Carbon: Introduction

with permission, based on Ian M. Watt’s book1 and article4

• Density: 2.2gm cm-3
• Melting Point: around 3550°C
• Evaporation Temp.: 2400°C
  • Temperature at which a substance has a vapor pressure of 1.33 x 10-2 mbar (arbitrary)
• Average Thickness: 23nm at 5µg cm-2 in nm
• Uses:
  • TEM substrates, replicas
  • SEM coating of non-conducting samples for x-ray analysis
  • EBSD: coating of non-conducting samples with an amorphous layer
  • Microprobe: coating of non-conducting samples with highly conductive layer

Carbon films are uniformly amorphous and highly transparent to electrons because carbon has a low atomic weight. Due to their strongly interconnected three-dimensional network structure, carbon films have remarkable mechanical stability, even when as thin as 1 - 2nm3.
At thicknesses used in electron microscopy from about 2nm upwards carbon films are chemically inert, continuous and basically free from self-structure except for a speckled appearance visible at only the highest magnifications.
Carbon depositions produce a film around the sides and back of specimens without specimen rotation (see EM picture to left). This may be explained by the deflection of light carbon atoms out of straight-line flight by collisions with gas molecules and with the walls or obstructions in a vacuum chamber.

References:
1. Watt IM, 1985. The principles and practice of electron microscopy. Cambridge University Press, ISBN 0-521-25557-0.
2. Bradley DE, 1954. Evaporated carbon films for use in electron microscopy. Br J Appl Phys, 5, 65-66.
3. Hayat MA, 1981. Principles and techniques of electron microscopy, Vol 1 (2nd Ed), University Park Press.
4. Watt IM, November, 1991. Carbon - The electron microscopist’s most useful element. Microscopy and Analysis, pp 21-23.

The carbon rods are offered in either spectroscopically pure or technical grades for standard electron microscopy evaporation requirements using carbon coaters. Due to the purification process used all carbon rods we supply have a graphite structure. The purification process removes impurities at a high temperature and causes the carbon to crystallize. The graphite structure has no adverse effects on the carbon coating or the carbon coating process. The user might perceive an additional advantage: the carbon rods (graphite rods) are less brittle than the carbon rods and are easier to handle and easier to shape.

Technical grade carbon rods have a typical impurity level of around 50ppm or greater. Typical applications are non critical SEM or SEM/EDS coating of non-conducting samples.

"Spec-Pure" (spectroscopically pure) grade is available for carbon (graphite) rods with impurities equal or less than 2ppm (single element 1 ppm or less). Typically the impurity level is more likely 1ppm or less. This grade should be used for TEM, FESEM, FIB, WDS, EBSD, Microprobe and more critical SEM/EDS applications.