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Molybdenum(IV) silicide
Encyclopedia
Molybdenum disilicide (2, or molybdenum silicide), an intermetallic compound, a silicide
of molybdenum
, is a refractory ceramic
with primary use in heating element
s. It has moderate density
, melting point 2030 °C, and is electrically conductive. At high temperatures it forms a passivation layer of silicon dioxide
, protecting it from further oxidation. Its CAS number is . It is a gray metallic-looking material with tetragonal crystal structure
(alpha-modification); its beta-modification is hexagonal and unstable. It is insoluble in most acids but soluble in nitric acid
and hydrofluoric acid
.
While MoSi2 has excellent resistance to oxidation and high Young's modulus
at temperatures above 1000 °C, it is brittle
in lower temperatures. Also, at above 1200 °C it loses creep
resistance. These properties limits its use as a structural material
, but may be offset by using it together with another material as a composite material
.
Molybdenum disilicide and MoSi2-based materials are usually made by sintering
. Plasma spraying can be used for producing its dense monolithic and composite forms; material produced this way may contain a proportion of β-MoSi2 due to its rapid cooling.
Molybdenum disilicide heating elements can be used for temperatures up to 1800 °C, in electric furnace
s used in laboratory and production environment in production of glass
, steel
, electronics
, ceramic
s, and in heat treatment
of materials. While the elements are brittle, they can operate at high power without aging, and their electrical resistivity does not increase with operation time. Their maximum operating temperature
has to be lowered in atmospheres with low oxygen content due to breakdown of the passivation layer.
Other ceramic materials used for heating elements are eg. silicon carbide
, barium titanate
, and lead titanate
composite material
s.
Molybdenum disilicide is used in microelectronics
as a contact material. It is often used as a shunt
over polysilicon lines to increase their conductivity and increase signal speed.
http://www.mhi-inc.com/PG3/high-temperature-heating-elements.html
Silicide
A silicide is a compound that has silicon with more electropositive elements.Silicon is more electropositive than carbon. Silicides are structurally closer to borides than to carbides....
of molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, is a refractory ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
with primary use in heating element
Heating element
A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....
s. It has moderate density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
, melting point 2030 °C, and is electrically conductive. At high temperatures it forms a passivation layer of silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
, protecting it from further oxidation. Its CAS number is . It is a gray metallic-looking material with tetragonal crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
(alpha-modification); its beta-modification is hexagonal and unstable. It is insoluble in most acids but soluble in nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
and hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
.
While MoSi2 has excellent resistance to oxidation and high Young's modulus
Young's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
at temperatures above 1000 °C, it is brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...
in lower temperatures. Also, at above 1200 °C it loses creep
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
resistance. These properties limits its use as a structural material
Structural material
Structural engineering depends on the knowledge of materials and their properties, in order to understand how different materials support and resist loads.Common structural materials are:-Wrought iron:...
, but may be offset by using it together with another material as a composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
.
Molybdenum disilicide and MoSi2-based materials are usually made by sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
. Plasma spraying can be used for producing its dense monolithic and composite forms; material produced this way may contain a proportion of β-MoSi2 due to its rapid cooling.
Molybdenum disilicide heating elements can be used for temperatures up to 1800 °C, in electric furnace
Furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
s used in laboratory and production environment in production of glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s, and in heat treatment
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
of materials. While the elements are brittle, they can operate at high power without aging, and their electrical resistivity does not increase with operation time. Their maximum operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
has to be lowered in atmospheres with low oxygen content due to breakdown of the passivation layer.
Other ceramic materials used for heating elements are eg. silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
, barium titanate
Barium titanate
Barium titanate is the inorganic compound with the chemical formula BaTiO3. Barium titanate is a white powder and transparent as larger crystals...
, and lead titanate
Lead titanate
Lead titanate is an inorganic compound with the chemical formula PbTiO3. It is the lead salt of titanic acid. Lead titanate is a yellow powder that is insoluble in water....
composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
s.
Molybdenum disilicide is used in microelectronics
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
as a contact material. It is often used as a shunt
Shunt (electrical)
In electronics, a shunt is a device which allows electric current to pass around another point in the circuit. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage.-Defective...
over polysilicon lines to increase their conductivity and increase signal speed.
External links
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