Sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure[1] or heat[2] without melting it to the point of liquefaction.
Examples of pressure-driven sintering are the compacting of snowfall to a glacier, or the formation of a hard snowball by pressing loose snow together.
In some special cases, sintering is carefully applied to enhance the strength of a material while preserving porosity (e.g. in filters or catalysts, where gas adsorption is a priority).
On a microscopic scale, material transfer is affected by the change in pressure and differences in free energy across the curved surface.
The change in energy is much higher when the radius of curvature is less than a few micrometers, which is one of the main reasons why much ceramic technology is based on the use of fine-particle materials.
[3] The ratio of bond area to particle size is a determining factor for properties such as strength and electrical conductivity.
To yield the desired bond area, temperature and initial grain size are precisely controlled over the sintering process.
[3] The source of power for solid-state processes is the change in free or chemical potential energy between the neck and the surface of the particle.
[5] Industrial procedures to create ceramic objects via sintering of powders generally include:[6] All the characteristic temperatures associated with phase transformation, glass transitions, and melting points, occurring during a sinterisation cycle of a particular ceramic's formulation (i.e., tails and frits) can be easily obtained by observing the expansion-temperature curves during optical dilatometer thermal analysis.
By matching the material and particle size to the ware being sintered, surface damage and contamination can be reduced while maximizing furnace loading.
In most cases, the density of a collection of grains increases as material flows into voids, causing a decrease in overall volume.
Mass movements that occur during sintering consist of the reduction of total porosity by repacking, followed by material transport due to evaporation and condensation from diffusion.
Sintered bronze in particular is frequently used as a material for bearings, since its porosity allows lubricants to flow through it or remain captured within it.
Sintered copper may be used as a wicking structure in certain types of heat pipe construction, where the porosity allows a liquid agent to move through the porous material via capillary action.
For materials that have high melting points such as molybdenum, tungsten, rhenium, tantalum, osmium and carbon, sintering is one of the few viable manufacturing processes.
Sintering of powders containing precious metals such as silver and gold is used to make small jewelry items.
Evaporative self-assembly of colloidal silver nanocubes into supercrystals has been shown to allow the sintering of electrical joints at temperatures lower than 200 °C.
[7] Particular advantages of the powder technology include: The literature contains many references on sintering dissimilar materials to produce solid/solid-phase compounds or solid/melt mixtures at the processing stage.
Sintered plastics are used in applications requiring caustic fluid separation processes such as the nibs in whiteboard markers, inhaler filters, and vents for caps and liners on packaging materials.
Liquid phase sintering was successfully applied to improve grain growth of thin semiconductor layers from nanoparticle precursor films.
[10][11] English engineer A. G. Bloxam registered in 1906 the first patent on sintering powders using direct current in vacuum.
The primary purpose of his inventions was the industrial scale production of filaments for incandescent lamps by compacting tungsten or molybdenum particles.
The starting boron–carbon or silicon–carbon powders were placed in an electrically insulating tube and compressed by two rods which also served as electrodes for the current.
The steps were: (i) molding the powder; (ii) annealing it at about 2500 °C to make it conducting; (iii) applying current-pressure sintering as in the method by Weintraub and Rush.
[12] Sintering that uses an arc produced via a capacitance discharge to eliminate oxides before direct current heating, was patented by G. F. Taylor in 1932.
In spark plasma sintering (SPS), external pressure and an electric field are applied simultaneously to enhance the densification of the metallic/ceramic powder compacts.
However, the grain size changes in other ceramic materials, like tetragonal zirconia and hexagonal alumina, were not statistically significant.
The power behind the densification is derived from the capillary pressure of the liquid phase located between the fine solid particles.
Mechanisms 4–6 are densifying – atoms are moved from the bulk material or the grain boundaries to the surface of pores, thereby eliminating porosity and increasing the density of the sample.
These improvements are in general in the form of a support made from an inert and thermally stable material such as silica, carbon or alumina.
