Negative thermal expansion
Encyclopedia
Negative Thermal Expansion (NTE) is a physicochemical
process in which some materials contract upon heating rather than expanding
as most materials do. Materials which undergo this unusual process have a range of potential engineering
, photonic
, electronic
, and structural applications. For example, if one were to mix a negative thermal expansion material with a "normal" material which expands on heating, it could be possible to make a zero expansion composite material
, such as Invar
.
and phase transitions.
Recently, Liu et al. showed that the NTE phenomenon originates from the existence of high pressure, small volume phases with higher entropy, with their configurations present in the stable phase matrix through thermal fluctuations.
to expand by an amount different from the teeth, for example when drinking a hot drink, causing toothache
. If dental fillings were made of a composite material
containing a mixture of materials with positive and negative thermal expansion then the overall expansion could be precisely tailored to that of tooth enamel
.
Glass-ceramic
is used for cooktops.
Ordinary ice shows NTE in its hexagonal and cubic phases at very low temperatures (below -200 °C). In its liquid form, water
also displays negative thermal expansivity below 3.984°C.
Quartz and a number of zeolites also show NTE over certain temperature ranges. Fairly pure silicon has a negative coefficient of thermal expansion for temperatures between about 18 K and 120 K.
Cubic Scandium trifluoride has this property which is explained by the quartic oscillation of the fluoride ions. The energy stored in the bending strain of the fluoride ion is proportional to the fourth power of the displacement angle, unlike most other materials where it is proportional to the square of the displacement. A fluorine atom is bound to two scandium atoms, and as temperature increases the fluorine oscillates more perpendicularly to its bonds. This draws the scandium atoms together throughout the material and it contracts. ScF3 exhibits this property from 10K to 1100K above which it shows the normal positive thermal expansion.
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...
process in which some materials contract upon heating rather than expanding
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
as most materials do. Materials which undergo this unusual process have a range of potential engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
, photonic
Photonics
The science of photonics includes the generation, emission, transmission, modulation, signal processing, switching, amplification, detection and sensing of light. The term photonics thereby emphasizes that photons are neither particles nor waves — they are different in that they have both particle...
, electronic
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...
, and structural applications. For example, if one were to mix a negative thermal expansion material with a "normal" material which expands on heating, it could be possible to make a zero expansion 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...
, such as Invar
Invar
Invar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion . The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.It was invented in 1896 by Swiss scientist...
.
Origin of Negative Thermal Expansion
There are a number of physical processes which may cause contraction with increasing temperature, including transverse vibrational modes, Rigid Unit ModesRigid Unit Modes
Rigid unit modes represent a class of lattice vibrations or phonons that exist in network materials such as quartz, cristobalite or zirconium tungstate. Network materials can be described as three-dimensional networks of polyhedral groups of atoms such as SiO4 tetrahedra or TiO6 octahedra...
and phase transitions.
Recently, Liu et al. showed that the NTE phenomenon originates from the existence of high pressure, small volume phases with higher entropy, with their configurations present in the stable phase matrix through thermal fluctuations.
Applications
There are many potential applications for materials with controlled thermal expansion properties, as thermal expansion causes many problems in engineering, and indeed in everyday life. One simple example of a thermal expansion problem is the tendency of dental fillingsDental fillings
This page is about types of dental restorative materials. For dental fillings see dental restorationsDental restorative materials are specially fabricated materials, designed for use as dental restorations , which are used to restore tooth structure loss, usually resulting from but not limited to...
to expand by an amount different from the teeth, for example when drinking a hot drink, causing toothache
Toothache
A toothache, also known as odontalgia or, less frequently, as odontalgy, is an aching pain in or around a tooth.-Causes:* Dental etiology, In most cases toothaches are caused by problems in the tooth or jaw, such as** Dental caries...
. If dental fillings were made of 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...
containing a mixture of materials with positive and negative thermal expansion then the overall expansion could be precisely tailored to that of tooth enamel
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
.
Glass-ceramic
Glass-ceramic
Glass-ceramics are polycrystalline material produced through controlled crystallization of base glass. Glass-ceramic materials share many properties with both glasses and ceramics...
is used for cooktops.
Materials
Perhaps one of the most studied materials to exhibit negative thermal expansion is Cubic Zirconium Tungstate (ZrW2O8). This compound contracts continuously over a temperature range of 0.3 to 1050 K (at higher temperatures the material decomposes). Other materials that exhibit this behaviour include: other members of the AM2O8 family of materials (where A = Zr or Hf, M = Mo or W) and ZrV2O7. A2(MO4)3 also is an example of controllable negative thermal expansion.Ordinary ice shows NTE in its hexagonal and cubic phases at very low temperatures (below -200 °C). In its liquid form, water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
also displays negative thermal expansivity below 3.984°C.
Quartz and a number of zeolites also show NTE over certain temperature ranges. Fairly pure silicon has a negative coefficient of thermal expansion for temperatures between about 18 K and 120 K.
Cubic Scandium trifluoride has this property which is explained by the quartic oscillation of the fluoride ions. The energy stored in the bending strain of the fluoride ion is proportional to the fourth power of the displacement angle, unlike most other materials where it is proportional to the square of the displacement. A fluorine atom is bound to two scandium atoms, and as temperature increases the fluorine oscillates more perpendicularly to its bonds. This draws the scandium atoms together throughout the material and it contracts. ScF3 exhibits this property from 10K to 1100K above which it shows the normal positive thermal expansion.