Yttralox is a transparent ceramic consisting of yttria (Y2O3) containing approximately 10% thorium dioxide (ThO2). [1] [2] It was one of the first transparent ceramics produced, [3] and was invented in 1966 by Richard C. Anderson at the General Electric Research Laboratory while sintering mixtures of rare earth minerals.
Yttralox is a solid solution of thorium dioxide in yttria. [4] The thorium dioxide additive affects the growth of grains during densification, leading to improved optical transparency. Uncontrolled grain growth allows a few grains to grow larger than the others, trapping pores inside them. The additive increases the grain boundary hardness more than the internal grain hardness. [5] This causes porosity to remain on grain boundaries rather than becoming trapped inside grains, allowing them to be eliminated later in the sintering process. This greatly improves the material's optical transparency, because porosity causes light scattering. [2] Porosities as low as one part per million were reported. [6] The resulting grain size was in the range 10–50 μm. [4]
Yttralox was marketed as being "transparent as glass", has a melting point twice as high, and transmits frequencies in the near infrared band as well as visible light. [4] [7] [8] However, it has little plasticity at high temperatures and low thermal conductivity, giving it a thermal shock performance little better than common glass. [5]
Commercialization was limited because Yttralox required high sintering temperatures of 2000–2200°C. Yttralox was proposed for use in lamp envelopes and high-temperature windows and lenses. [1] It was investigated for use as a low-loss window material for lasers, [9] for example in conjunction with a laser Doppler velocimeter for ramjet research. [5] It was also investigated for use with infrared equipment in missiles. [2] Neodymium oxide–doped Yttralox was used as a proof of concept for laser gain in a polycrystalline oxide ceramic, but was not commercialized due to low efficiency. [1]
Yttralox's competing materials were an yttria containing lanthanum oxide manufactured by GTE, and a pure yttria material manufactured by Raytheon. [2]
Yttralox was invented in 1966 by Richard C. Anderson at the General Electric Research Laboratory while sintering mixtures of rare earth minerals. [3] [4] The initial objective of the research was to develop ionic conductors for fuel cells using yttria–zirconium dioxide materials. Although the zirconium dioxide-rich versions were of more interest for ionic conductors, the yttria-rich versions unexpectedly produced transparent samples. [10] Further research established that other oxides of Group 4 elements, thorium dioxide and hafnium dioxide, were also effective at producing transparent yttria, and the thorium dioxide system became the most extensively studied. [5] [11] [10] Further work at GE was performed by Paul J. Jorgensen, Joseph H. Rosolowski, and Douglas St. Pierre. [8] Fabrication of Yttralox was reported by Greskovich and Woods. [12]
As of 1982, Yttralox was no longer being produced. [5]
A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, and brick.
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Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing process used with metals, ceramics, plastics, and other materials. The nanoparticles in the sintered material diffuse across the boundaries of the particles, fusing the particles together and creating a solid piece.
Zirconium dioxide is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabilized cubic structured zirconia, cubic zirconia, is synthesized in various colours for use as a gemstone and a diamond simulant.
Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturated composition with a formulation of 74% In, 8% Sn, and 18% O by weight. Oxygen-saturated compositions are so typical that unsaturated compositions are termed oxygen-deficient ITO. It is transparent and colorless in thin layers, while in bulk form it is yellowish to gray. In the infrared region of the spectrum it acts as a metal-like mirror.
Industrial processes are procedures involving chemical, physical, electrical, or mechanical steps to aid in the manufacturing of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry.
Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production. Thorianite is the name of the mineralogical form of thorium dioxide. It is moderately rare and crystallizes in an isometric system. The melting point of thorium oxide is 3300 °C – the highest of all known oxides. Only a few elements (including tungsten and carbon) and a few compounds (including tantalum carbide) have higher melting points. All thorium compounds, including the dioxide, are radioactive because there are no stable isotopes of thorium.
In materials science, a refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, inorganic, non-metallic, porous, and heterogeneous. They are typically composed of oxides or carbides, nitrides etc. of the following elements: silicon, aluminium, magnesium, calcium, boron, chromium and zirconium.
Many ceramic materials, both glassy and crystalline, have found use as optically transparent materials in various forms from bulk solid-state components to high surface area forms such as thin films, coatings, and fibers. Such devices have found widespread use for various applications in the electro-optical field including: optical fibers for guided lightwave transmission, optical switches, laser amplifiers and lenses, hosts for solid-state lasers and optical window materials for gas lasers, and infrared (IR) heat seeking devices for missile guidance systems and IR night vision.
In materials science, the sol–gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides, especially the oxides of silicon (Si) and titanium (Ti). The process involves conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network of either discrete particles or network polymers. Typical precursors are metal alkoxides. Sol-gel process is used to produce ceramic nanoparticles.
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In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element. The heating element is usually an electrical filament heated by a separate electric current passing through it. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area. Cold cathodes rely on field electron emission or secondary electron emission from positive ion bombardment, and do not require heating. There are two types of hot cathode. In a directly heated cathode, the filament is the cathode and emits the electrons. In an indirectly heated cathode, the filament or heater heats a separate metal cathode electrode which emits the electrons.
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A ceramic knife is a knife with a ceramic blade typically made from zirconium dioxide (ZrO2; also known as zirconia), rather than the steel used for most knives. Ceramic knife blades are usually produced through the dry-pressing and firing of powdered zirconia using solid-state sintering. The blades typically score 8.5 on the Mohs scale of mineral hardness, compared to 4.5 for normal steel and 7.5 to 8 for hardened steel and 10 for diamond. The resultant blade has a hard edge that stays sharp for much longer than conventional steel blades. However, the blade is brittle, subject to chipping, and will break rather than flex if twisted. The ceramic blade is sharpened by grinding the edges with a diamond-dust-coated grinding wheel.
Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components and the study of their structure, composition and properties.
Yttria-stabilized zirconia (YSZ) is a ceramic in which the cubic crystal structure of zirconium dioxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" (ZrO2) and "yttria" (Y2O3), hence the name.
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