Glass is an amorphous (non-crystalline) solid material. Glasses are typically brittle and opticallytransparent.
The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, made of about 75% silica (SiO2) plus Na2O, CaO, and several minor additives. Often, the term glass is used in a restricted sense to refer to this specific use.
In science, however, the term glass is usually defined in a much wider sense, including every solid that possesses a non-crystalline (i.e. amorphous) structure and that exhibits a glass transitionwhen heated towards the liquid state. In this wider sense, glasses can be made of quite different classes of materials: metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers. For many applications (bottles, eyewear) polymer glasses (acrylic glass, polyethylene terephthalate) are a lighter alternative to traditional silica glasses.
Glass, as a substance, plays an essential role in science and industry. Its chemical, physical, and in particular optical properties make it suitable for applications such as flat glass, container glass, optics and optoelectronics material, laboratory equipment, thermal insulator (glass wool), reinforcement materials (glass-reinforced plastic, glass fiber reinforced concrete), and glass art (art glass, studio glass).
Silicate glass
Silica (the chemical compound SiO2) is a common fundamental constitute of glass. In nature, vitrification of quartz occurs when lightningstrikes sand, forming hollow, branching rootlike structures called fulgurite.
[edit]History
Main article: History of glass
The history of creating glass can be traced back to 3500 BCE in Mesopotamia.[1] The term glass developed in the late Roman Empire. It was in the Roman glassmaking center at Trier, now in modern Germany, that the late-Latin term glesum originated, probably from a Germanicword for a transparent, lustrous substance.[2]
[edit]Glass ingredients
While fused quartz (primarily composed of SiO2) is used for some special applications, it is not very common due to its high glass transition temperature of over 1200 °C.[3] Normally, other substances are added to simplify processing. One is sodium carbonate (Na2CO3), which lowers the glass transition.[clarification needed] However, the soda makes the glass water soluble, which is usually undesirable, so lime (calcium oxide (CaO), generally obtained from limestone), somemagnesium oxide (MgO) and aluminium oxide (Al2O3) are added to provide for a better chemical durability. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass.[4] Soda-lime glasses account for about 90% of manufactured glass.
Most common glass has other ingredients added to change its properties. Lead glass or flint glassis more 'brilliant' because the increased refractive index causes noticeably more specular reflectionand increased optical dispersion. Adding barium also increases the refractive index. Thorium oxidegives glass a high refractive index and low dispersion and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eye glasses.[citation needed] Iron can be incorporated into glass to absorb infrared energy, for example in heat absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs UV wavelengths.[5]
Borosilicate glasses (e.g. Pyrex) have as main constituents silica and boron oxide. They have very low coefficients of thermal expansion (7740 Pyrex COE is 32.5×10–7/°C as compared to 8.36×10−5/°C for one type of soda-lime glass),[6] making them more dimensionally stable. The lower COE also makes them less subject to stress caused by thermal expansion, thus less vulnerable to cracking from thermal shock. They are commonly used for reagent bottles, optical components and household cookware.
Another common glass ingredient is "cullet" (recycled glass). The recycled glass saves on raw materials and energy. However, impurities in the cullet can lead to product and equipment failure.
Fining agents such as sodium sulfate, sodium chloride, or antimony oxide may be added to reduce the number of air bubbles in the glass mixture.[4] Glass batch calculation is the method by which the correct raw material mixture is determined to achieve the desired glass composition.
[edit]Contemporary glass production
Following the glass batch preparation and mixing, the raw materials are transported to the furnace.Soda-lime glass for mass production is melted in gas fired units. Smaller scale furnaces for specialty glasses include electric melters, pot furnaces, and day tanks.[4]
After melting, homogenization and refining (removal of bubbles), the glass is formed. Flat glass for windows and similar applications is formed by the float glass process, developed between 1953 and 1957 by Sir Alastair Pilkington and Kenneth Bickerstaff of the UK's Pilkington Brothers, who created a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity. The top surface of the glass is subjected to nitrogen under pressure to obtain a polished finish.[7] Container glass for common bottles and jars is formed by blowing and pressing methods. Further glass forming techniques are summarized in the table Glass forming techniques.
Once the desired form is obtained, glass is usually annealed for the removal of stresses. Surface treatments, coatings or lamination may follow to improve the chemical durability (glass container coatings, glass container internal treatment), strength (toughened glass, bulletproof glass, windshields), or optical properties (insulated glazing, anti-reflective coating).
[edit]Architecture
Main articles: Architectural glass and Glazing
The use of glass in buildings is a transparent feature to allow light to enter into rooms and floors, illuminating enclosed spaces and framing an exterior view through a window. It is also a material for internal partitions and external cladding.
[edit]Glassmaking in the laboratory
New chemical glass compositions or new treatment techniques can be initially investigated in small-scale laboratory experiments. The raw materials for laboratory-scale glass melts are often different from those used in mass production because the cost factor has a low priority. In the laboratory mostly pure chemicals are used. Care must be taken that the raw materials have not reacted with moisture or other chemicals in the environment (such as alkali oxides and hydroxides, alkaline earth oxides and hydroxides, or boron oxide), or that the impurities are quantified (loss on ignition).[8] Evaporation losses during glass melting should be considered during the selection of the raw materials, e.g., sodium selenite may be preferred over easily evaporating SeO2. Also, more readily reacting raw materials may be preferred over relatively inertones, such as Al(OH)3 over Al2O3. Usually, the melts are carried out in platinum crucibles to reduce contamination from the crucible material. Glass homogeneity is achieved by homogenizing the raw materials mixture (glass batch), by stirring the melt, and by crushing and re-melting the first melt. The obtained glass is usually annealed to prevent breakage during processing.[8][9]
In order to make glass from materials with poor glass forming tendencies, novel techniques are used to increase cooling rate, or reduce crystal nucleation triggers. Examples of these techniques include aerodynamic levitation (cooling the melt whilst it floats on a gas stream), splat quenching (pressing the melt between two metal anvils) and roller quenching (pouring the melt through rollers).
No comments:
Post a Comment