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Introduction to Forming

Roughly into following categories:Dry pressing; die pressing, isostatic

pressing, hot pressingExtrusion Injection moldingCasting: slip casting, tape casting, gel

castingFactors to be considered: complexity of object, size, quantity (production rate) etc.

Che5700 陶瓷粉末處理

2Taken from TA Ring, 1996

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Processing Aids

For different forming techniques, the additives may be different; e.g. in the following table, the molecular weight is not the same different viscosity, yield strength etc.

Che5700 陶瓷粉末處理

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Comparison of system characteristics

Solvent content not same for different techniques; corresponding packing fraction and its changes also different, large change significant contraction; contraction occurs at some stage; forming, drying, or sintering; Vo =volume of organics in green body

Che5700 陶瓷粉末處理

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Stress during forming

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Strength of green body

If requires machining on green body (e.g. shape modification), hence need strength of green body, if not enough strength, easy to appear cracks, as following table, taken from JS Reed, 1995

Che5700 陶瓷粉末處理

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General considerations

Both technical and economic factors should be considered

For advanced ceramics, microstructure control is a critical issue

Process window reflects the easiness of control; (reliability, reproducibility issue)

Cost ~ 1/ yield

There are often more than one process to choose for a specific ceramic product. meaning comparison of various factors

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Process Window

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Dimensional Control

Tight control of dimension superior control of process through forming and sintering plus final machining (polishing) – an expensive step

Total volume shrinkage = (drying stage) + (calcination stage) + (sintering stage) ….. Precise control depends on the control at this three stagesDry pressed, injection molded drying stage shrinkage ~ 0; extruded and slip casted parts drying stage shrinkage ~ 3-12%

Shrinkage during calcination: high for injection molding and tape casting

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Shrinkage may not be isotropic!

Che5700 陶瓷粉末處理

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Shaping method on grain size dependence of strength

• Source: J. Euro. Cer. Soc., 16, 1189-1200, 1996.• Methods tested: uniaxial pressing, cold isostatic pressing, pressure filtration, gel casting, and combinations; • High strength: high purity material for high fracture toughness; small size of critical flaw at instability (crack growth); grain size & flaw size both important• Any microstructure which reduce subcritical crack growth is helpful to strength

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Experimental results from this work,showing effects of shaping methods

• Different fabrication technique – different flaw size & distributions• Gelcasting: gel formation – acrylamide + N,N’-methylene bis-acrylamide (use ammonium persulfate as initiator) + alumin powder (need several hours to complete polymerization) (final step: CIP before sintering

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• Some techniques applied: (e.g.) ultrasonic dispersion + vacuum de-gassing of slurry + uniaxial pressing + cold isostatic pressing• Ultrasonic dispersion + pressure filtration + slow drying: to get samples with low flaw density; or pressure filtration + CIP;• Different processing techniques achieve packing homogeneity low sintering temperature to achieve high density• Casting and pressure filtration: may give rise to “cooperative alignment” of particles hence good results

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•Batch 1, 2, 3: from gelcasting• General trend: increasing strength with decreasing grain size•

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• 1 – 8: ranking of strength• obvious relation between strength and subcritical crack growth (flaw density or distribution – strongly related to forming techniques)

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Lightest Object in 2002

Aerogel density as low as 0.003 g/cc (air density around 0.0014 g/cc) by NASA(source: 2002 Time Magazine)