Glass is a non-crystalline, amorphous solid, transparent and has widespread practical and decorative uses in, for example, window panes and optoelectronics. The most familiar, the oldest, types of manufactured glass are "silicate glasses" based on the chemical compound silica, the primary constituent of sand; the term glass, in popular usage, is used to refer only to this type of material, familiar from use as window glass and in glass bottles. Of the many silica-based glasses that exist, ordinary glazing and container glass is formed from a specific type called soda-lime glass, composed of 75% silicon dioxide, sodium oxide from sodium carbonate, calcium oxide called lime, several minor additives. Many applications of silicate glasses derive from their optical transparency, giving rise to their primary use as window panes. Glass will transmit and refract light. Glass can be coloured by adding metallic salts, can be painted and printed with vitreous enamels; these qualities have led to the extensive use of glass in the manufacture of art objects and in particular, stained glass windows.
Although brittle, silicate glass is durable, many examples of glass fragments exist from early glass-making cultures. Because glass can be formed or moulded into any shape, it has been traditionally used for vessels: bowls, bottles and drinking glasses. In its most solid forms it has been used for paperweights and beads; when extruded as glass fiber and matted as glass wool in a way to trap air, it becomes a thermal insulating material, when these glass fibers are embedded into an organic polymer plastic, they are a key structural reinforcement part of the composite material fiberglass. Some objects were so made of silicate glass that they are called by the name of the material, such as drinking glasses and eyeglasses. Scientifically, the term "glass" is defined in a broader sense, encompassing every solid that possesses a non-crystalline structure at the atomic scale and that exhibits a glass transition when heated towards the liquid state. Porcelains and many polymer thermoplastics familiar from everyday use are glasses.
These sorts of glasses can be made of quite different kinds of materials than silica: metallic alloys, ionic melts, aqueous solutions, molecular liquids, polymers. For many applications, like glass bottles or eyewear, polymer glasses are a lighter alternative than traditional glass. Silicon dioxide is a common fundamental constituent of glass. In nature, vitrification of quartz occurs when lightning strikes sand, forming hollow, branching rootlike structures called fulgurites. Fused quartz is a glass made from chemically-pure silica, it has excellent resistance to thermal shock, being able to survive immersion in water while red hot. However, its high melting temperature and viscosity make it difficult to work with. Other substances are added to simplify processing. One is sodium carbonate; the soda makes the glass water-soluble, undesirable, so lime, some magnesium oxide and aluminium oxide are added to provide for a better chemical durability. The resulting glass is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass.
Most common glass contains other ingredients to change its properties. Lead glass or flint glass is more "brilliant" because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. Adding barium increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion and was used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eyeglasses. Iron can be incorporated into glass to absorb infrared radiation, for example in heat-absorbing filters for movie projectors, while cerium oxide can be used for glass that absorbs ultraviolet wavelengths; the following is a list of the more common types of silicate glasses and their ingredients and applications: Fused quartz called fused-silica glass, vitreous-silica glass: silica in vitreous, or glass, form. It has low thermal expansion, is hard, resists high temperatures, it is the most resistant against weathering. Fused quartz is used for high-temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc.
Soda-lime-silica glass, window glass: silica + sodium oxide + lime + magnesia + alumina. Is transparent formed and most suitable for window glass, it has a high thermal expansion and poor resistance to heat. It is used for windows, some low-temperature incandescent light bulbs, tableware. Container glass is a soda-lime glass, a slight variation on flat glass, which uses more alumina and calcium, less sodium and magnesium, which are more water-soluble; this makes it less susceptible to water erosion. Sodium borosilicate glass, Pyrex: silica + boron trioxide + soda + alumina. Stan
Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an excellent example of this phenomenon, but thermochromism has more practical uses, such as baby bottles which change to a different color when cool enough to drink, or kettles which change when water is at or near boiling point. Thermochromism is one of several types of chromism; the two common approaches are based on liquid crystals and leuco dyes. Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation. Leuco dyes allow wider range of colors to be used, but their response temperatures are more difficult to set with accuracy; some liquid crystals are capable of displaying different colors at different temperatures. This change is dependent on selective reflection of certain wavelengths by the crystallic structure of the material, as it changes between the low-temperature crystallic phase, through anisotropic chiral or twisted nematic phase, to the high-temperature isotropic liquid phase.
Only the nematic mesophase has thermochromic properties. The twisted nematic phase has the molecules oriented in layers with changing orientation, which gives them periodic spacing; the light passing through the crystal undergoes Bragg diffraction on these layers, the wavelength with the greatest constructive interference is reflected back, perceived as a spectral color. A change in the crystal temperature can result in a change of spacing between the layers and therefore in the reflected wavelength; the color of the thermochromic liquid crystal can therefore continuously range from non-reflective through the spectral colors to black again, depending on the temperature. The high temperature state will reflect blue-violet, while the low-temperature state will reflect red-orange. Since blue is a shorter wavelength than red, this indicates that the distance of layer spacing is reduced by heating through the liquid-crystal state; some such materials are cholesteryl cyanobiphenyls. Mixtures with 3–5 °C span of temperatures and ranges from about 17–23 °C to about 37–40 °C can be composed from varying proportions of cholesteryl oleyl carbonate, cholesteryl nonanoate, cholesteryl benzoate.
For example, the mass ratio of 65:25:10 yields range of 17–23 °C, 30:60:10 yields range of 37–40 °C. Liquid crystals used in dyes and inks come microencapsulated, in the form of suspension. Liquid crystals are used in applications where the color change has to be defined, they find applications in thermometers for room, refrigerator and medical use, in indicators of level of propane in tanks. A popular application for thermochromid liquid crystals are the mood rings. Liquid crystals are difficult to require specialized printing equipment; the material itself is typically more expensive than alternative technologies. High temperatures, ultraviolet radiation, some chemicals and/or solvents have a negative impact on their lifespan. Thermochromic dyes are based on mixtures of leuco dyes with suitable other chemicals, displaying a color change in dependence on temperature; the dyes are applied on materials directly. An illustrative example is the Hypercolor fashion, where microcapsules with crystal violet lactone, weak acid, a dissociable salt dissolved in dodecanol are applied to the fabric.
In this case the apparent thermochromism is in fact halochromism. The dyes most used are spirolactones, fluorans and fulgides; the acids include bisphenol A, parabens, 1,2,3-triazole derivates, 4-hydroxycoumarin and act as proton donors, changing the dye molecule between its leuco form and its protonated colored form. Leuco dyes have less accurate temperature response than liquid crystals, they are suitable for various novelty items. They are used in combination with some other pigment, producing a color change between the color of the base pigment and the color of the pigment combined with the color of the non-leuco form of the leuco dye. Organic leuco dyes are available for temperature ranges between about −5 °C and 60 °C, in wide range of colors; the color change happens in a 3 °C interval. Leuco dyes are used in applications where temperature response accuracy is not critical: e.g. novelties, bath toys, flying discs, approximate temperature indicators for microwave-heated foods. Microencapsulation allows their use in wide range of products.
The size of the microcapsules ranges between 3–5 µm, which requires some adjustments to printing and manufacturing processes. An application of leuco dyes is in the Duracell battery state indicators. A layer of a leuco dye is applied on a resistive strip to indicate its heating, thus gauging the amount of current the battery is able to supply; the strip is triangular-shaped, changing its resistance along its length, therefore heating up a proportionally long segment with the amount of current flowing through it. The length of the segment above the threshold temp
Chemistry is the scientific discipline involved with elements and compounds composed of atoms and ions: their composition, properties and the changes they undergo during a reaction with other substances. In the scope of its subject, chemistry occupies an intermediate position between physics and biology, it is sometimes called the central science because it provides a foundation for understanding both basic and applied scientific disciplines at a fundamental level. For example, chemistry explains aspects of plant chemistry, the formation of igneous rocks, how atmospheric ozone is formed and how environmental pollutants are degraded, the properties of the soil on the moon, how medications work, how to collect DNA evidence at a crime scene. Chemistry addresses topics such as how atoms and molecules interact via chemical bonds to form new chemical compounds. There are four types of chemical bonds: covalent bonds, in which compounds share one or more electron; the word chemistry comes from alchemy, which referred to an earlier set of practices that encompassed elements of chemistry, philosophy, astronomy and medicine.
It is seen as linked to the quest to turn lead or another common starting material into gold, though in ancient times the study encompassed many of the questions of modern chemistry being defined as the study of the composition of waters, growth, disembodying, drawing the spirits from bodies and bonding the spirits within bodies by the early 4th century Greek-Egyptian alchemist Zosimos. An alchemist was called a'chemist' in popular speech, the suffix "-ry" was added to this to describe the art of the chemist as "chemistry"; the modern word alchemy in turn is derived from the Arabic word al-kīmīā. In origin, the term is borrowed from the Greek χημία or χημεία; this may have Egyptian origins since al-kīmīā is derived from the Greek χημία, in turn derived from the word Kemet, the ancient name of Egypt in the Egyptian language. Alternately, al-kīmīā may derive from χημεία, meaning "cast together"; the current model of atomic structure is the quantum mechanical model. Traditional chemistry starts with the study of elementary particles, molecules, metals and other aggregates of matter.
This matter can be studied in isolation or in combination. The interactions and transformations that are studied in chemistry are the result of interactions between atoms, leading to rearrangements of the chemical bonds which hold atoms together; such behaviors are studied in a chemistry laboratory. The chemistry laboratory stereotypically uses various forms of laboratory glassware; however glassware is not central to chemistry, a great deal of experimental chemistry is done without it. A chemical reaction is a transformation of some substances into one or more different substances; the basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through a chemical equation, which involves atoms as subjects; the number of atoms on the left and the right in the equation for a chemical transformation is equal. The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.
Energy and entropy considerations are invariably important in all chemical studies. Chemical substances are classified in terms of their structure, phase, as well as their chemical compositions, they can be analyzed using the tools of e.g. spectroscopy and chromatography. Scientists engaged in chemical research are known as chemists. Most chemists specialize in one or more sub-disciplines. Several concepts are essential for the study of chemistry; the particles that make up matter have rest mass as well – not all particles have rest mass, such as the photon. Matter can be a mixture of substances; the atom is the basic unit of chemistry. It consists of a dense core called the atomic nucleus surrounded by a space occupied by an electron cloud; the nucleus is made up of positively charged protons and uncharged neutrons, while the electron cloud consists of negatively charged electrons which orbit the nucleus. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons.
The nucleus is dense. The atom is the smallest entity that can be envisaged to retain the chemical properties of the element, such as electronegativity, ionization potential, preferred oxidation state, coordination number, preferred types of bonds to form. A chemical element is a pure substance, composed of a single type of atom, characterized by its particular number of protons in the nuclei of its atoms, known as the atomic number and represented by the symbol Z; the mass number is the sum of the number of neutrons in a nucleus. Although all the nuclei of all atoms belonging to one element will have the same
The term phase transition is most used to describe transitions between solid and gaseous states of matter, as well as plasma in rare cases. A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium, certain properties of the medium change discontinuously, as a result of the change of external conditions, such as temperature, pressure, or others. For example, a liquid may become gas upon heating to the boiling point, resulting in an abrupt change in volume; the measurement of the external conditions at which the transformation occurs is termed the phase transition. Phase transitions occur in nature and are used today in many technologies. Examples of phase transitions include: The transitions between the solid and gaseous phases of a single component, due to the effects of temperature and/or pressure: A eutectic transformation, in which a two component single phase liquid is cooled and transforms into two solid phases.
The same process, but beginning with a solid instead of a liquid is called a eutectoid transformation. A peritectic transformation, in which a two component single phase solid is heated and transforms into a solid phase and a liquid phase. A spinodal decomposition, in which a single phase is cooled and separates into two different compositions of that same phase. Transition to a mesophase between solid and liquid, such as one of the "liquid crystal" phases; the transition between the ferromagnetic and paramagnetic phases of magnetic materials at the Curie point. The transition between differently ordered, commensurate or incommensurate, magnetic structures, such as in cerium antimonide; the martensitic transformation which occurs as one of the many phase transformations in carbon steel and stands as a model for displacive phase transformations. Changes in the crystallographic structure such as between ferrite and austenite of iron. Order-disorder transitions such as in alpha-titanium aluminides.
The dependence of the adsorption geometry on coverage and temperature, such as for hydrogen on iron. The emergence of superconductivity in certain metals and ceramics when cooled below a critical temperature; the transition between different molecular structures of solids, such as between an amorphous structure and a crystal structure, between two different crystal structures, or between two amorphous structures. Quantum condensation of bosonic fluids; the superfluid transition in liquid helium is an example of this. The breaking of symmetries in the laws of physics during the early history of the universe as its temperature cooled. Isotope fractionation occurs during a phase transition, the ratio of light to heavy isotopes in the involved molecules changes; when water vapor condenses, the heavier water isotopes become enriched in the liquid phase while the lighter isotopes tend toward the vapor phase. Phase transitions occur when the thermodynamic free energy of a system is non-analytic for some choice of thermodynamic variables.
This condition stems from the interactions of a large number of particles in a system, does not appear in systems that are too small. It is important to note that phase transitions can occur and are defined for non-thermodynamic systems, where temperature is not a parameter. Examples include: quantum phase transitions, dynamic phase transitions, topological phase transitions. In these types of systems other parameters take the place of temperature. For instance, connection probability replaces temperature for percolating networks. At the phase transition point the two phases of a substance and vapor, have identical free energies and therefore are likely to exist. Below the boiling point, the liquid is the more stable state of the two, whereas above the gaseous form is preferred, it is sometimes possible to change the state of a system diabatically in such a way that it can be brought past a phase transition point without undergoing a phase transition. The resulting state is metastable, i.e. less stable than the phase to which the transition would have occurred, but not unstable either.
This occurs in superheating and supersaturation, for example. Paul Ehrenfest classified phase transitions based on the behavior of the thermodynamic free energy as a function of other thermodynamic variables. Under this scheme, phase transitions were labeled by the lowest derivative of the free energy, discontinuous at the transition. First-order phase transitions exhibit a discontinuity in the first derivative of the free energy with respect to some thermodynamic variable; the various solid/liquid/gas transitions are classified as first-order transitions because they involve a discontinuous change in density, the first derivative of the free energy with respect to pressure. Second-order phase transitions are continuous in the first derivative but exhibit discontinuity in a second derivative of the free energy; these include the ferromagnetic phase transition in materials such as iron, where the magnetization, the first derivative of the free energy with respect to the applied magnetic field strength, increases continuously from zero as the temperature is lowered below the Curie temperature.
The magnetic susceptibility, the second derivative of the free energy with the field, changes discontinuously. Under the Ehrenfest classification sche
Liquid crystals are a state of matter which has properties between those of conventional liquids and those of solid crystals. For instance, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of liquid-crystal phases, which can be distinguished by their different optical properties; the contrasting areas in the textures correspond to domains where the liquid-crystal molecules are oriented in different directions. Within a domain, the molecules are well ordered. LC materials may not always be in a liquid-crystal phase. Liquid crystals can be divided into thermotropic and metallotropic phases. Thermotropic and lyotropic liquid crystals consist of organic molecules, although a few minerals are known. Thermotropic LCs exhibit a phase transition into the liquid-crystal phase. Lyotropic LCs exhibit phase transitions as a function of both temperature and concentration of the liquid-crystal molecules in a solvent. Metallotropic LCs are composed of both inorganic molecules.
Examples of liquid crystals can be found both in the natural world and in technological applications. Most contemporary electronic displays use liquid crystals. Lyotropic liquid-crystalline phases are abundant in living systems but can be found in the mineral world. For example, many proteins and cell membranes are liquid crystals. Other well-known examples of liquid crystals are solutions of soap and various related detergents, as well as the tobacco mosaic virus, some clays. In 1888, Austrian botanical physiologist Friedrich Reinitzer, working at the Karl-Ferdinands-Universität, examined the physico-chemical properties of various derivatives of cholesterol which now belong to the class of materials known as cholesteric liquid crystals. Other researchers had observed distinct color effects when cooling cholesterol derivatives just above the freezing point, but had not associated it with a new phenomenon. Reinitzer perceived that color changes in a derivative cholesteryl benzoate were not the most peculiar feature.
He found that cholesteryl benzoate does not melt in the same manner as other compounds, but has two melting points. At 145.5 °C it melts into a cloudy liquid, at 178.5 °C it melts again and the cloudy liquid becomes clear. The phenomenon is reversible. Seeking help from a physicist, on March 14, 1888, he wrote to Otto Lehmann, at that time a Privatdozent in Aachen, they exchanged samples. Lehmann examined the intermediate cloudy fluid, reported seeing crystallites. Reinitzer's Viennese colleague von Zepharovich indicated that the intermediate "fluid" was crystalline; the exchange of letters with Lehmann ended on April 24, with many questions unanswered. Reinitzer presented his results, with credits to Lehmann and von Zepharovich, at a meeting of the Vienna Chemical Society on May 3, 1888. By that time, Reinitzer had discovered and described three important features of cholesteric liquid crystals: the existence of two melting points, the reflection of circularly polarized light, the ability to rotate the polarization direction of light.
After his accidental discovery, Reinitzer did not pursue studying liquid crystals further. The research was continued by Lehmann, who realized that he had encountered a new phenomenon and was in a position to investigate it: In his postdoctoral years he had acquired expertise in crystallography and microscopy. Lehmann started a systematic study, first of cholesteryl benzoate, of related compounds which exhibited the double-melting phenomenon, he was able to make observations in polarized light, his microscope was equipped with a hot stage enabling high temperature observations. The intermediate cloudy phase sustained flow, but other features the signature under a microscope, convinced Lehmann that he was dealing with a solid. By the end of August 1889 he had published his results in the Zeitschrift für Physikalische Chemie. Lehmann's work was continued and expanded by the German chemist Daniel Vorländer, who from the beginning of 20th century until his retirement in 1935, had synthesized most of the liquid crystals known.
However, liquid crystals were not popular among scientists and the material remained a pure scientific curiosity for about 80 years. After World War II work on the synthesis of liquid crystals was restarted at university research laboratories in Europe. George William Gray, a prominent researcher of liquid crystals, began investigating these materials in England in the late 1940s, his group synthesized many new materials that exhibited the liquid crystalline state and developed a better understanding of how to design molecules that exhibit the state. His book Molecular Structure and the Properties of Liquid Crystals became a guidebook on the subject. One of the first U. S. chemists to study liquid crystals was Glenn H. Brown, starting in 1953 at the University of Cincinnati and at Kent State University. In 1965, he organized the first international conference on liquid crystals, in Kent, with about 100 of the world's top liquid crystal scientists in attendance; this conference marked the beginning of a worldwide effort to perform research in this field, which soon led to the development of practical applications for these unique materials.
Liquid crystal materials became a focus of research in the development of flat panel electronic displays beginning in 1962 at RCA Laboratories. When physical che