Thermal expansion Thermal Expansion is the change in volume of
a material when heated. Generally, materials increase in volume
when heated. A materials coefficient of thermal expansion tells you
how much its volume changes with temperature over time. A select
group of materials actually decrease in size when heated. This is
called Negative Thermal Expansion (NTE).
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Uses
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Why do cyanides undergo NTE? These cyanide-bridged compounds,
such as Zn(CN) 2 and Cd(CN) 2 have tough octahedral units with
strong metal carbon and metalnitrogen bonds joined by loose cyanide
bridges. At a given temperature, the C and N atoms oscillate around
the M-M axis. The displacement of these atoms can either be in the
same direction or in the opposite direction. Both of these modes of
oscillation cause the bonds to shorten as the metal atoms are
anchored closer together. This increases as the temperature rises,
as the oscillations are greater.
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Displacement of C and N atoms (a)single-atom and (b) diatomic
linkages * Black circle= heavy atoms White circle=bridging atoms
e.g cyanide Displacement same direction as axis Displacement
different directions to axis
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Computer modelling of the lattice To investigate this
behaviour, we used the supercomputer to run simulations of Zinc and
Cadmium cyanide crystal structures at different temperatures. The
computer simulates the various interactions between the particles
in the lattice. Surface effects are avoided by simulating a cube
where a particle leaving though one side will emerge through the
opposite. We interfaced with the supercomputer using the DL_POLY
module through Linux. As well as Zinc and Cadmium cyanides,
mixtures of zinc and cadmium cyanides were modelled (Zn x Cd 1-x
(CN) 2 ). We used different ratios of Zn and Cd (x =
0.2,0.4,0.6,0.8).
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The Results
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Zn40
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Zn20 Zn40 Zn60Zn80 600K
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Zn20 at 300K
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Zn20 at 400K As temperature increases, so does bond energy. The
greater the bond energy, the more vibrations so more displacement
in either of the modes, the shorter the bond length.
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Zn20 1000
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Zn40 1000
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Zn60 1000
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Zn80 1000
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Calculations 100k -0.02395 500k -0.01726
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Better Formats
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Conclusion Many difficulties needed to learn Linux quickly
However managed to get data despite time pressure. Many areas for
further research such as introduction of guests/different
geometries. Interested to see experimental results Many thanks