Freeze gelation

Last updated December 15, 2021

Freeze-gelation, is a form of sol-gel processing of ceramics that enables a ceramic object to be fabricated in complex shapes, without the need for high-temperature sintering. The process is similar to freeze-casting.

The process is simple, but the science is, as of 2005, not well understood. The most common process involves the mixing of a silica solution with a filler powder. For example, if we were making a component out of alumina, aluminium oxide, then we would still use a silica sol, but alumina filler powder. The relative amounts used differ, normally between 3 and 4 times more filler than sol is added by weight.

A wetting agent is added, such that the filler powder disperses properly in the sol, which is mostly water. This makes the mixture doughy and stiff. The mixture is, however, highly thixotropic, so that when vibrated it turns liquid. The stiff dough is placed in a mold and the mold vibrated to liquefy the mixture, filling the mold and releasing any trapped air.

The filled mold is then frozen. On freezing, silica precipitates from the sol, forming a gel. This gel holds the filler powder together in something approximating a sintering greenform. The component is then dried in a furnace, leaving the component.

The advantages of freeze-geleation over sintering are essentially cost-based. It doesn't require high pressure equipment or powerful furnaces (drying temperatures are only just above water's boiling point), yet it creates a useful product which takes the shape of the mold very accurately.

History

In terms of being simply a process by which powder can be made into a monolith, freeze casting could be as old as the earth. A material called laminar opaline silica or LOS is believed to be formed by the freeze casting of volcanic ash, some soils containing the required sols to make the gel.

Artificially it is also an old process, having been known and studied for 100 years or more, but never brought to significant industrial application. Lottermoser, a German, wrote a paper on 'das Ausfrieren von Hydrosolen' (the Freezing of Hydrosols) in 1908. Through the 20th century various people have patented techniques using freeze-gelation, most being centred on the use of ceramics as refractory materials. A furnace lining brick, or an investment casting mold, can be easily fabricated using this method.

Recently there was a flurry of interest in freeze-casting at the University of Bath, UK, which led most significantly to two doctoral theses, by J. Laurie in 1995 and by M. Statham in 1998. Taken together in chronological order, these form a good introduction to the technique for the interested party.

Applications

To consider the applications of freeze-casting, we should consider the properties of the freeze-cast component. First, and critically, it is not fully dense. It contains only about 60–70% solid matter, the remainder being air in the form of porosity. This in turn leads to an interesting property of freeze-castings – they are often porous, not merely at the surface, but throughout their thickness. A fluid will penetrate through the pores in the casting and eventually soak through, like a sponge. This is because at porosity percentages above the 'pore percolation threshold', pores link up into continuous channels. The pore percolation threshold depends on the characteristics of the material, but it is normally very roughly around 20%. A 60% dense component has 40% porosity.

As we might expect, this amount of air in the component reduces its strength a lot. Pure, fully dense alumina, for example, is as strong as steel – far stronger if processed carefully – but freeze-cast alumina components are of similar strength to concrete. The freeze-cast component also tends to be brittle, fracturing easily.

It is unlikely then that freeze-cast components could be used structurally (without further processing – more later), but they have other properties that make them useful. They are rather light, with freeze-cast alumina components having a density somewhere in the region of 2.5 g/cm³, similar to aluminium. They are easy and cheap to make, from inexpensive and safe ingredients and using no dangerous equipment. They can take complex shapes, as they are cast, rather than machined. They can also be very large, probably larger than monolithic ceramic components made by any other process. Finally, and crucially, their porosity means that they can be infiltrated by materials with useful properties, or processed with other materials in. For example, the component could be dipped in molten copper, such that the copper is drawn up by capillary action into the porosity, increasing the conductivity of the component vastly. Alternatively, copper powder could be used as a filler powder in place of some alumina to the same end.

Freeze-cast components, in their basic form, are ideal for use as heat-resisting objects. In this way, they can be useful in metalwork, as molds or as substrates for metal spray-forming. However, with suitable post-processing, they could fulfil many other applications, such as silicon chip mounts, or even engine blocks.

Theory

The science is not particularly well understood. It has been known for years that silica sols (also known as colloidal silica, silicic acid, polysilicic acid) will gel when exposed to temperatures around 0 °C (32 °F). The theoretical mechanism is quite simple:

Colloidal silica is produced by the polymerisation of monosilicic acid, Si(OH)₄, until the chains of polysilicic acid become so long they form silica particles with hydroxylated surfaces. On freezing of the sol, the silica particles are rejected away from the solidifying interface and forced into the interstices between the ice crystals. Here, they come into contact with each other, and link via the condensation of their surface hydroxyl groups into siloxane bonds. This, happening throughout the sol, forms a gel.

In a filled sol, the ceramic powder is trapped within the gel, and forms a monolith.

Related Research Articles

In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a mold that contains a negative impression of the intended shape. The metal is poured into the mold through a hollow channel called a sprue. The metal and mold are then cooled, and the metal part is extracted. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods.

Sintering Process of forming and bonding material by heat or pressure

Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.

Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes can avoid, or greatly reduce, the need to use metal removal processes, thereby drastically reducing yield losses in manufacture and often resulting in lower costs.

$Refractory$

A refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, inorganic, non-metallic, porous, and heterogeneous. They are typically composed of oxides or carbides, nitrides etc. of the following materials: silicon, aluminium, magnesium, calcium, and zirconium. Some metals with melting points >1850 °C like niobium, chromium, zirconium, tungsten, rhenium, tantalum, molybdenum etc. are also considered refractories.

Many ceramic materials, both glassy and crystalline, have found use as optically transparent materials in various forms from bulk solid-state components to high surface area forms such as thin films, coatings, and fibers. Such devices have found widespread use for various applications in the electro-optical field including: optical fibers for guided lightwave transmission, optical switches, laser amplifiers and lenses, hosts for solid-state lasers and optical window materials for gas lasers, and infrared (IR) heat seeking devices for missile guidance systems and IR night vision.

In materials science, the sol–gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides, especially the oxides of silicon (Si) and titanium (Ti). The process involves conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network of either discrete particles or network polymers. Typical precursors are metal alkoxides.

Ceramic forming techniques are ways of forming ceramics, which are used to make everything from tableware such as teapots to engineering ceramics such as computer parts. Pottery techniques include the potter's wheel, slip casting and many others.

Metal injection molding (MIM) is a metalworking process in which finely-powdered metal is mixed with binder material to create a "feedstock" that is then shaped and solidified using injection molding. The molding process allows high volume, complex parts to be shaped in a single step. After molding, the part undergoes conditioning operations to remove the binder (debinding) and densify the powders. Finished products are small components used in many industries and applications.

A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminum and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.

Ceramic engineering Science and technology of creating objects from inorganic, non-metallic materials

Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components and the study of their structure, composition and properties.

Investment casting industrial process based on lost-wax casting

Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes.

A frit is a ceramic composition that has been fused, quenched, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble by causing them to combine with silica and other added oxides. However, not all glass that is fused and quenched in water is frit, as this method of cooling down very hot glass is also widely used in glass manufacture.

Porosity sealing is done through the process of vacuum impregnation. Vacuum impregnation is a preferred OEM process that seals porosity and leak paths in metal castings, sintered metal parts and electrical castings that form during the casting or molding process. Vacuum impregnation stops casting porosity

Porous glass is glass that includes pores, usually in the nanometre- or micrometre-range, commonly prepared by one of the following processes: through metastable phase separation in borosilicate glasses (such as in their system SiO₂-B₂O₃-Na₂O), followed by liquid extraction of one of the formed phases; through the sol-gel process; or simply by sintering glass powder.

Ceramic nanoparticle is a type of nanoparticle that is composed of ceramics, which are generally classified as inorganic, heat-resistant, nonmetallic solids that can be made of both metallic and nonmetallic compounds. The material offers unique properties. Macroscale ceramics are brittle and rigid and break upon impact. However, Ceramic nanoparticles take on a larger variety of functions, including dielectric, ferroelectric, piezoelectric, pyroelectric, ferromagnetic, magnetoresistive, superconductive and electro-optical.

Compaction of ceramic powders is a forming technique for ceramics in which granular ceramic materials are made cohesive through mechanical densification, either by hot or cold pressing. The resulting green part must later be sintered in a kiln. The compaction process permits an efficient production of parts to close tolerances with low drying shrinkage. It can be used for parts ranging widely in size and shape, and for both technical and nontechnical ceramics.

Freeze-casting, also frequently referred to as ice-templating, or freeze alignment, is a technique that exploits the highly anisotropic solidification behavior of a solvent in a well-dispersed slurry to template controllably a directionally porous ceramic. By subjecting an aqueous slurry to a directional temperature gradient, ice crystals will nucleate on one side of the slurry and grow along the temperature gradient. The ice crystals will redistribute the suspended ceramic particles as they grow within the slurry, effectively templating the ceramic.

Titanium foams exhibit high specific strength, high energy absorption, excellent corrosion resistance and biocompatibility. These materials are ideally suited for applications within the aerospace industry. An inherent resistance to corrosion allows the foam to be a desirable candidate for various filtering applications. Further, titanium's physiological inertness makes its porous form a promising candidate for biomedical implantation devices. The largest advantage in fabricating titanium foams is that the mechanical and functional properties can be adjusted through manufacturing manipulations that vary porosity and cell morphology. The high appeal of titanium foams is directly correlated to a multi-industry demand for advancement in this technology.

Gel casting is a direct foaming technique used to produce ceramic and polymeric scaffolds.

Liquid phase sintering is a sintering technique that uses a liquid phase to accelerate the interparticle bonding of the solid phase. In addition to rapid initial particle rearrangement due to capillary forces, mass transport through liquid is generally orders of magnitude faster than through solid, enhancing the diffusional mechanisms that drive densification. The liquid phase can be obtained either through mixing different powders—melting one component or forming a eutectic—or by sintering at a temperature between the liquidus and solidus. Additionally, since the softer phase is generally the first to melt, the resulting microstructure typically consists of hard particles in a ductile matrix, increasing the toughness of an otherwise brittle component. However, liquid phase sintering is inherently less predictable than solid phase sintering due to the complexity added by the presence of additional phases and rapid solidification rates. Activated sintering is the solid-state analog to the process of liquid phase sintering.

Freeze gelation

Contents

History

Applications

Theory

Related Research Articles

References

Further reading