Single Crystal Diamond Filaments
Micro Star Technologies Inc. (MST) has developed a process to produce very thin and long single crystal diamond filaments with potential applications in research and nano technology. The most noticeable property is the filament’s flexibility. Following is a description plus a direct measurement of diamond Young modulus.
The diamond filaments range in thickness from 100nm to 10µ and in length from 80 to 300µ. The cross section has a four sided symmetry in alignment with the filament crystal orientation. Most filaments have some thickness taper from one end to the other, up to 40%. Fig. 1 is an optical microscope image of a typical filament. The thickness measurements and cross section are measured with the TEM and SEM.
Fig. 1 Diamond filament with typical dimensions.
Each filament is a single diamond crystal, which is oriented exactly along the 100 crystal direction. No matter how long the filament is, the orientation is maintained such that in the absence of torsion both ends of the filament have exactly the same crystal orientation. This is shown in Fig. 2.
Fig. 2 Diamond filament crystal orientation.
The most noticeable property of the filaments is their flexibility. The thinner the filament the more flexible it is. Fig. 3 and Fig. 4 show a 250nm filament bent into a loop. The radius in Fig. 4 is 5µ.
The fact that the filaments bend to a small radius without breaking indicates a high degree of perfection on their crystal structure. If there were defects like cracks or discontinuities, the filament would break rather than bend.
Fig. 3 Diamond filament 250nm thick bent in a loop.
Fig. 4 Diamond filament bent in a 5µ loop.
Coupled with the filaments flexibility is what appears to be a total absence of fatigue. At MST several filaments have been subjected to more than a thousand bending cycles of about 20° without breakage. This will of course require more extensive testing.
Diamond Young Modulus Measurements.
In view of the filament flexibility, the first obvious experiment is a direct measurement of the Young modulus of elasticity. In most materials this is measured with a simple test. For instance, a steel wire held firmly on one end as a cantilever with a weight at the other end, deflects a certain amount. Measuring the deflection and knowing the length, cross section and weight, the Young modulus can be readily calculated.
The Young modulus has not been measured directly in diamond for several reasons. One is, there have not been diamond crystals available with a large aspect ratio, which is the ratio of length to thickness. Another is, the extreme hardness and brittleness of diamond. If a large enough force is applied to a commonly available diamond crystal, it will break or shatter before any measurable deflection can be observed.
The Young modulus has been measured in diamond with indirect methods. One way measures the speed of sound or compression waves in a diamond crystal. This is described by J. Wilks and E. Wilks (Properties And Applications Of Diamond. Butterworth-Heineman Oxford 1991). They give an approximate value of 1,050 GPa. Due to anisotropy of diamond the Young modulus varies with crystal orientation by approximately 10%, according to the same source.
Having diamond filaments with aspect ratios as high as 1,000, a direct measurement of the Young modulus is a straight forward procedure although some practical limitations arise from the microscopic scale involved. The following is the procedure used at MST to measure the Young modulus of the diamond filaments.
A filament is accurately measured with a transmission electron microscope (TEM) to determine its length, thickness and cross section dimensions. The filament is firmly held on one end and the other end stays free. It is then positioned on an optical microscope stage such that both ends of the filament are precisely located on the focal plane. This establishes the initial position of the filament before it is deflected.
Next, a weight is attached at the free end without disturbing the filament position on the stage. The weights used are small spheres of sapphire or tungsten carbide. The specific gravities of these materials are known. The diameter of the spheres is verified in the optical microscope. The spheres are also weighted with a precision scale to verify that the actual weight agrees with the calculated weight. The sphere is attached to the filament with a small drop of glue whose weight is also taken into account.
The deflection of the filament at a given point is measured with a micrometer gage coupled to the microscope focus. The deflected filament is also photographed at a known magnification scale.
Fig. 5 Optical microscope image of the diamond filament of Fig. 1 with a 300µ sapphire sphere attached.
Fig. 6 The same filament of Fig. 5 with the parameters used to calculate the Young modulus. L: sphere’s moment arm; x: distance to the point where the deflection v and the slope v’ are measured.
The microscope images provide the parameters needed to calculate the Young modulus. Fig. 5 is a microscope photograph of a filament being deflected by the weight of a sphere. Fig. 6 is the same image with the parameters used to calculate the Young modulus.
From the tests performed at MST the average value for the Young modulus of diamond in the 100 crystal direction was measured at 900 GPa. Due to various experimental limitations the error margin is estimated at ± 15%.
To verify the validity of the test, a 5µ diameter carbon fiber was measured the same way. The calculated modulus for the carbon fiber was 516 GPa, which agrees with available data (Materials Handbook, Brady, Clauser and Vaccari, McGraw-Hill 1997).
MST has considerable resources to manufacture, manipulate and measure items at micrometer and nanometer scales. Nevertheless other laboratories with better research and scientific resources should be able to verify and refine the above numbers providing higher precision. The availability of these diamond filaments should lead to other experiments and tests of diamond properties and possibly new applications in research and industry.
MST has several sizes of diamond filaments available at nominal prices, or can produce them on request. MST also can attach the filaments to any desired holder or to scanning probe microscope probes. Email Bernard Mesa (see the bottom our home page).