Agglomerates in nanopowders and ceramic technology

Abstract

Questions are considered connected with use in ceramic technology of ultrafine powder raw material with agglomerated particles. It is shown that presence of agglomerates in nanopowders unavoidably leads to formation in sintered material of structural defects in the form of residual porosity and microcracks. Data are provided for a quantitative dependence of pore structure characteristics for a powder compact on the content of agglomerated particles and the connection of pore structure characteristics with sintering kinetics and the properties of the sintered material obtained.One of the main tendencies in the development of contemporary ceramic material technology is the extensive use of powders with a submicron particle size. Use of powder with particles of this size makes it possible to intensify sintering considerably, to promote preparation of material with a fine grain structure and improved physicomechanical properties.Recently quite productive technology has been developed for preparing ultrafine powders, however, attempts to use them in mass production often does not lead to the expected effect. In spite of the predicted theory, compacts of submicron powders after sintering do not reach an acceptable density, they have coarse unsintered pores and other defects within them, thus worsening material properties [1, 2]. A powder system of nanoparticles should not be considered as a simple collection of individual particles. As a result of the high specific surface in these powder systems the finest individual particles are collected into well-formed, sometimes strongly sintered groups of particles, that have been given the name agglomerates. An example is shown in Fig. 1 of an electron microscope picture of agglomerates in submicron powders of transformation-strengthened structural ceramic of the composition ZrO 2 + 3 mol.% Y 2 O 3 . With an average size of individual particles of 0.036 mm the average size of agglomerates is 5.8 mm, i.e. it exceeds the size of individual particles by more than two orders of magnitude. The pore structure of a compact of agglomerated powder is extremely nonuniform. Apart from the finest pores between individual submicron powder particles (intra-agglomerate) they contain a significant number of coarse high-coordination 2 inter-agglomerate pores. These pores in size exceed the intra-agglomerate sizes by a factor of several tens or hundreds. Presence of two types of pores is confirmed by the nature of the distribution of pores in a compact. This distribution of the pore volume within a compact is shown in Fig. 2, compacted from ultrafine agglomerated ZrO 2 (Y 2 O 3 ) powder with an avFig. 1. Agglomerates in submicron powders of the composition ZrO 2 + 3 mol.% Y 2 O 3 : a) collection of agglomerates in powder at low magnification; b ) structure of an individual agglomerate. * The coordination number of a pore equals the number of particles surrounding it.

Powder Compact Structure. Part 1. Particle Packing Inhomogeneity

“…Pictures are shown in Fig. 2 of agglomerates in submicron powders [2]. Presence of agglomerates within raw material often directly determines inhomogeneity for a powder compact obtained from them.…”

“…12) [2]. In the initial sintering stage, when there is removal of fine little coordinated pores within a volume of densely packed compact made from individual particles, and within intra-agglomerate regions of a compact with agglomerates, the compaction process proceeds by a simple "scenario."…”

HCBS ceramic concretes in the XXI century — problems and prospects for applying technology in the field of silicate materials science. Part 1

“…In spite of the fact that currently adequate production methods have been developed for preparing nano and ultrafine powders, their use in mass production of engineering ceramics has not produced the expected effect [26]. As applied to ceramic technology this is due to two important situations.…”

“…Here the pore structure of compacts is extremely nonuniform as a result of the fact that alongside fine pores between nanoparticles (intra-agglomerate) there is also formation of coarse inter-agglomerate pores. In addition, the average diameter of the latter is greater by a factor of ten or one hundred than for intra-agglomerate pores [26]. A result of the features in question nanocompacts of nanopowders after sintering do not achieve high density as a result of the fact that they have internal coarse unsintered pores whose development is due to the agglomerated structure of the starting materials [26].…”

Sintering of nanostructured powders

“…This feature involves the fact that they are dense, unagglomerated particles, as a rule of regular spherical shape with a specific choice of synthesis parameters, and they are polycrystalline formations with a grain size of the order of several nanometers and a developed network of intraparticle intergranular boundaries [1]. With comparatively low fineness (average particle size 1 -3 mm) the unique energy saturation of these polycrystalline particles provides high activity during sintering [2]. The regular shape and absence of agglomerate formations promotes preparation of dense particle packing in a powder mold, sintering at comparatively low temperature without unjustified active development of recrystallization, leading to undesirable grain growth.…”