The mechanism of sintering is schematically illustrated : （1）After pressing，many of the powder particles touch one another (Figure a). （2）During the initial sintering stage，necks form along the contact reg

29、ions between adjacent particles; in addition，a grain boundary forms within each neck，and every interstice between particles becomes a pore (Figure b). （3）As sintering progresses， the pores become smaller and more spherical in shape (Figure c).,Scanning electron micrograph of an aluminum oxide powder compact that was sintered at 1700 for 6min 5000. The driving forc