Osmium Alloy Four Point Probe Tips vs Tungsten Carbide Tips
Due to the limitations of Osmium materials, Jandel Engineering now only offers tungsten carbide probe tips. Occasionally we are asked about the relative merits of the osmium alloy tips compared to the much more commonly used tungsten carbide tips. In a nutshell, the osmium alloy tips are somewhat softer and less durable. Here are the comments from Jandel’s managing director John Clark regarding the usage of osmium alloy tips compared with the much more common and more durable tungsten carbide tips:
“The limit of our knowledge of osmium as an ELEMENT is that it is the most dense, and that it cannot be fabricated. The material we use for probe tips is used for ‘pen-balls’ – those used on gold pen nibs to give a long writing life. The ones we use are a mixture of platinum group metals including osmium, platinum, rhodium, palladium etc. The specification we had merely stated “not less than 50% osmium alloy”. We have had this checked by a laboratory and it was true at that time, some years ago, and this was for a proprietary alloy “Irit 57” made by the Degussa company. From our point of view “osmium” (alloy) tips are a lot softer than TC and inclined to crumble when we shape them. For this reason we are cautious when we accept requests for less than 100 microns x 100g, and really don’t like the idea of making 40 microns. I am fairly sure it is more conductive, and may possibly offer less contact resistance in some circumstances. Our output of osmium tipped probes is less than 2% of the total we make and I guess this is on account of their failure to maintain a tip profile for very long. Going back to the early days of 4-point probing, people used to make their own probe heads with ‘gramophone needles’ (not fibre!). Such needles were often osmium alloy tipped, so are the needle points used in moving iron and moving coil meters. I think the use of osmium alloy here was more against corrosion than anything else. Cynically it is my belief that the use of osmium alloy tips originated from these early attempts and has persisted to the present day.”
More information regarding pure osmium, the element:
Mineral hardness: A fingernail has a hardness of 2.5, a copper cent coin is 3, a knife made of steel is 5.5, and osmium is 7. The hardness of tungsten carbide on the Mohs scale is 8.5 to 9.0.
The scale is roughly logarithmic, that is, a material with a hardness of 8 is roughly 10 times as hard as another material whose hardness is 7, which in turn is 10 times as hard as a material whose hardness is 6, and so on. The minerals chosen by Mohs for this scale are:
10: diamond
9: corundum
8: topaz
7: quartz
6: orthoclase
5: apatite
4: fluorite
3: calcite
2: gypsum
1: talc
Osmium metal is lustrous, bluish white, extremely hard, and brittle even at high temperatures. It has the highest melting point and lowest vapour pressure of the platinum group. The metal is very difficult to fabricate, but the powder can be sintered in a hydrogen atmosphere at a temperature of 2000oC. The solid metal is not affected by air at room temperature, but the powdered or spongy metal slowly gives off osmium tetroxide, which is a powerful oxidising agent and has a strong smell. The tetroxide is highly toxic, and boils at 130oC (760 mm). Concentrations in air as low as 10-7gm-3 can cause lung congestion, skin damage, or eye damage. Although diamond is much harder than osmium, osmium can withstand compression better than any known material. A research team at Lawrence Livermore National Laboratory squeezed a tiny sample of the metal to extremely high pressure and found that it shrunk even less than diamond. (text courtesy of www.webelements.com)
Four-Point-Probes is a division of Bridge Technology. To request further information please call Bridge Technology at (480) 219-9007 or send e-mail to Joshua Bridge at: sales@bridgetec.com
