Introduction of Glasses

Introduction of Glasses

• The History of Glasses:

It is totally uncertain to talk about who prepared the first glass and how [1]. But it is thought that firstly glasses were prepared in Mesopotamia (middle-east) nearly 3600 years ago. There is a story about origin of glasses that they were discovered accidently by merchants. They were travelling on a ship loaded with ammonium nitrate and stopped on a sea coast. They were cooking their meal and they need some underprop for holding their pots. So, they took some nitrate blocks to place them under their pots on fire. They observed that nitrate blocks got melted and mixed with the sand of sea coast and thus converted into a clear shiny liquid, perhaps known as glass [2]. Almost in 1500 B.C an artist from Egypt employed a process to prepare glass craft. Glasses were used in making pearls, toys, jewelry. In the 16^th century, due to good reflectance property, glasses were used in mirror making in European countries but that trend was not common in eastern countries like India, China etc. Before that in 13^th century eye glasses were prepared for better sight property in European countries [1].

On industrial scale, glass was first used in making a building of crystal palace in 1850’s and that extraordinary building gave an insight to the architectures to make the household and other crafts from glass [2].

• Glass:

The term “GLASS” comes from a Late Latin term “Glaseum” that stands for transparency and lustrous property of a material. Due to different types of glasses and their different characteristics, glasses may be defined in a variety of the ways.  In 1930, it was defined as a material without any periodicity and non- crystalline or amorphous in nature. In almost 1940’s glass was defined as an inanimate material which is similar to the liquid phase of that material, but its viscosity is opposite to that of a liquid as its viscosity is increased in a sudden cooling process. Glasses may be used in preparing hard rigid materials.

By American Society for Testing Materials (ASTM), glass is an inanimate substance of fusion that was cooled to a rigid condition without any proper crystallization [3]. A glass can also be defined as a non- crystalline material with a structure that has no long range regularity in arrangement of atoms. Glasses also show a time dependent transformation phenomena at a certain value of temperature called glass transition temperature T_g.

There are wide spread applications of glasses depending upon their excellent qualities which include rigidity, translucency, brittleness, high resistance against corrosion and appropriate electric current insulator.

So, glass is not a crystalline solid, without any periodicity of structure, hence it is an amorphous material that is usually translucent. It has a lot of applications such as in decorative material and in optoelectronic devices [4].

• Nature of Glass:

Mostly glasses are made by a process which involves first converting the crystalline solid into its molten form or liquid. This is accomplished at a very large value of temperature and then finally on quenching or sudden cooling of this melt, a glassy solid is formed. But in the whole process liquid must not be crystalized. Due to this sudden cooling one can say glasses are super cooled melts. There are also some other techniques of glass formation like sol gel technique, chemical vapor deposition method and by radiotherapy of heavy ions. But mostly we use melt and quench technique for glass preparation. As long as atomic structure is related, in glasses atomic arrangement changes in each unit cell. So in order to define glass structure, complete study of whole glass structure is required. Study of single unit cell does not give complete information about structure of glass.

Angell defined glass as, “A glass is an amorphous solid which is capable of passing continuously into viscous liquid state, usually but not necessarily accompanied by an abrupt increase in heat capacity.” Usually glasses are inorganic solids but if there are other materials like organic or metallic solids which have properties of glasses i-e irregular atomic arrangement, have rigidity, transparency and similar properties of glass transition so that materials also come in glassy materials category. Glasses are mechanically hard and have high value of compression strength. Their stress to strain behavior is linear in elastic region. They exhibit high value of thermal insulation and they are excellent dielectric materials as they have high value of dielectric constant [5].

• Transformation Range of Glass:

The conversion of proper liquid phase of material to a solid one is a step by step continuous process and this conversion occurs in a specific temperature range which is called transformation range of glass. So the value of temperature in this range where a glassy state without any crystallization phase is achieved is called transition temperature T_g of glass. In glass formation, there is continuous change in volume of super cooled liquid but in crystallization, this change of volume is discontinuous [6].

• Types of Glasses:

Different types of glasses are found depending on their different composition, material used for their preparation. Also the glasses prepared by same material have different characteristics like structure, mechanical strength, chemical behavior and other electrical properties. This is due to change in composition of glass. There are a lot of inorganic glasses discussed below:

1.5.1- Vitreous silica:

Vitreous silica also known as quartz glass or fused quartz is mostly prepared by a single material that is silica. It has a good level of transparency for near UV radiation. As it is made from purified silica only so it melts at a higher temperature as compared to other glasses. Its mechanical strength is high as its hardness number is high. Vitreous silica has no color and widely used in electronics parts [7]. Vitreous silica looks simple as based on a single component but it has a drawback of high melting temperature and also shows other complex behavior [8].Vitreous silica has high electrical resistivity; value of thermal expansion coefficient is given by approximately 550×10^-9 /⁰C. But preparation of vitreous silica is too much expensive hence it has less number of applications as in fiber optics etc.

1.5.2- Borosilicate glasses:

As the name indicates borosilicate glasses consist of boron oxide and silicon oxide, in which silicon oxide is in higher percentage than borate. Also some alkali oxides K, Na are added in small quantities. Such glasses highly resist corrosion which may be due to some chemical reactions. Borosilicate glasses work in temperature range of 1823 ^°K to 1873 ^°K, which is a little higher than that of soda lime glasses. Their coefficient of thermal expansion is quiet low i-e approximately 300×10^-8 to 600×10^-8 / ^°C. These glasses are being applied in vehicle headlights, laboratory glass equipment. They are also employed in nuclear reactors to provide a medium so that unnecessary irradiated species like ions are prevented to interfere with reaction under progress in reactor [9]. They are also used in kitchen utensils and in textile industry as a useful fiber [5].

1.5.3- Soda Lime glasses:

Soda lime glasses also known as commercial glasses as they are cost friendly so find applications in many fields like in glass made window panes, liquor vessels and fluorescent bulb covers, this commercial glass is used. This glass is composed of silica, calcium carbonate, sodium carbonate and traces of alumina and magnesium oxide up to 3 wt%. When all such compounds are mixed and melted at a high temperature of almost 1500 ^°C then soda lime glass is obtained. There are many good properties of soda lime glasses which make them commercial glasses. They have excellent transparency in visible region of spectrum. Because of their low electrical conductivity, they can be used as an insulating material. It also occupies good chemical endurance. But its uses are limited due to high value of thermal expansion coefficient i-e approximately 1000×10^-8 / ^°C and can’t withstand high temperatures which results into mechanical fracture.

1.5.4- Lead Silicate glasses:

Such glasses consist of lead oxide and silica as primary components of glass formation and also traces of potassium hydroxide. These glasses have very useful properties like high value of luminosity, large value of operating temperature and low electrical conductivity. Due to which these glasses find applications in electrical feedback circuits for example in feedback providing components. Fluidness of glass is increased by adding PbO in silica, due to which such glasses are used in microelectronic components as a dielectric layer.

1.5.5- Aluminosilicate glasses:

Aluminosilicate glasses belong to such commercial family of glasses which consist of higher percentage of silica than alumina. They also contain some amount of alkali and alkaline earth metals in average ranges like MgO, CaO, B₂O_3, and NaO etc. The value of modulus of elasticity and resistance to chemical attack are increased by adding alkali metals in smaller amount in such glasses. There is close correspondence in value of electrical resistance in between vitreous silica and aluminosilicate glasses containing only alkaline earth components. The thermal properties of these glasses like value of coefficient of thermal expansion and refractory nature lie in between fused silica and soda lime glasses [9]. These glasses have a higher value of melting point as compared to borosilicate glasses.

Such glasses have a wide range of applications in technical fields like steam boilers, halogen- tungsten bulbs etc. Such glasses can be transparent to visible light when ZnO is added to them. Aluminosilicate glasses are not prepared with ease and they are not economically friendly glasses so require large expenditures for their preparation [5]. Such glasses are also used in fiber glass reinforced plastics in compositional terms known as S-glass and E-glass [9].

1.5.6- Borate glasses:    

Borate glasses consists of boron trioxide B₂O₃ but added usually in the form of H₃BO₃. Borate glasses have importance because these glasses prevent crystallization process and directly converted to glassy state on melting. Borate glasses have trigonal and tetrahedral structure which all depends on varying compositions [3]. On industrial scale these glasses find applications in photonic switches preparation, as radiation screening material and to provide protective sealing for TV tubes and light bulbs etc. It is noticed that on adding heavy metallic oxides, these glasses become denser. But on commercial scale it is less important due to its high water solubility [5].

1.5.7- Glass Ceramics:

Glass ceramics usually exist in one of the three compositions:

LAS: Lithium, aluminium-silicate glasses.

MAS: Magnesium, aluminium-silicate glasses.

ZAS: Zinc, aluminium-silicate glasses.

Such glasses have high mechanical strength, low value of thermal expansion, can highly resist thermal loads or shocks and their impact resistance is also high. Glass ceramics find applications in telescope mirrors, as an insulating material and also in cooking utensils etc. An important part about glass ceramics is that it is partially in glassy state and partially in crystalline state [10].

1.5.8- Optical Glasses:

Optical glasses are prepared with different compositions and when composition is changed then their properties and applications are also changed. But mostly optical glasses have good value of refractive index and dispersion property. Such glasses are used in scientific equipment, optical microscopes and fighter aircrafts. Usually they are also used in sunglasses and lenses [11].

1.5.9- Rare Earth Doped Glasses:

Glasses doped with some rare earth ions find many applications in solid state lasers. They are also used in field of optics i-e in optical sensors, as fibers in optical transmission system. Actually rare earth metals have sharp edged and distinct energy levels. By giving some structural changes to these energy levels, such ions doped glasses with useful properties can be achieved [5].

1.5.10- Sealing Glasses:

There are some glasses which provide a protective covering or sealing to metal oxides in many devices. Like borosilicate alkaline earth aluminosilicate glasses are used as a sealant to tungsten wire in incandescent bulbs and electric discharge tubes. PbO glasses are used as a protective layer in many vacuum discharge tubes and to enclose semiconductor diodes for better insulation. Some metal alloys like fernico (iron- nickel-cobalt alloy) are also sealed by using sodium borosilicate glasses [11]. The requirement for sealing glasses is coefficient of thermal expansion of both metal and glass must be in close correspondence so that protective sealing of glass doesn’t break with changing temperature [12].

1.5.11- Fiber Glass:

Fiber glass is actually in the form of reinforced plastic or fiber glass strengthened plastic so it is basically a combination of glass and plastic. Due to this combination, properties of glass are modified that is fiber glass is weightless or has less weight than glass but still with large mechanical strength. Fiber glass has low value of coefficient of linear thermal expansion and is not dependent on changing temperature. Electrically fiber glass is not a conductor but it is a good insulator even with low density. Best part of fiber glass is that it doesn’t produce any harmful byproduct when in contact with heat, as it is not flammable. Fiber glass is also used in making buildings on industrial scale because its thermal conductivity is low. Fiber glass has good electric properties so it is a good choice for electromagnetic window panes etc.

As glass-fiber has good resistance to corrosion (due to water or some chemical) so it is used as resistive covering to protect harbor and boat basin. Fiber glass grills are also used in metallic mining process to avoid any corrosion due to chemical attack. Fiber glass is also used in aerospace technology for making testing apparatus, enclosing parts of devices and also in ducting. Fiber glass is also widely applied in transport industry like in making vehicle’s body parts [13].

1.5.12- Other Silica Based Oxide Glasses: 

In this glass family optical glasses find major applications in optical devices and their elements, also including ophthalmics. The glasses such as borosilicate and soda lime with addition of ZnO, BaO and La₂O₃ as compositional components have excellent optical properties. They have high refractive index and optical dispersion, which are important for many applications in field of optics.

1.5.13- Other Non-Silica Based Oxide Glass Families:

Commercially such glasses without including silica as primary component are less useful. Borate glasses and lead oxide based glasses immediately react with moisture. So they are not water resistant. But they are equally important as they help to understand the structure and properties of glasses. There are also some non-silica based glasses which are commercially useful like alkaline earth aluminium oxide glasses. They have high temperature sealing power and strong optical transmission in infrared region. Electrical resistivity of boroaluminate glasses is far greater than that of silica based glasses. There are a lot of glasses which consist of V₂O₅, Sb₂O₃ and TeO₂ as major components along with minute amount of silica, which are well known for their fluidity with very low melting point.

These types of glasses are used in electronic components to provide proper metal to glass seal. Apart from these glasses there are also some TeO₂ based glasses with useful optical properties like high refractive index even greater than two [9].

1.5.14- Lead Glasses:

When calcium oxide is replaced by PbO in potash glasses then, refractive index is raised from 1.5 to 1.8. Due to this high value of refractive index, lead glasses have prism like property i-e it can disperse light. Fluidity of the glass is increased by adding PbO and operating temperature range is decreased. PbO glasses are being used as decorative material in glazes and colorful enamels. Lenses made up of lead glasses have property to absorb X-rays, ɣ- rays or other dangerous radiations so, lead lenses are used in laboratories where such hazardous radiations are used. On adding Pb oxide, glasses become dense owing to higher atomic mass of lead. But PbO has hazardous effects on health and it readily reacts with water [14].

1.5.15- Germanate glasses:

In optical transmission or especially in optical fiber system, there are some losses of transmitting signal. So, to overcome this a new system of glasses, which consist of germanium oxide as a component of glass system were introduced. Coefficient of thermal expansion for germanate glasses is higher than that of borate and silicate glasses which finds application in high vacuum technology for sealing purposes. Optical properties of GeO₂ containing glasses are far better than borate and silicate glasses so, they are extensively used in optical lasers. Their refractive index is approximately 1.8 or even larger with small content of heavy metal oxides (lead oxide, bismuth oxide, tellurium oxide) [15].

Germanate glasses are also used in optical sensors, for signal processing in optical systems, and in laser technology [16].

1.5.16- Zinc Glasses:

Zinc oxide is an inanimate compound which is usually symbolize as ZnO. It usually does not dissolve in water. It exists in nature in the form of zincite a mineral. ZnO is a semiconductor with broad forbidden gap. It can be used in LCD as translucent electrode plates, in microelectronics i-e thin film transistors and in LED. ZnO glasses are relatively softer with hardness number of 4.5 on Mohs scale. ZnO have good mechanical properties like good thermal conductivity, low value of coefficient of thermal expansion and high melting point so it can be used in glasses and ceramics. ZnO is also used as a medicine for its antiseptic property, to cure skin itching, rashes etc. ZnO is also layered in nuclear reactors for preventing corrosion due to any chemical reaction [17].

1.5.17- Phosphate glasses:  

P₂O₅ is the primary component of phosphate glasses. Such glasses are not chemically stable as they readily absorb moisture from air. However by adding heavy metal oxides i-e silica, lead oxide, bismuth oxide etc, they can achieve better stability. Their optical transmission spectra lie in UV region. Phosphate glasses with traces of iron are used to handle nuclear waste materials [5]. Phosphate glasses are also used in bio-medical field, as replacement for natural bone mineral in the form of calcium-soda phosphate glass. Calcium phosphate glasses when dissolved in water show antibacterial properties. Phosphate is also used in agriculture as a fertilizer [11].

1.5.18- Halide Glasses:      

In halide glasses, heavy metal inanimate fluoride glasses are very important in telecommunication system. Because losses of the transmitted optical signals are less even for larger distances, which is otherwise not possible in silica or other oxide based glasses. Due to this property HMF glasses are well known by the name of repeaterless telecommunication glasses. But their application is limited as these HMF glasses are hygroscopic in nature and immediately react with water so they are not compatible to be used in telecommunication system.

1.5.19- Oxy-halide, Oxy-carbide, Oxy-nitride Glasses:

In these types of glasses, halogen, carbon and nitrogen are replaced by oxygen. These glasses are chemically stable. Oxy-halide glasses are far better than oxide glasses because of lower value of viscosity. So they find application as ultra-low temperature sealants for many devices, especially lead based oxy-halides. As compared to oxide glasses, these glasses have very high value of anionic electrical conductivity due to which they are used in solid electrolytes. Commercially, oxy-nitrides and oxy-carbides are important due to their high mechanical strength.

1.5.20- Chalcogenide and Chalcohalide Glasses:

Chalcogenide glasses consist of group VI elements and some elements of group IV and V, and by adding halogen in these compositions of chalcogenides, a new class of glass called chalcohalide is formed. They are well known photoconductors and hence utilized for photocopying purpose. These glasses find their optical transmission spectrum in IR region, hence they are used as optical glass fiber in laser aided microsurgery for transmitting CO₂ based laser light. Chalcogenides are superior to halide glasses because the former are not readily attacked by water. Hence it is better to use chalcogenides instead of halide glasses for mid-IR sensing purpose.

1.5.21- Metallic Glass:

Metallic glasses are also known as glassy metals are amorphous metals which are made up of metals and some non-metallic elements. But they can also be prepared by metals only. They have properties of both metals and glasses. In aspect of magnetic, optical and electrical properties, the glassy metals are similar to metals but their electrical conductivity usually increases by increasing temperature. Very high speed quenching methods are used to form thin fibers or thin films of metallic glasses. As they are in the form of thin films so iron based glassy metals are utilized in core laminations of electrical power transformers. They are also applied in pliable magnetic insulating materials. The core laminations made up of Fe-based glassy metals are energy efficient as they save up to 30% of total transformer power. This is attributed to their high electrical resistivity.

Such glasses also show superconducting behavior to be used in thermonuclear reactors as superconducting magnet. Hence radiation losses are very much decreased by using such amorphous metals. Such glasses are also utilized in aerospace i-e high speed space rocket, airjets etc owing to the extraordinary super mechanical strength of metallic glasses [9].

1.6- Glass Composition:

1.6.1 Glass Former:

The major component in glass with highest wt % is the glass former. It acts as a backbone for the whole interconnection of glass network. Mostly glass former of the network has high valency and is connected to oxygen atom by covalent bond for example, in silicate and soda lime glasses, SiO₂ acts as a former. It is noticed that bond strength for covalent bond of positive ion and oxygen atom must be larger than 80 kcal/mol. commonly used network formers in glasses are:

• Silicon dioxide
• Boric oxide
• Vanadium oxide
• Germanium oxide, etc.

Former stabilize the structure of glass in short range order whether it is tetrahedral, trigonal or any other else [3].

1.6.2 Intermediates:

Intermediate lies somewhere between the network former and glass modifier. As network formers establish and maintain the structure of glass in short range order but intermediate don’t form the glass structure by itself. But it helps in making glass network. Intermediate does not initiate the formation of glass but once glass network is started so it helps in maintaining it more appropriately. Intermediate can enhance or reduce the properties of network former. For example when Al₂O₃ is added to some alkali based glass so increases its hardness, rigidity and resistance to chemical attack but due to its addition melting temperature is increased.

Intermediate functions in both ways as former and modifier but it depends on its number of nearest neighbors and also on glass type under consideration. For oxides which act as intermediates, the covalent bond strength must lie between 60 to 80 kcal/mol. commonly used intermediates are:

• Lead oxide
• Alumina
• Bismuth oxide
• Zinc oxide
• Cadmium oxide
• Zirconium oxide, etc.

1.6.3 Glass Modifier:

Modifier is such a component of glass which actually does not take part in glass formation process but it is added to alter the properties of glass former. The maximum bond strength attained by the modifier is 40 kcal/mol and maximum nearest neighbors are 10. A modifier usually decreases the melting point of glass because it also decreases the bond strength of glass network and hence changes the properties of glass. For example alkali oxide i-e Na₂O when added to silicate glasses so it modifies its thermal and physical properties like viscosity etc. Mostly oxides of alkali and alkaline earth elements perform network modifier role in glasses [18]. By the addition of suitable modifiers, glasses can be used in many important fields like optoelectronics and solid state ionic conduction mechanism. Mostly these modifiers are used in glasses depending upon their applications which are:

• Na₂O
• CaO
• PbO₂
• ZnO
• BaO
• K₂O
• Li₂O

1.7- Importance of Glasses:

We use glasses in our everyday life. There are large number of products that cannot be manufactured without adding glass as glass is compulsory ingredient for such products. Most importantly, glasses are used in buildings in the form of window panes for purpose of insulation of heat. Glasses are also important in electronics and household appliances for providing a protective screen for TV, laptop, mobile phones etc. It is possible to receive any immediate information via phones, mails etc. All this is possible by glass fibers and optical waveguides made up of glass. Telecommunication and transfer of information to faraway places is possible through such glass fibers reinforced plastics.

Glasses are also important from the point of view of safety that they act as protective screen(in the form of lens) for some harmful radiations like X-ray, ɣ ray etc. In automobile industry, glasses play an important role in making such parts which are not only light in weight but are also mechanically strong e.g aerospace, motor cars, airplanes etc. Even glasses are equally useful in medical field and biotechnology [19].

All this is possible due to manifold properties of glasses which include their high value of elasticity modulus, low value of thermal conductivity, excellent transparency in visible region for most of the glasses, resistance to chemical attack, electrical insulation etc [20].

1.8- Advantages and Disadvantages of Glasses:

There is an overview of advantages and disadvantages of glasses at the end of this chapter [21]:

Advantages: 

• Good light transparency.
• Ease of polishing.
• Excellent electric current insulator.
• Perfect heat insulation.
• Good chemical stability.
• Light in weight.
• Excellent molding power.
• Excellent lamination property.
• Good resistance to corrosion.
• Ease of cleaning.

Disadvantages:

• Poor ductility.
• High maintenance expenses.
• Poor resistance to mechanical stress.
• Prone to attacks easily
• Subjected to dielectric losses.
• High value of dielectric losses.