The primary benefit of these diamond anvils is their simplicity of use. They never need to be polished by the users or serviced by the manufacturer.
CRYO ANVIL APPLICATIONS
The whole field of cryo specimen preparation has developed primarily to assure the investigator that chemical materials of interest remain where they are naturally found following the rigors of the preparation procedure. DDDK has made cryo anvils with single crystal diamond inserts for several different slamming devices. These anvils were placed in the field for evaluation and experimental results have demonstrated the advantages outlined above. Depth-of-freeze has proved to be as good as copper and our factory- polished surface cannot be scratched and is permanently free from oxidation. The durability of our copper-diamond bond is shown by the hundreds of impacts it has survived.
Because of the limited sizes of single crystal diamond available, a slamming area of 4mm square is the largest practical. The new CVD diamond film technology is another approach but the ability to grow films on copper, film adhesion and particle size are all problems not yet resolved. Other materials, such as sapphire, have been tried unsuccessfully because of their relatively poor heat transfer capabilities.
A number of methods have been used to initially freeze specimen with the idea that faster is better. These include simple immersion in liquid cryogens of their slushes, contact with jets of cryogens, and contact with a pre-chilled metal mirror. Typical results have given up to 20 microns of properly frozen specimen.
Metal mirror fixation is generally done with either pliers or a press using highly polished metal surfaces. Copper is most often used because its thermal properties are good. It is, however, readily oxidized and scratched reducing its ability to conduct heat. This oxide layer must be removed, usually with a wet abrasive slurry which is slow and messy.
The two thermal properties which are significant in this situation are heat capacity and thermal conductivity. The heat capacity of the involved materials affects the ultimate temperature achieved when the materials come in contact.
For a simply system:
m1cp1T1 + m2cp2T2 = m1+2cp1+2T1+2
(m = mass, cp = heat capacity, and T = temperature).
Since the initial conditions of the tissue are somewhat fixed, it is important that the anvil have a high heat capacity, high mass and low temperature in order to have the lowest ending temperature possible.The heat transfer rate is proportional to the overall heat transfer coefficient, U. This coefficient is related to the thermal conductivity of the materials involved this way:
1 = 1 + 1
U k1 k2
(k is thermal conductivity)
The higher the thermal conductivity of the anvil material, the greater the depth-of- freeze. In our case, however, the tissue controls the results because it’s thermal conductivity is very low relative to that of any anvil material.
Here are the significant thermal properties of copper and several materials that don’t oxidize too rapidly at room temperature. All data was taken at – 150°C:
Silver 1.0 .056
Gold .7 .029
Copper .9 .088
Sapphire .06 .10
Diamond 5.7 .147
Diamond seems to be a natural fit in this application for a number of reasons:
- Diamond conducts heat away from the sample 6 times as fast as copper.
- Diamond absorbs twice as much heat as copper per degree of temperature change.
- Diamond is impervious to both scratching and oxidation (unlike copper or other metals).