Chapter 1 Introduction
1.2 Material Groups and Structure of TiO 2
Titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania, is the naturally occurring oxide of titanium, chemical formula TiO2.
Natural occurrence
Titanium dioxide occurs in four forms:
z rutile, a tetragonal mineral usually of prismatic habit, often twinned;
z anatase or octahedrite, a tetragonal mineral of dipyramidal habit;
z brookite, an orthorhombic mineral. Both anatase and brookite are relatively rare minerals;
z Titanium dioxide (B) or TiO2(B), a monoclinic mineral.
z Titanium Oxide or TiO, as present in K and M spectral type stars.
Titanium dioxide occurrences in nature are never pure; it is found with contaminant metals such as iron. The oxides can be mined and serve as a source for commercial titanium. The metal can also be mined from other minerals such as ilmenite or leucoxene ores, or one of the purest forms, rutile beach sand.
z Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2, with two rarer polymorphs anatase (sometimes
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known by the obsolete name 'octahedrite'), a tetragonal mineral of pseudo-octahedral habit; and brookite, an orthorhombic mineral.
Rutile has among the highest refractive indices of any known mineral and also exhibits high dispersion. Natural rutile may contain up to 10% iron and significant amounts of niobium and tantalum.
Rutile derives its name from the Latin rutilus, red, in reference to the deep red color observed in some specimens when viewed by transmitted light.
Occurrence :
Rutile is a common accessory mineral in high-temperature and high-pressure metamorphic rocks and in igneous rocks.
Rutile is the preferred polymorph of TiO2 in such environments because it has the lowest molecular volume of the three polymorphs; it is thus the primary titanium bearing phase in most high pressure metamorphic rocks, chiefly eclogites. Brookite and anatase are typical polymorphs of rutile formed by retrogression of metamorphic rutile.
Within the igneous environment, rutile is a common accessory mineral in plutonic igneous rocks, although it is also found occasionally in extrusive igneous rocks, particularly those which have deep mantle sources such as kimberlites and lamproites. Anatase and brookite are found in the igneous environment particularly as products autogenic alteration during the cooling of plutonic rocks; anatase is also found formed within placer deposits sourced from primary rutile.
The occurrence of large specimen crystals is most common in pegmatites, skarns and particularly granite greisens.
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Rutile is found as an accessory mineral in some altered igneous rocks, and in certain gneisses and schists. In groups of acicular crystals it is frequently seen penetrating quartz as in the "fléches d'amour" from Grisons, Switzerland.
Uses and economic importance:
Rutile, when present in large enough quantities in beach sands, forms an important constituent of heavy mineral sands ore deposits. It is primarily extracted for use in refractory manufacture or use as a base for paints. Rarely is it extracted as an ore of titanium.
Finely powdered rutile is a brilliant white pigment and is used in paints, plastics, papers, foods, and other applications that call for a bright white color. Titanium dioxide pigment is the single greatest use of titanium worldwide. Nanoscale particles of rutile are transparent to optical light but remain highly reflective to UV light.
Hence, they are used in sunscreens.
Small rutile needles present in gems are responsible for an optical phenomenon known as asterism. Asteriated gems are known as "star" gems. Star sapphires, star rubies, and other "star" gems are highly sought after and often more valuable than their normal equivalents.
Synthetic rutile
Synthetic rutile was first produced in 1948 and is sold under a variety of names.
Very pure synthetic rutile is transparent and almost colorless (slightly yellow) in large pieces. Synthetic rutile can be made in a variety of colors by doping, although the purest material is almost colorless. The high refractive index gives an adamantine lustre and strong refraction that leads to a diamond-like appearance. The
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near-colorless diamond substitute is sold under the name Titania, which is the old-fashioned chemical name for this oxide. However, rutile is seldom used in jewellery because it is not very hard (scratch-resistant), measuring only about 6 on the Mohs hardness scale.
Figure 1-1 The unit cell of rutile
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Figure 1-2 The crystal structure of rutile . z Anatase[24]
Anatase is one of the three mineral forms of titanium dioxide (the other two being brookite and rutile). It is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but, although the degree of symmetry is the same for both, there is no relation between the interfacial angles of the two minerals, except, of course, in the prism-zone of 45° and 90°. The common pyramid of anatase, parallel to the faces of which there are perfect cleavages, has an angle over the polar edge of 82°9', the corresponding angle of rutile being 56°52½'It was on account of this steeper pyramid of anatase that the mineral was named, by RJ Haüy in 1801, from the Greek anatasis, "extension," the vertical axis of the crystals being longer than in rutile. There are also important differences between the physical characters of anatase and rutile; the former is not quite so hard (H=5½-6) or dense
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(specific gravity 3.9); it is optically negative, rutile being positive; and its lustre is even more strongly adamantine or metallic-adamantine than that of rutile.
Two types or habits of anatase crystals may be distinguished. The commoner occurs as simple acute double pyramids with an indigo-blue to black colour and steely lustre. Crystals of this kind are abundant at Le Bourg-d'Oisans in Dauphiné, where they are associated with rock-crystal, feldspar, and axinite in crevices in granite and mica-schist. Similar crystals, but of microscopic size, are widely distributed in sedimentary rocks, such as sandstones, clays, and slates, from which they may be separated by washing away the lighter constituents of the powdered rock.
Crystals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit; the colour is honey-yellow to brown. Such crystals closely resemble xenotime in appearance and, indeed, were for a long time supposed to belong to this species, the special name wiserine being applied to them. They occur attached to the walls of crevices in the gneisses of the Alps, the Binnenthal near Brig in canton Valais, Switzerland, being a well-known locality.
When strongly heated, anatase is converted into rutile, changing in specific gravity to 4.1; naturally occurring pseudomorphs of rutile after anatase are also known. Crystals of anatase have and continue to be artificially prepared in laboratories by introducing the moisture-sensitive titanium tetrachloride, TiCl4, to water at very cold temperatures (the process is very exothermic) to produce TiO2 and HCl gas. Such synthetic forms of anatase are currently under scrutiny in the field of semiconductors and photovoltaic materials.
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Another name commonly in use for this mineral is octahedrite, a name which, indeed, is earlier than anatase, and given because of the common (acute) octahedral habit of the crystals. Other names, now obsolete, are oisanite and dauphinite, from the well-known French locality.
Figure 1-3 Ball-and-stick model of anatase's unit cell
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Figure 1-4 The crystal structure of anatase .
z Brookite
Brookite is a mineral consisting of titanium oxide, TiO2, and hence identical with rutile and anatase in composition, but crystallizing in the orthorhombic system (see crystal structure).
It was named for Henry James Brooke, English mineralogist, 1771 - 1857.
z Titanium dioxide (B) [25] [26] [27] [28]
Titanium dioxide (B) or TiO2(B) is the monoclinic form of titanium dioxide.
The mineral is found in weathering rims on tektites and perovskite and as lamellae in
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anatase from hydrothermal veins and has a density lower than that of the other three polymorphs.
In the laboratory anatase can be converted in a hydrothermal route to TiO2(B) nanotubes and nanowires which are of potential interest as catalytic supports and photocatalysts. For this to happen anatase is mixed with 15M sodium hydroxide and heated at 150 ° C for 72 hours. The reaction product is washed with dilute hydrochloric acid and heated at 400 °C for another 15 hours. the yield of nanotubes is quantitative and the tubes have an outer diameter of 10 to 20 nanometres and an inner diameter of 5 to 8 nanometres and have a length of 1 micrometres. A higher reaction temperature (170 °C) and less reaction volume gives the corresponding nanowires.
1.3 Characteristic of TiO
2-xN
xUV-VIS light absorption
In physics, absorption is the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make transition between two electronic energy levels. The photon is destroyed in the process. The absorbed energy may be re-emitted as radiant energy or transformed into heat energy. The absorption of light during wave propagation is often called attenuation.
The absorbance of an object quantifies how much light is absorbed by it. This may be related to other properties of the object through the Beer-Lambert law.
For most substances, the amount of absorption varies with the wavelength of the light, leading to the appearance of colour in pigments that absorb some wavelengths but not others. For example, an object that absorbs blue, green and
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yellow light will appear red when viewed under white light. More precise measurements at many wavelengths allow the identification of a substance via absorption spectroscopy.
Contact angle [29][31]
The contact angle is the angle at which a liquid/vapor interface meets the solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat horizontal solid surface. The shape of the droplet is determined by the Young-Laplace equation. The contact angle plays the role of a boundary condition. Contact angle is measured using a contact angle goniometer. The contact angle is not limited to a liquid/vapour interface; it is equally applicable to the interface of two liquids or two vapours.
Figure 1-5 Image from a video contact angle device which water drops on glass.
In near years , the other people report about the change in water contact angle on
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semiconductor oxides such as ZnO, Ta2O5, In2O3, InTaO4 or indium–tin oxides (ITO) and the unexpected response of some of them to irradiation with visible light.
TiO2 has been used as a reference material. These oxides are known because they present some kind of photoactivity and have band gaps within the UV region of the spectrum. Except for ZnO, no results about hydrophilic adjustment of contact angles upon light irradiation have been reported for the other investigated oxides. Meanwhile, for InTaO4 it has been reported recently that this photoactive material doped with nickel is able to split the water into H2 and O2 when irradiated with visible light . Wetting [30]
Wetting is the contact between a fluid and a surface, when the two are brought into contact. When a liquid has a high surface tension (strong internal bonds), it will form a droplet, whereas a liquid with low surface tension will spread out over a greater area (bonding to the surface). On the other hand, if a surface has a high surface energy (or surface tension), a drop will spread, or wet, the surface. If the surface has a low surface energy, a droplet will form. This phenomenon is a result of the minimization of interfacial energy. If the surface is high energy, it will want to be covered with a liquid because this interface will lower its energy, and so on.
The primary measurement to determine wettability is a contact angle measurement. This measures the angle between the surface and the surface of a liquid droplet on the surface. For example, a droplet would have a high contact angle, but a liquid spread on the surface would have a small one. The contact angle and the surface energies of the materials involved are related by the Young–Dupré equation
where is the surface tension between two substances and S, V, and L correspond to the solid, vapor, and liquid substances in a contact angle experiment respectively.
A contact angle of 90° or greater generally characterizes a surface as not-wettable, and one less than 90° means that the surface is wettable. In the context of water, a wettable surface may also be termed hydrophilic and a non-wettable
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surface hydrophobic. Superhydrophobic surfaces have contact angles greater than 150°, showing almost no contact between the liquid drop and the surface. This is sometimes referred to as the "Lotus effect". This characteristic of spreading out over a greater area is sometimes called 'wetting action' when discussing solders and soldering.
Wetting is often an important factor in the bonding (adherence) of two materials.
It is also the basis for capillary action, the ability of a narrow tube to draw a liquid, even against the force of gravity.
Hydrophile
Hydrophile, from the Greek (hydros) "water" and φιλια (philia) "friendship,"
refers to a physical property of a molecule that can transiently bond with water (H2O) through hydrogen bonding. This is thermodynamically favorable, and makes these molecules soluble not only in water, but also in other polar solvents. There are hydrophillic and hydrophobic parts of the cell membrane.
A hydrophilic molecule or portion of a molecule is one that is typically charge-polarized and capable of hydrogen bonding, enabling it to dissolve more readily in water than in oil or other hydrophobic solvents. Hydrophilic and hydrophobic molecules are also known as polar molecules and nonpolar molecules, respectively.
Soap has a hydrophilic head and a hydrophobic tail which allows it to dissolve in both waters and oils, therefore allowing the soap to clean a surface.
Super hydrophilicity [32]
Under light irradiation, water dropped onto titanium dioxide forms no contact angle (almost 0 degrees). This effect, called super hydrophilicity, was discovered in 1995. Super hydrophilic material has various advantages. For example, it can defog glass, and it can also enable oil spots to be swept away easily with water. Such
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materials are already commercialized as door mirrors for cars, coatings for buildings, etc.
Several mechanisms of this super hydrophilicity have been proposed by researchers. One is the change of the surface structure to a metastable structure, and another is cleaning the surface by the photodecomposition of dirt such as organic compounds adsorbed on the surface, after either of which water molecules can adsorb to the surface. The mechanism is still controversial, and it is too soon to decide which suggestion is correct. To decide, atomic scale measurements and other studies will be necessary.
1.4 Applications of TiO
2 [22] [23]As a pigment of high refringence, titanium dioxide is the most widely used white pigment because of its brightness and very high refractive index (n=2.4), in which it is surpassed only by a few other materials. When deposited as a thin film, its refractive index and colour make it an excellent reflective optical coating for dielectric mirrors and some gemstones, for example "mystic fire topaz". TiO2 is also an effective opacifier in powder form, where it is employed as a pigment to provide whiteness and opacity to products such as paints, coatings, plastics, papers, inks, foods, and most toothpastes. Used as a white food coloring, it has E number E171. In cosmetic and skin care products, titanium dioxide is used both as a pigment and a thickener. It is also used as a tattoo pigment and styptic pencils.
This pigment is used extensively in plastics and other applications for its UV resistant properties where it acts as a UV reflector.
In ceramic glazes titanium dioxide acts as an opacifier and seeds crystal formation. In almost every sunscreen with a physical blocker, titanium dioxide is
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found both because of its refractive index and its resistance to discoloration under ultraviolet light. This advantage enhances its stability and ability to protect the skin from ultraviolet light.
Titanium oxide is also used as a semi-conductor.
Photocatalyst
Titanium dioxide, particularly in the anatase form, is a photocatalyst under ultraviolet light. Recently it has been found that titanium dioxide, when spiked with nitrogen ions, is also a photocatalyst under visible light. The strong oxidative potential of the positive holes oxidizes water to create hydroxyl radicals. It can also oxidize oxygen or organic materials directly. Titanium dioxide is thus added to paints, cements, windows, tiles, or other products for sterilizing, deodorizing and anti-fouling properties and is also used as a hydrolysis catalyst. It is also used in the Graetzel cell, a type of chemical solar cell.
Titanium dioxide has potential for use in energy production: as a photocatalyst, it can carry out hydrolysis, i.e., break water into hydrogen and oxygen. Were the hydrogen collected, it could be used as a fuel. The efficiency of this process can be greatly improved by doping the oxide with carbon, as described in "Carbon-doped titanium dioxide is an effective photocatalyst" .
As TiO2 is exposed to UV light, it becomes increasingly hydrophilic; thus, it can be used for anti-fogging coatings or self-cleaning windows. TiO2 incorporated into outdoor building materials can substantially reduce concentrations of airborne pollutants such as volatile organic compounds and nitrogen oxides.
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1.5 Polyethylene Terephthalate
[33]1.5.1 Introduction
Polyethylene terephthalate (aka PET, PETE or the obsolete PETP or PET-P) is a thermoplastic polymer resin of the polyester family that produced by the chemical industry and is used in synthetic fibers; beverage, food and other liquid containers;
thermoforming applications; and engineering resins often in combination with glass fiber. It is one of the most important raw materials used in man-made fibers.
Depending on its processing and thermal history, it may exist both as an amorphous (transparent) and as a semi-crystalline (opaque and white) material. Its monomer can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct or the transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a byproduct.
Polymerization is through a polycondensation reaction of the monomers (done immediately after esterification/transesterification) with ethylene glycol as the byproduct (the ethylene glycol is recycled in production).
The majority of the world's PET production is for synthetic fibers (in excess of 60%) with bottle production accounting for around 30% of global demand. In discussing textile applications, PET is generally referred to as simply "polyester"
while "PET" is used most often to refer to packaging applications.
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Chemical structure :
Figure 1-6 Chemical structure of polyethylene terephthalate.
Light absortion spectrum :
Figure 1-7 Light absortion spectrum of polyethylene terephthalate measured using a light spectrophotometer .
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Table 1-1 Properties of PET.
[33]
PET
Density 1370 kg/m3
Young modulus(E) 2800–3100 MPa
Tensile strength(σt) 55–75 MPa
Elongation @ break 50–150%
notch test 3.6 kJ/m2
Glass temperature 75 °C
melting point 260 °C
Vicat B 170 °C
Thermal conductivity 0.24 W/m.K linear expansion coefficient (α) 7×10−5/K
Specific heat (c) 1.0 kJ/kg.K Water absorption (ASTM) 0.16
Price 0.5–1.25 €/kg
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1.5.2 Crystals
Crystallization occurs when polymer chains fold up on themselves in a repeating, symmetrical pattern. Long polymer chains tend to become entangled on themselves, which prevents full crystallization in all but the most carefully controlled circumstances. PET is no exception to this rule; 60% crystallization is the upper limit for commercial products, with the exception of polyester fibers.
PET in its natural state is a crystalline resin. Clear products can be produced by rapidly cooling molten polymer to form an amorphous solid. Like glass, amorphous PET forms when its molecules are not given enough time to arrange themselves in an
PET in its natural state is a crystalline resin. Clear products can be produced by rapidly cooling molten polymer to form an amorphous solid. Like glass, amorphous PET forms when its molecules are not given enough time to arrange themselves in an