Material Structure of Alumina

Structure of material focuses more on relationships between physical and chemical properties of certain material. Material is a physical substance that things can be made from. It can be any equipment or physical substance that need in daily life. (Cambridge University Press, 2012) Structure is a construction or framework of identifiable elements includes components, entities, factors, members, parts, steps and more which gives form and stability, and resists stresses and strains. Structures have defined boundaries within each element is physically or functionally connected to the other elements, the elements themselves and their interrelationships are taken to be either fixed or changing only occasionally or slowly. (Business Dictionary, 2016)

The structure of materials can be classified by the general magnitude of various features. There are three most common major classifications that listed generally in increasing size. Atomic structure has special features of cannot be seen, for example the bonding types between the atoms and its arrangement. Microstructure has features that can be seen easily using a microscope while macrostructure can be seen with the naked eye. The atomic structure primarily affects the chemical, physical, thermal, electrical, magnetic, and optical properties. The microstructure and macrostructure can also affect these properties but they generally have a larger effect on mechanical properties and on the rate of chemical reaction.

Alumina(Al2O3) normally appear to be white, is the chemicals composition of aluminium and oxygen. It is the main component in production of aluminium metal also serves as the raw material in making ceramics and an active agent in chemical processing. (Encyclopaedia Britannica, 2016) Alumina, the white crystalline powder, is made of bauxite which is naturally occur in various amount of hydrous aluminium oxides as the mineral corundum and some of it form the precious gemstones ruby and sapphire. Corundum is a rock-forming mineral appears as solid crystalline form of aluminium oxide containing traces of iron, titanium, vanadium and chromium.

Rubies and sapphires are gem forms of corundum, which has special characteristic in colour to trace impurities. Rubies are given their characteristic deep red colour and their laser qualities by traces of chromium. Sapphires come in different colours given by various other impurities, such as iron and titanium. (Wikipedia, 2016) Alumina has advantages such as its thermal, chemical, and physical properties when compared with several ceramics materials, and is widely used for firebricks, abrasives and integrated circuit (IC) packages.

Alumina is widely used compare to other materials in ceramic production. This is because it has very good electrical insulation, mechanical strength from moderate to extremely high also very high compressive strength which is basically from 2,000 to 4,000 MPa. Other than that, it is naturally hard and has moderate thermal conductivity. Alumina also can stand high resistance and less corrosive. Besides, it has good gliding properties, low density, bio-inert and food compatible.


Figure above shows a photograph of three thin disk specimens of alumina which have been placed over a printed page in order to demonstrate their differences in light-transmittance characteristics. The disk on the left is transparent, whereas the one in the middle is translucent and, the disk on the right is opaque. All the light that reflected from the printed page is passes through the transparent disk which are made of monocrystalline.

Polycrystalline materials, are solids that are comprised of a collection of crystallites, separated from one another by areas of relative disorder known as grain boundaries. Translucent disk has the structure of numerous and very small single crystals that are all connected. The boundaries between these small crystals scatter a portion of the light reflected from the printed page. Thus, some of the light reflected from the printed page is transmitted through the disk. The grain boundaries of high purity of alumina disks are very clean. Exceptional for the opaque disk, none of the light passes through it. 

Opaque disk composed of amorphous structure not only of many small, interconnected crystals, but also of a large number of very small pores or void spaces. These pores also effectively scatter the reflected light and render this material opaque. These differences in optical properties are a consequence of differences in structure of these materials, which have resulted from the way the materials were processed. (Specimen preparation, P.A. Lessing; photography by S. Tanner.) (William D. Callister, Jr., 2007)

Both monocrystalline and polycrystalline are crystalline solid. Monocrystalline structure shows periodic across the whole volume and perfect repeated arrangement of atoms that extends throughout the entirety of the specimen without interruption. All unit cells have the same orientation and interlock in the same way. It has crystal a regular geometric shape with flat faces. This transparent material is highly used in semiconductor industries, optics, material engineering and electrical conductors, refractories, insulators and heat radiation plates, metal matrix composites, high temperature materials and heat sinks, integrated circuit packages and substrate. (G.P. Thomas, 2016)

Polycrystalline composed of a collection of many small crystals or grains. It has small crystals or nuclei form at various positions. These have random crystallographic orientations. There is some atomic mismatch between the grains within the region called grain boundary. Grain boundaries disrupt the motion of dislocations through a material. Dislocation propagation is impeded because of the stress field of the grain boundary defect region and the lack of slip planes and slip directions and overall alignment across the boundaries. Reducing grain size is therefore a common way to improve strength. Translucent material usually used in potteries and porcelains. Amorphous is a type of non-crystalline solid that is less systematic and has irregular arrangement of atoms over relatively large atomic distances. Amorphous materials are characterized by atomic or molecular structures that are relatively complex and become ordered only with some difficulty. (William D. Callister, Jr., 2007)

Focusing on optical properties of Alumina, will explain more on its light transmittance based on three principle effects which are Reflection (R), Absorption (A) and Transmittance (T). Reflection defined as the portion of light that bounces-off an alumina surface. Absorption means light that is captured-by and dissipated as heat within a material as it passes through while transmittance is a measure of light transmitted through alumina without being absorbed or reflected but only scattered. (Dow Corning, 2012)

Optical property means a material’s response to exposure to electromagnetic radiation and visible light. When light travels from one medium into another, several things happen. Some of the light radiation may be transmitted through the medium, some will be absorbed, and some might be reflected at the interface between the two medias. (William D. Callister, Jr., 2007) Lord Rayleigh, a British physicist has been developed a theory of light scattering to describe the light transmission properties on materials’ surface. It defines as elastic scattering of light or other electromagnetic radiation by particles that is smaller than the wavelength of the radiation. Rayleigh scattering does not change the state of material hence it is a parametric process. The particles may be individual atoms or molecules. It can occur when light travels through transparent solids and liquids. The transmission is varied due to the presence of grain boundaries, pores and type of surfaces. (Wikipedia, 2016)

A transmitted light beam is deflected in direction and appears diffuse as a result of multiple scattering events. Opacity results when the scattering is so extensive that virtually none of the incident beam is transmitted, not deflected, to the back surface. The index of refraction for polycrystalline material normally appeared translucent. Both reflection and refraction occur at grain boundaries, which causes a diversion in the incident beam.

This results from a slight difference in index of refraction between adjacent grains that do not have the same crystallographic orientation. Scattering of light also occurs in two-phase materials in which one phase is finely dispersed within the other. The beam dispersion occurs across phase boundaries when there is a difference in the refractive index for the two phases. The greater its difference, the more efficient is the scattering. Monocrystalline material will appear highly transparent.

The sizes of the crystallites are smaller than the wavelength of visible light, and the indices of refraction of the two phases are nearly identical. As a consequence of fabrication or processing, many ceramic pieces contain some residual porosity in the form of finely dispersed pores. These pores also effectively scatter light radiation. The single crystal is totally transparent, polycrystalline and porous materials are respectively, translucent and opaque. Some scattering of visible light occurs at the boundaries between crystalline and amorphous regions, again as a result of different indices of refraction. For highly crystalline specimens, this degree of scattering is extensive, which leads to translucency, and even opacity. Highly amorphous polymers are completely transparent. (William D. Callister, Jr., 2007)

Besides the variety on optical properties, these disks material were produced with different processing technique. Processing of materials and manufacturing can take place in open atmosphere, ovens, high temperature furnaces or in specifically designed reactors. Monocrystalline material of optical sapphire is known to possess highly desirable optical and physical characteristics, such as high strength, high density and a high degree of transparency, as well as the ability to retain these properties, to a large degree, at elevated temperatures. In recent years, many advances have been made in production of transparent ceramics by various processing and sintering strategies. Powder granulation and dry pressing for transparent ceramic applications has been demonstrated. (Michael Stuer, Zhe Zhao, Paul Bowen, 2012)

The polycrystalline translucent ceramic material is formed by pressing a powder material, such as high purity alumina, into a desired shape, sintering the shaped material to provide closed porosity, and subjecting the sintered material to hot isostatic pressing to yield a single phase material having substantially zero porosity and an average grain size of no greater than 1.0 micrometre. This small grain size contributes to a stronger material than conventional ceramic materials without detrimentally effecting translucency. This is surprising as small grain size is often considered to prevent relatively high translucency. Preferably, the material has a flexure strength of at least about 400 MPa. (Sumitomo Chemical, 1993)

In order for these amorphous powders of alumina to be made into bulk samples, it is necessary to develop new compositions that will require a much lower critical cooling rate and high thermal stability. To achieve this, amorphous powders can be produce by radiant heating. The radiation sources can be either electric or gas fired. Amorphous alumina coatings were processed by metalorganic chemical vapour deposition (MOCVD) with certain chemical properties. The processing temperature should be in the range of 350 oC–700 oC at low pressure of 5 Torr. The surface of low temperature processed amorphous films is hydrophobic where water contact angled 106 degrees. It is concluded that amorphous alumina coatings can be used as oxidation and corrosion barriers at ambient or moderate temperature. The process involves nitrogen as a carrier of dilution gas. (J. Nanosci. Nanotechnol, 2011)

In conclusion, the transparent disk of alumina absorbs, reflects and transmits large amount of light compare to translucent and opaque disk. This is due to the difference in structure of monocrystalline structure in the transparent disk which helps to reduce the scattering of lights. Monocrystalline structure is well arranged without any grain boundaries, no spaces between the crystalline that form good surface of alumina. Thus, the printed page placed under the transparent alumina disk can be seen clearly.


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