SPHERICAL DIAMOND AND MANUFACTURING METHOD FOR SAME

Abstract

Among all the materials available on earth, diamond has demonstrated outstanding properties for general-purpose applications. Nevertheless, due to the total lack of processability, artificial diamonds have never captured large industrial markets for the recognized performance. However, theoretical chemists recently paid attention to an old but highly efficient way of producing new facets on gem diamonds by manual self-abrasion. They found by using molecular dynamics calculations that the rate-determining step in the self-abrasion sp.sup.3-sp.sup.2 order-disorder transition on the crystal surface. The product of such a transition is an amorphous layer, which chemically decomposes to produce a new facet. Taking advantage of the self-abrasion mechanism thus found, we designed a novel spheroidization method and experimental apparatuses, wherein the self-abrasion works preferentially on mechanically weak portions like vertices and edges but hardly on stronger surfaces. Spherical diamonds lack self-aggregation properties, are resistant against shocks, have mechanically strong surface and offer a new material.

Claims

1. A method of manufacturing spherical diamond particles by subjecting irregular polyhedral single-crystalline diamond powder to an improved spheroidization process, wherein asperities on the crystal surface like apexes and edges are preferentially abraded upon light but direct collision with the neighboring diamond particles to approach spherical surface morphology.

2. A method of manufacturing spherical diamond particles as mentioned in claim 1 but characterized by controling the spheroidization process of self-abrasion using one or a plural number of the following four auxiliary measures including (1) pressing, (2) rolling, (3) heating or cooling, (4) lining of the inner wall of the cylindrical abrasion container with CVD polycrystalline diamond thin-film.

3. Spherical diamond particles manufactured as described in claim 2, each particle consisting of the known internal diamond core and spherical surface comprising of a large number of partial facets. The particles are further characterized by appropriately high sphericity index, or circularity index derived from the analysis of two-dimensional images. When the spherical diamond particles are smaller than micron sizes, it is desirable that they have circularity index of greater than 90%, or more favorably greater than 95%.

4. Spherical diamonds as described in claim 3, and characterized by having the unsaturated valence of surface carbon atoms formed by the abrasion process saturated by adding hydrogen, fluorine, oxygen, water and other substances.

5. Manufacturing method of spherical diamonds as described in claim 2, and characterized by adopting the pressing process, one of the auxiliary processes claimed to accelerate the spheroidization, in the following manner. Vertical pressure is created by weights placed on top of the cover disk and applied to a layer of single-crystalline diamond particles, loosely packed in the abrasion cylinder in such a way that each particles can be readily roll with the revolving movements of cylinder. Such an arrangement works to increase the force acting between asperities on the surface of diamond particles in direct or shearing contacts to accelerate their destruction by wearing.

6. Manufacturing method of spherical diamonds as described in claim 2, but characterized by adopting the rolling process, one of the auxiliary processes claimed to accelerate the spheroidization, in the following manner. In this invention, rolling is introduced in order for spheroidization to occur evenly over the entire surface of diamond particle to reach the desired high sphericity in the shortest possible operation time. This purpose is fulfilled by horizontally revolving the cover and cylinder of self-abrasion apparatus in opposite directions or revolving only the cylinder and fixing the cover at a static configuration. In this way all the diamond particles always keep rolling to achieve uniform abrasion of surface and reach high sphericity.

7. Manufacturing method of spherical diamonds as described in claim 2, but characterized by adopting the heating or cooling, one of the auxiliary processes claimed to accelerate the spheroidization, as mentioned above, in the following manner. In this invention, heating is introduced in order to accelerate the spheroidization reaction by heating the space of self-abrasion chamber to 100 to 300 C., or cooling is introduced in order to retard the reaction by cooling the same space to below room temperature, with the purpose of reaching the desired sphericity in the shortest possible operation time.

8. Manufacturing method of spherical diamond as described in claim 2, and characterized by adopting lining of the inner wall with high-quality polycrystalline diamond film, one of the auxiliary methods of accelerating the spheroidization, namely, as follows. In this invention, the purpose of lining is to prevent wearing damage of inner wall by collision with the diamond particles being abraded. Especially vulnerable material of inner wall will be iron, which will form brittle iron carbides. We will use readily available 3 nm diamond particles as the nucleation seeds for the lining with polycrystalline CVD diamond film.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 Illustration showing basic concept of combined pressing, rolling and self-abrasion actions. A layer of diamond particles is inserted between a pair of hard and concentric disk. Disks are pushed toward each other, so that all the particles make close contacts with neighboring particles. At the same time, the two disks are rotated in opposite direction in order for the whole diamond particles to engage in rolling.

[0050] FIG. 2 Left: A commercial motor-driven Chinese ink-stick grinder. A shallow circular ink-stone is filled with water and revolved slowly, and a pair of ink-stick are fixed to holders by screwed pinches and pressed vertically onto the revolving ink-stone. Used for producing fresh aqueous ink for calligraphy. Center: The first version of spheroidization apparatus. Shallow and circular ink-stone is replaced by a shallow cylinder with a base disk with 103 mm in the inner diameter, 30 mm in depth and 6 mm in thickness, made of SUS 304, and fixed to the ink-stone motor, and revolved at a rate of about 30 rpm. At the bottom of cylinder is spread 20 g of microdiamond powder. A SUS 304 cover disk with a diameter of 100 mm, a thickness of 5 mm, a weight of 620 g, is inserted into the cylinder. The cover disk also works as a weight (see FIG. 4). Right: The second version of spheroidization apparatus. A simple device that is able to adjust the positioning of cover disc is attached at the right end of apparatus. In addition, the original plastic shield was removed to assist external cooling by means of an electric fan and avoid overheating.

[0051] FIG. 3 A digital microscopic image of microdiamond before spheroidization. Nominal diameter of diamond powder=22-36 m. Microscope is manufactured by Tokyo Hirox Co., Type KH1300.

[0052] FIG. 4 Dimensions of the first version of spheroidization apparatus in mm.

[0053] FIG. 5 Histogram of Heywood diameter distribution in microdiamond at various stages of spheroidization process. Top: Before abrasion (sample size=119). Middle: After execution of Example 1 (sample size=119). Bottom: After completion of Example 4 (sample size=512).

[0054] FIG. 6 Histogram of circularity index distribution in microdiamond at various stages of spheroidization process. Details are same as FIG. 5.

[0055] FIG. 7 A digital microscopic photograph of microdiamond after the completion of Example 2. Note the pulverized layer of microdiamond particles.

[0056] FIG. 8 Comparison of the distribution in Heywood diameters of microdiamonds before spheroidization and after the completion of Example 2. About 30% of the whole sample became pulverized to reduce the average diameter from 29.155.65 m before abrasion to 23.987.34 m. No significant change was observed in circularity index.

[0057] FIG. 9 Digital microscopic photographs of flattened microdiamond particles after the completion of Example 4.

EMBODIMENTS OF THE INVENTION

[0058] The present invention can be better understood by reading the following explanations while looking at the Figures. Although individual details are given in Examples, this invention is not limited to the particular methods, conditions, devices and illustrations mentioned below.

EXAMPLE 1

[0059] The ink-stone revolving mechanism of a commercial Chinese ink-stick motor grinder (FIG. 2 right) was removed and a SUS304 self-abrasion cylinder with an inner diameter of 103 mm, a depth of 30 mm and a thickness 6 mm was attached as shown in FIG. 4. In addition, the ink-stick holding mechanism was replaced with a SUS304 disk with a diameter of 100 mm, thickness 5 mm and a weight of 620 g, which was slid horizontally into the inside wall of abrasion cylinder, thus acting as a weight as well as cover. The modified set-up is called here as the second version of spheroidization apparatus. Twenty g of commercial microdiamond powder having an average diameter of 29 m was placed in a thin space between the cover and bottom disc of the abrasion cylinder, which was then subjected continuous revolving by turning on the motor. However, the motor proved too small to drive heavy cylinder for a long time, and evolved much heat. When the temperature of outer wall of cylinder reached 70 C. after six hours, the operation was suspended.

[0060] After leaving the spheroidization apparatus to room temperature, a few portions of abraded microdiamond were sampled from near the center of cylinder bottom and observed under a digital microscopy (constructed by Tokyo HIROX Co., Type KH3000). In the beginning no visible change could be discerned except for slightly darkened color of the particle surface, but 119 isolated particles that showed continuous periphery were selected under the microscope and subjected to the analysis of Heywood diameters and circularity index using commercial image analysis software (MacView, 4.sup.th Version, from Tokyo Mountech Co.). Comparison of histogram distribution of these parameters before and after the spheroidization operation revealed much difference (FIGS. 5 & 6): Heywood diameters decreased whereas circularity index increased. In the latter a few new peaks having circularity indices higher than 90% appeared. As these shifts were much smaller than the distribution widths, significance of the changes was studied by F- and t-tests. The changes in both parameters before and after self-abrasion proved to have equal dispersibility and siginificant difference, respectively (Table 1). Results of statistical analysis are described in the previous section DETAILED DISCLOSURE OF THE PRESENT INVENTION.

EXAMPLE 2

[0061] Using the same spheroidization apparatus as mentioned above, we managed to hang the heavy cover in exactly parallel position with the base of cylinder to avoid excessive friction between them and suppress heat evolution to allow longer and continuous operation. In the course of adjusting and running, we had a bad case of direct and strong contact between cover and base disks, which kept revolving for a few hours making sharp noise. Abraded microdiamond powder had developed intense black color, indicating contamination of SUS304 from the inner wall. Inspection under the digital microscope showed a large proportion of pulverized microdiamond particles (FIG. 7).

[0062] We sampled 177 pieces of microdiamond randomly and analyzed the distribution of Heywood diameter (FIG. 8). The histogram showed that about one third of the powder was fragmented into much smaller pieces and formed a second broad distribution centered at 14 m in diameter (FIG. 8). The rest comprises the major peak at the same position as before the abrasion. Namely, fragmentation preceded abrasion when too high pressure was applied.

EXAMPLE 3

[0063] In contrast to Example 2, we encountered with an opposite result, wherein neither Heywood diameter nor circularity coefficient changed significantly before and after 12 hours of continuous operation. The t-Test confirmed this conclusion of no change. Although we did not measure the pressure, it seems that in this case the applied pressure was out of range. It is likely that the applied pressure was somewhat lower than the critical value. We will take advantage of this lesson in the design of the third and higher models in order to realize the desired spheroidization.

EXAMPLE 4

[0064] In another experiment in which we wanted to reproduce and extend the results of Example 1, we encountered still different result. After the most careful adjustments of the parallel disposition between cover and base disks, we succeeded in running the abrasion continuously for five days. However, the results were surprising: diameter increased and circularity decreased, both by small margins. Furthermore, observation under the digital microscope revealed extremely large particles with diameters up to 50 m and remarkably flattened in shape (FIG. 9). The interpretation of these observations was given in the section DETAILED DISCLOSURE OF THE PRESENT INVENTION in great detail.

[0065] We have demonstrated various possibilities of manipulating shapes of small diamond particles by means of pressure- and rolling-assisted self-abrasion method, with special attention to the spheroidization which should add much higher value to the small artificial diamond particles for industrial applications. However, this invention is not limited by the few examples given here, but should give many more variations in the shapes of diamond within the claimed scope of invention.