The thermal conductivity of diamond
While not a metal, diamond is an excellent conductor of heat, and its thermal conductivity can be increased by increasing isotopic purity.[1][9] The effect is obviously based on some kind of crystal resonance, which is weakened by the 13C atoms randomly scattered between the 12C atoms.
Aluminum and beryllium oxide?
Aluminum oxide[3] and beryllium oxide[2] are also excellent conductors of heat. While natural aluminum[11] and beryllium[8] are isotopically pure, oxygen is not.[10] The crystals of these oxides consist of two (chemically) different atoms, not one. But oxygen and the other element are not randomly scattered around - they have a fixed position in the crystal. On the other hand, 18O and 17O are randomly scattered around, as they randomly occupy oxygen positions, usually occupied by a 16O. Any possibility of a crystal resonance is reduced by the randomly scattered 18O and 17O atoms.
If the oxygen in the crystal would be enriched in 16O, then could this increase the thermal conductivity of the material?
Silicon and other semiconductors?
Natural silicon[5] has three isotopes,[12] gallium has two,[14] while phosphorus[13] and arsenic[15] are monoisotopic. Silicon and the gallium compounds are not pure when in actual use, but the concentration of dopants is often low,[16] much lower than the concentration of 29Si, 30Si or 71Ga.
If a semiconductor would only contain 28Si and dopants (or 69Ga, 31P or 75As and dopants), then could this increase the thermal conductivity and make it easier to manage waste heat?
The thermal conductivity of metals
This possibility (proven in the case of diamond[1] ) only applies to non-metallic materials. The position and state of atoms in metals in not as clearly defined as in the crystal structure of a covalent compound, and the usually high thermal conductivity is caused by a different mechanism. The thermal conductivity of the non-monoisotopic copper exceeds that of aluminum[11] or gold, both monoisotopic.[17]
References:
[1]
Compared to natural diamonds that are composed of a mixture of 12C and 13C isotopes, isotopically pure diamonds possess improved characteristics such as increased thermal conductivity.[1] Thermal conductivity of isotopically enriched diamonds is at a minimum when 12C and 13C are in a ratio of 1:1 and reaches a maximum when the composition is 100% 12C or 100% 13C.[1]
The 12C isotopically pure, (or in practice 15-fold enrichment of isotopic number, 12 over 13 for carbon) diamond gives a 50% higher thermal conductivity than the already high value of 900-2000 W/(m·K) for a normal diamond, which contains the natural isotopic mixture of 98.9% 12C and 1.1% 13C. This is useful for heat sinks for the semiconductor industry.[4]
[2]
Thermal conductivity 330 W K−1 m−1
[3]
Thermal conductivity 30 W·m−1·K−1[3]
[4]
Thermal conductivity 12 (|| c-axis), 6.8 (⊥ c-axis), 1.4 (am.) W/(m⋅K)[1](p12.213)
[5]
Thermal conductivity 149 W/(m·K)
[6]
Thermal conductivity 1.1 W/(cm*K) (300 K)
[7]
Thermal conductivity 0.55 W/(cm·K) (at 300 K)
[8]
http://ift.tt/2keApXy (TL;DR monoisotopic, 9 only)
[9]
12C: 98.9%
13C: 1.1%
[10]
16O: 99.757%
17O: 0.038%
18O: 0.205%
[11]
http://ift.tt/2xHVG1f (TL;DR monoisotopic, 27 only)
[12]
28Si: 92.2%
29Si: 4.7%
30Si: 3.1%
[13]
http://ift.tt/2xHysZ9 (TL;DR monoisotopic, 31 only)
[14]
69Ga: 60.11%
71Ga: 39.89%
[15]
http://ift.tt/2xH7AZg (TL;DR monoisotopic, 75 only)
[16]
Small numbers of dopant atoms can change the ability of a semiconductor to conduct electricity. When on the order of one dopant atom is added per 100 million atoms, the doping is said to be low or light. When many more dopant atoms are added, on the order of one per ten thousand atoms, the doping is referred to as heavy or high. This is often shown as n+ for n-type doping or p+ for p-type doping. (See the article on semiconductors for a more detailed description of the doping mechanism.)
[17]
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