Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Dispersions and stability of ceramic powders in liquids
YKI – Ytkemiska institutet.
1993 (English)In: Surface and Colloid Chemistry in Advanced Ceramics Processing / [ed] Pugh, R.J. & Bergström, L., Marcel Dekker, 1993, p. 127-192Chapter in book (Refereed)
Abstract [en]

The homogenization, dispersion, and stability of particles in liquids are primary important steps in the processing of highperformance ceramics produced by conventional slurry consolidation methods such as tape casting and slip casting. In industry today, it is generally accepted that the driving force and efficiency of sintering is influenced by the basic powder properties such as purity, grain size, and chemical heterogeneity. However, more recent research studies suggest that the packing and distribution of particles throughout the green body control the porosity and microstructure and pX an important role in determining the reliability of the final product [1 -4]. Essentially, the aim of the dispersion process is to achieve a high solids (in some cases today >60 vol %) homogeneous suspension with a welldefined rheological behavior and possibly an intermediate (but at present not well defined) degree of stability. This enables the ceramic to be formed into complex geometrics with a defect free microstructure after sintering. In single component systems, the uniformity of packing generally depends not only on the particle size distribution, but also the number of secondary agglomerates which should be reduced to a minimum, since they cause defects such as voids and cracks in the composite. In other cases, where two, three, or even four different colloidal components are mixed together in the fluid, the situation becomes more complex, and it is difficult to obtain a homogeneous dispersion of particles. For example, a typical slurry may contain matrix material plus one or two sintering aids, plus fibers. Under such circumstances, segregation, coagulation, and separation of components —or worse, some degree of heterocoagulation—can occur during processing, and this is obviously detrimental, generating cracks and waping during sintering. From a fundamental point of view, the process of homogenization and dispersion of a powder in a liquid may be dealt with in several stages [5]. Initially, wetting must occur, in which the liquid phase wets and spreads on the powder's external surface, and also air must be displaced from the internal pores during immersion. At a later stage, secondary units (clusters or agglomerates) must be broken down or fractured into essentially primary units. Finally, the primary particles must remain dispersed through the liquid medium and reagglomeration prevented by some type of stabilization mechanism. In the case of the wetting of a high energy ceramic powder surface (such as a metal or oxide) with a lower energy liquid (such as water or a hydrocarbon) then the wett i ng and im m e rsion processes occur alm ost sim ul taneously. Generally, these processes are both irreversible and rapid. However, the dispersion and stability steps (in the obsence of any forrn of stabilization barrier) are more complex and may occur on different time scales, depending on the particle concentration in suspension. For example, in a concentrated colloidal suspension, the dispersion of the components in a ball mill may occur relatively fast, but after the slip is poured into the mold this "breakup step" may be rapidly reversed by coagulation. This can be illustrated from the classical theory of second order perikinetic coagulation kinetics as described by Smoluchowski [6], dealing with the rate of rapid coogulation of monodispersed colloidal particles. The rapid coagulation rate that occurs in the absence of any stabilization mechanism can be expressed in terms of the half life t1/2^ or the time to reduce the number of particles in the system to half the original value; it may be represented by 3X} 2 x lOll t1Q = = seconds in water at 20°C (1) 4kBTNo N0(cm~3) where No is the original particle concentration of the fully dispersed system expressed in particles/cm3, rl is the viscosity of the liquid medium, kB is the Boltzmann constant, and T is the temperature. For a dilule colloidal suspension in water, where for example No = 107 particles/cm3, then at room temperature tlQ would be several hours, such that the system could be regarded as being relatively stable, whereas for a concentrated suspension, then No " 1014 particles/cm3, and t1/2 would be reduced to milliseconds (the process would be reversed extremely rapidly). In ceramic powder dispersions where the size and density of the particles can be fairly well defined, values of tig can be easily determined and compared to the sedimentation rate. Generally, the concentration, particle size, and viscosity (as controlled by the addition of binder) can all have a pronounced influence on stability and sedimentation. This is clearly illustrated in Table 1. In concentrated suspensions, it is essential therefore to introduce some type of stabilizing agent, especially in highly concentrated suspensions, to prevent reagglomeration from occurring. In the case of slow coagulation, the process is essentially controlled by the interparticle interactions that have the net effect in controlling the overall state of the dispersion. In systems where attra clive forces dom i nate the inte raction, the system become unstable, and the particles coagulate. This causes an increase in viscosity and sedimentation, and in concentrated dispersions it may produce a structured green body. In cases where repulsive forces are strong, then a relatively stable, well dispersed (lower viscosity) suspension of individual particles will be formed. In recent years, the importance of colloidal processing has been strongly emphasized, and research studies suggest that defect free composites with st ructural rel iabil i ty can be produced from a highly co nce ntrated suspension, provided the stability is critically controlled [7-9]. For example, the suspension must be sufficiently colloidally stable and free from agglomerates and unwanted foreign bodies; yet it must be sufficiently fluid to be easily formed into the desired shape. Also, the packing should be sufficiently dense to give a minimum pore volume. This can only be achieved by control of the particle size distribution and a balance of the interparticle surface forces. Too strong an interparticle attraction must be avoided, since particles tend to stick on contact, causing flocs or open structures; but also strong repulsion causes a packed structure with a low density, which causes difficulties in removing the liquid during heat treatment. It would therefore seem reasonable to aim for an intermediate degree of stability. By use of surface active processing chemicals for controlling the interparticle forces, it would appear feasible to meet this requirement. In practice, however, the situation is often more complicated, since if a suitable dispersant is found, it may not stabilize all the particles to the same extent in a multicomponent system. In the case of not finding a single dispersant, then several dispersants may be added; but this may cause competitive adsorption to occur on the different components. Before these types of problems can be discussed further, it is necessary to review the theoretical background to the wetting, dispersion, and stability steps in the following sections. However, to begin it is important to devote some effort to considering the types of particles and organizations that may occur in ceramic powders.

Place, publisher, year, edition, pages
Marcel Dekker, 1993. p. 127-192
Series
Surfactant Science Series ; 51
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:ri:diva-13595OAI: oai:DiVA.org:ri-13595DiVA, id: diva2:987358
Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2016-09-29Bibliographically approved

Open Access in DiVA

No full text in DiVA

By organisation
YKI – Ytkemiska institutet
Natural Sciences

Search outside of DiVA

GoogleGoogle Scholar

urn-nbn

Altmetric score

urn-nbn
Total: 611 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
v. 2.34.0