Direct Coagulation Casting is a new colloidal forming technique. Double layer stabilized, concent... more Direct Coagulation Casting is a new colloidal forming technique. Double layer stabilized, concentrated alumina suspensions are solidified by shifting the suspensions pH from 4 towards the isoelectric point of 9 using the in situ enzyme-catalyzed decomposition of urea. This reaction minimizes the repulsive forces between the suspended particles. The remaining, attractive Van der Waals forces form a stiff particle network. Suspensions with low viscosities (0.3 Pa*s, 59 vol%) were prepared at pH 4. Deagglomeration of the suspensions by ball milling reduced the agglomerate size below 5 µm. The coagulation kinetics could be influenced either by the urease concentration or by the suspension temperature. Process variables were establised, providing long idle times, which allowed additional filtration and degassing steps. Coagulation was followed by drying and sintering, whereby densities of more than 3.97 g/cm 3 , a 4-point bending strength of 685 MPa (HIPed) and a high reliablility (m = 40) for high purity alumina were achieved. DCC has the potential to improve the reliability of alumina components of complex shape, as well as to avoid expensive molding.
This paper is devided into two parts: The first part covers the fundamentals of ceramic processin... more This paper is devided into two parts: The first part covers the fundamentals of ceramic processing in general, in the second part some examples of electroceramic components are given. Our overview is limited mostly to components made via the powder route.
Advances in biochemical engineering/biotechnology, 2012
Originally developed to fill and restore bone defects, bioactive glasses are currently also being... more Originally developed to fill and restore bone defects, bioactive glasses are currently also being intensively investigated for bone tissue engineering applications. In this chapter, we review and discuss current knowledge on porous bone tissue engineering scaffolds made from bioactive silicate glasses. A brief historical review and the fundamental requirements in the field of bone tissue engineering scaffolds will be presented, followed by a detailed overview of recent developments in bioactive glass-based scaffolds. In addition, the effects of ionic dissolution products of bioactive glasses on osteogenesis and angiogenic properties of scaffolds are briefly addressed. Finally, promising areas of future research and requirements for the advancement of the field are highlighted and discussed.
ABSTRACT In this work we describe how stable ceramic suspensions can be destabilized by time dela... more ABSTRACT In this work we describe how stable ceramic suspensions can be destabilized by time delayed reactions. We also describe how this method can be used in case of high solids loaded suspensions for forming complex and reliable ceramic components. The method uses an aqueous, electrostatically stabilized ceramic suspension with high solids loadings free of agglomerates and of low viscosity. It is cast into a mold and then coagulates forming a stiff, wet green body. Coagulation is performed by changing the pH of the suspension and/or by creating salt directly inside the suspension using a controlled time-delayed reaction. Enzyme catalyzed reactions that are decomposing a substrate, or self-propagating decomposition reactions of a substrate, can be used. After the coagulation reaction in the suspension, the wet green body shows good mechanical properties and can be demolded, dried and sintered. Polymers show also sol-gel transitions upon changing pH, ionic strength or temperature. In case such polymer sols are combined with particles in a suspension, we obtain very rigid ceramic green bodies. Since the forming process takes place without pressure and at ambient temperatures, inexpensive molds and tools can be used. Ceramic bodies with homogeneous, defect poor microstructures can be cast using almost any ceramic powder or even powder combinations with different IEP behavior. Complex shaped components with thin and thick cross sections can be cast as there is no gradient in density and/or temperature during consolidation of the green body. High strength (680 MPa) alumina parts of high reliability (Weibull-module m = 40) have been demonstrated.
ABSTRACT In this work we describe a new ceramic green body fabrication method using ceramic collo... more ABSTRACT In this work we describe a new ceramic green body fabrication method using ceramic colloidal suspensions that allow the production of complex shaped parts of high strength and reliability. The method uses an aqueous, electrostatically stabilized ceramic suspension free of agglomerates and of low viscosity. It is cast into a mold and then coagulates forming a stiff, wet green body. Coagulation is performed by changing the pH of the suspension and/or by creating salt directly inside the suspension using a controlled time-delayed reaction. Enzyme catalyzed reactions that are decomposing a substrate, or self-propagating decomposition reactions of a substrate, can be used. After the coagulation reaction in the suspension, the wet green body shows good mechanical properties and can be demolded, dried and sintered. This new forming process DCC ( Direct Coagulation Casting ) is characterized by near net shape capability of complex shaped parts with high quality and homogeneity in the green as well as in the sintered state. Only small amounts of organic additives which amount to less than 0.5 wt-% based on ceramic powder are needed for the catalytic reactions. Since the forming process takes place without pressure and at ambient temperatures, inexpensive molds and tools can be used. Ceramic bodies with homogeneous, defect poor microstructures can be cast using almost any ceramic powder. Complex shaped components with thin and thick cross sections can be cast as there is no gradient in density and/or temperature during consolidation of the green body. Alumina parts of average bend strength (680 MPa) and high reliability (Weibull-module m=40 ) have been demonstrated.
Direct Coagulation Casting is a new colloidal forming technique. Double layer stabilized, concent... more Direct Coagulation Casting is a new colloidal forming technique. Double layer stabilized, concentrated alumina suspensions are solidified by shifting the suspensions pH from 4 towards the isoelectric point of 9 using the in situ enzyme-catalyzed decomposition of urea. This reaction minimizes the repulsive forces between the suspended particles. The remaining, attractive Van der Waals forces form a stiff particle network. Suspensions with low viscosities (0.3 Pa*s, 59 vol%) were prepared at pH 4. Deagglomeration of the suspensions by ball milling reduced the agglomerate size below 5 µm. The coagulation kinetics could be influenced either by the urease concentration or by the suspension temperature. Process variables were establised, providing long idle times, which allowed additional filtration and degassing steps. Coagulation was followed by drying and sintering, whereby densities of more than 3.97 g/cm 3 , a 4-point bending strength of 685 MPa (HIPed) and a high reliablility (m = 40) for high purity alumina were achieved. DCC has the potential to improve the reliability of alumina components of complex shape, as well as to avoid expensive molding.
This paper is devided into two parts: The first part covers the fundamentals of ceramic processin... more This paper is devided into two parts: The first part covers the fundamentals of ceramic processing in general, in the second part some examples of electroceramic components are given. Our overview is limited mostly to components made via the powder route.
Advances in biochemical engineering/biotechnology, 2012
Originally developed to fill and restore bone defects, bioactive glasses are currently also being... more Originally developed to fill and restore bone defects, bioactive glasses are currently also being intensively investigated for bone tissue engineering applications. In this chapter, we review and discuss current knowledge on porous bone tissue engineering scaffolds made from bioactive silicate glasses. A brief historical review and the fundamental requirements in the field of bone tissue engineering scaffolds will be presented, followed by a detailed overview of recent developments in bioactive glass-based scaffolds. In addition, the effects of ionic dissolution products of bioactive glasses on osteogenesis and angiogenic properties of scaffolds are briefly addressed. Finally, promising areas of future research and requirements for the advancement of the field are highlighted and discussed.
ABSTRACT In this work we describe how stable ceramic suspensions can be destabilized by time dela... more ABSTRACT In this work we describe how stable ceramic suspensions can be destabilized by time delayed reactions. We also describe how this method can be used in case of high solids loaded suspensions for forming complex and reliable ceramic components. The method uses an aqueous, electrostatically stabilized ceramic suspension with high solids loadings free of agglomerates and of low viscosity. It is cast into a mold and then coagulates forming a stiff, wet green body. Coagulation is performed by changing the pH of the suspension and/or by creating salt directly inside the suspension using a controlled time-delayed reaction. Enzyme catalyzed reactions that are decomposing a substrate, or self-propagating decomposition reactions of a substrate, can be used. After the coagulation reaction in the suspension, the wet green body shows good mechanical properties and can be demolded, dried and sintered. Polymers show also sol-gel transitions upon changing pH, ionic strength or temperature. In case such polymer sols are combined with particles in a suspension, we obtain very rigid ceramic green bodies. Since the forming process takes place without pressure and at ambient temperatures, inexpensive molds and tools can be used. Ceramic bodies with homogeneous, defect poor microstructures can be cast using almost any ceramic powder or even powder combinations with different IEP behavior. Complex shaped components with thin and thick cross sections can be cast as there is no gradient in density and/or temperature during consolidation of the green body. High strength (680 MPa) alumina parts of high reliability (Weibull-module m = 40) have been demonstrated.
ABSTRACT In this work we describe a new ceramic green body fabrication method using ceramic collo... more ABSTRACT In this work we describe a new ceramic green body fabrication method using ceramic colloidal suspensions that allow the production of complex shaped parts of high strength and reliability. The method uses an aqueous, electrostatically stabilized ceramic suspension free of agglomerates and of low viscosity. It is cast into a mold and then coagulates forming a stiff, wet green body. Coagulation is performed by changing the pH of the suspension and/or by creating salt directly inside the suspension using a controlled time-delayed reaction. Enzyme catalyzed reactions that are decomposing a substrate, or self-propagating decomposition reactions of a substrate, can be used. After the coagulation reaction in the suspension, the wet green body shows good mechanical properties and can be demolded, dried and sintered. This new forming process DCC ( Direct Coagulation Casting ) is characterized by near net shape capability of complex shaped parts with high quality and homogeneity in the green as well as in the sintered state. Only small amounts of organic additives which amount to less than 0.5 wt-% based on ceramic powder are needed for the catalytic reactions. Since the forming process takes place without pressure and at ambient temperatures, inexpensive molds and tools can be used. Ceramic bodies with homogeneous, defect poor microstructures can be cast using almost any ceramic powder. Complex shaped components with thin and thick cross sections can be cast as there is no gradient in density and/or temperature during consolidation of the green body. Alumina parts of average bend strength (680 MPa) and high reliability (Weibull-module m=40 ) have been demonstrated.
In this \Vork \ YC <fescribe a new ceramic green body fabrication method using ceran1ic colloidal... more In this \Vork \ YC <fescribe a new ceramic green body fabrication method using ceran1ic colloidal suspensions that allow the production of complex shaped pan.s of high strength and rel iability. The method uses. ao aqueous, electrostatically stabilized ccran1ic suspension free o f agglomerates and of lo\v viscosity. his cast into a n1old and Lhen coagulates forming a stiff, wet g reen body. C' oagulation is performed by changing the pH of 1hc suspension and/or by creating salt directly inside the suspension µsi ng a eon1rolled time 4 delayed reaction Unzyme catalyzed reactions that are decomposing a substrate, or setf .. propagating decomposition rcaeLions of a substrate, can be used. After the coagulation reaction in the suspension, the \Vet green body shows good me<:hanical properties and can be demolded, dried and sintered. This nc\v forming process DCC (Direct Coagulation Casting) ls characterized by near net shape capability of cornp1cx shaped pans with high quality and homogeneity in the green as well as in the sintered state. Only sn1all atnounLs of org(lnic additives which a1nount to les:-than 0.5 \vt-% based on ceramic po' vder are ne-cdcd for the catalytic rt!actions. Since the forming process takes pltt"ec \vithout pressure and at ambient temperatures. inexpensive 1nolds and tools can be us:ed. Ccran1ie bodies with homogeneous. defect poor 1nicros1rue1ures can be cast using aln1os1 ainy cera1nie powder. Complex shaped contponcn1s with thin and thick cross seerions can be east a.~ there is no gradient in density and/or rcrnperature during oonsolidarion of the green body. Alumina pans of average bend strength (680 MPa) and high reliability (Weibul14 nlodule m-=-40) have been demonstraled.
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