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About High Temperature Processing

It has been understood for many centuries that heat can be used to transform materials from one state to another. This knowledge helped launch civilization with the development of ceramics and metals, fueled the speculations of alchemists, assisted the blacksmith in his trade and forms the basis of most modern techniques for producing useful materials including processes for producing steel, glass, cement, aluminium, copper and porcelain. The use of heat and the subsequent high temperatures that result is also a feature of process routes for “new” materials such as diamond coatings, semi-conductors, super conductors and fullerenes.

Steel structure.

Figure 1: Steel is an essential building block of modern civilization (Photo © openphoto.net.)

In the last two centuries, our practical knowledge of these techniques have been coupled with a profound revolution in the scientific understanding of high temperature processes. These developments have lead to greatly improved control and improved productivity for traditional materials, such as glass and steel, but have also allowed new materials processing routes to be developed, such as spin casting of metallic glasses and microwave sintering of ceramics. Our ancestors did not have the science or technology to make semi-conductors and nor could they manipulate the chemistry of steel within the range of a few parts per million.

Figure 2: Electric steelmaking plant for recycling steel (Image from www.industry.siemens.com...)

These successes aside, engineers and scientists working in the area of high temperature materials processing face new and significant challenges. Of particular importance is the need to reduce the environmental impact of these processes and develop the appropriate routes for recycling materials. This area has been a major focus for technologists since the middle of the 20th century and remains an important research and development topic. Another major area of development is in the manipulation of the structure of materials at the nanometer scale. This approach, strongly linked to developments in microscopy and fundamental physics, is reflected in the development of vapor deposition processes that can produce materials with important nanometer scale based properties and functions. Nanometer scale processing routes are still embryonic and it is unclear precisely how these materials and the processes to produce them will develop.

Why do so many processes for producing and manipulating materials involve high temperatures? The simple answer is that at high temperatures, physical and chemical changes in materials occur that may be very difficult or be indeed impossible to achieve at lower temperatures.

A good example of these important aspects of high temperature processing can be found in the manufacturing of glass. Most commercial glasses have SiO2 as their major component, other oxides such as Na2O, B2O3, CaO, MgO, Al2O3 and PbO are added in varying amounts to produce particular properties [1,2]. Glassmaking essentially involves three stages: melting, refining and cooling. Typically, the raw materials and recycled glass (“cullet”) are fed into a large rectangular furnace heated by either gas burners or electrodes with one end of the furnace being used for melting and other end for refining. The melting of recycled glass at one end of the furnace, provides a pool of material for which the feed materials such as silica (SiO2) and lime (CaO) can dissolve into. Some of the raw materials such as soda ash (Na2CO3) and limestone (CaCO3) undergo decomposition reactions as they heat up. In the refining end of the furnace, the fine bubbles formed from these reactions are allowed to separate from the melt, though the high viscosity of the melt works against the effects of bouyancy and gravity on removing these bubbles.

Figure 3: Melting of materials in glassmaking (image from ajzonca.tripod.com... )

In the case of limestone, its decomposition will only proceed above 850 °C, according to the reaction below:

  • CaCO3 (s) = CaO (s) + CO2 (g)

Like most reactions, increasing the temperature above the critical limit will greatly accelerate the kinetics of this reaction. In fact, when the kinetics of reactions, such a limestone decomposition, are not limited by the movement of the reacting and product species (“mass transfer”) and rather controlled by the actual dynamics of the reacting molecules (“reaction controlled”), the rate of the reaction generally follows an exponential relationship with temperature.

The silica (SiO2) feed, normally common sand, undergoes a series of phase transformations as it is heated[1]. The stable form of silica in ambient conditions is α-quartz, which has a trigonal crystal structure that will transform to hexagonal β-quartz at 573 °C, followed by a transformation to  β-tridymite at 870 °C, another hexagonal crystal structure. Tridymite transforms into β-cristobalite, a cubic crystal structure, at 1470 °C, before melting at 1713 °C. These various phase transformation have their own characteristic kinetics dependant on the rate of heating and cooling. For example, upon cooling of molten silica, it is possible to form both cristobalite and tridymite at room temperatures due to the very slow kinetics of their transformations. These polymorphs of silica that can co-exist with α-quartz, an equilibrium phase at room temperatures, are called metastable phases.

Figure 4: Structure of crystallised silica (A), of fused silica (B) and of sodium silicate glass (C). (Image from www.svarak.cz...)

The dissolution of Na2O, CaO and other oxides into silica will lower the melting point of the solid solution formed and consequently allow glassmaking furnaces to operate several hundred degrees below the melting point of pure silica. Dissolution of compounds into one another is generally enhanced by high temperatures because of two related effects:

  1. high temperatures generally increase the limits of solubility, and
  2. high temperatures promote rapid dissolution.

In the production of plate glass, the molten glass is drawn from the furnace by water cooled rolls that rapidly cool the molten glass below its softening temperature and to be mechanically handled into a subsequent continuous annealing furnace called a “lehr”[2]. In the float glass process the glass strand is drawn onto a molten bath of tin, the molten tin provides a low friction and flat interface that encourages a high surface quality in the plate glass produced.

The rate of cooling after being drawn from the furnace is crucial to the formation of a glass. Rapid cooling of the very viscous melt prevents the molecules within the melt from re-arranging into defined crystal structures. Instead, an amorphous solid is formed, commonly called glass, which has a liquid like molecular structure (often described as “short range order”) and vastly different properties compared to the crystalline phases formed from slower cooling. Glasses are an example of metastable materials and their unique properties illustrate the importance of rate of cooling from high temperatures on the properties of materials.

In summary, glassmaking demonstrates some important general characteristics of high temperature processes, namely:

  • High temperatures allow certain reactions and phase transformation to take place that can not occur at lower temperatures.
  • High temperatures generally promote dissolution of compounds into one another.
  • High temperatures promote rapid rates of reaction.
  • Manipulating the rate of cooling from high temperatures states can allow metastable phases to be formed.

It is interesting to consider that glass materials can also be formed from low temperature processing routes, for example, through mechanical milling [3], but these processes are very slow and difficult to scale up from the laboratory to the industrial level. High temperature processing remains the most important route for glass production.

References

  1. Swaddle T W 1997 Inorganic Chemistry, An Industrial and Environmental Perspective (San Diego: Academic Press) p 142-144.
  2. Evans J W and DeJonghe L C 1991 The Production of Inorganic Materials (MacMillan Publishing) p 279-281.
  3. Murty B S and Ranganathan S 1998 International Materials Reviews 43-3 p 101-141.

G.Brooks, February 2008