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Basic Properties of Diamond

Hanjiang Wang — Tue, 12 Apr 2011 05:59:20 +0000

Diamond can be either natural or synthetic, and has the following basic properties:

		Description
Mechanical Properties	Hardness	Diamond is almost the hardest material on the earth. On the Mohs scale of mineral hardness, its hardness is 10 — the hardest. Its microhardness is about 100000 MPa. Its Rockwell hardness is about 1.56 times of CBN’s, 3.7 times of tungsten carbide’s, 4.4 times of corundum’s and 8.5 times of quartz’. Diamond’s hardness is different on its different orientations (crystal planes). Because of its very high hardness, diamond can be used to make various diamond tools such as diamond saw blades, diamond core drill bits, diamond cup wheels and diamond polishing pads, to cut, grind or polish hard or abrasive materials, e.g., stone, concrete, asphalt, ceramics, bricks and carbide alloy.
	Brittleness	Though diamond is very hard, it is relatively brittle. It splits along its crystal cleavage planes when receiving a certain impact. The brittleness of diamond is also related to its crystal completeness. Crystal defects can generate internal stress and even lead to natural split. While complete crystal has a fairly high toughness and its split pressure can reach 30 – 100 MPa. So “impact toughness” is one of the main indexes to measure the quality of industrial diamonds.
	Density	The density of diamond is normally 3.47 – 3.56 g/cm³. Pure and complete-crystal diamond’s density is 3.52 g/cm³.
	Cleavage	The cleavage of diamond happens most likely along the <111> plane (a triangle that consists of the three diagonals of the three adjacent planes of a cubic diamond lattice).
	Fracture	The fracture surface of diamond can have a complex structure, and is often conchoidal or irregular-shaped.
Optical Properties	Color	Pure diamond has no color, but it is fairly rare. The color of diamond is often yellow, green, brown, rose, blue, grey and black. This is related with the impurities it contains and its crystal defects.
	Luster	Diamond often has the so-called “diamond gloss”, which means quite strong reflected light, shining but not metallic. A few diamonds have oil sheen, glass gloss or even no luster, which is usually caused by long-term chemical corrosion, radiation effects, invasive alien substances or other materials coated on their surface.
	Transparency	Pure diamond is transparent, and impure diamond can be translucent or even opaque.
	Refractive index	Pure diamond’s refractive index is 2.40 – 2.48, which is the highest in transparent minerals. The higher refractive index, the higher reflectivity. Diamonds that are specially designed and processed can reflect almost all the light that comes to their sides and interior.
	Dispersion	Diamond’s dispersion coefficient is 0.063, which is also the biggest in transparent minerals. The bigger dispersion coefficient, the better spectroscopy result. When a white light comes into a well-cut diamond, the light can be split into different colors of light because of the diamond’s dispersion effect, and the diamond looks dazzling.
	Luminescence	Diamond can shine distinct green, sky-blue or blue fluorescence under cathode ray, medium-brightness or weak sky-blue fluorescence under X-ray, distinct or medium-brightness sky-blue, purple or yellow-green fluorescence under ultraviolet, and light green-blue phosphorescence in darkroom after exposure under sunlight.
Thermal Properties	Thermal conductivity	Diamond is a good heat conductor. Different types of diamonds have different thermal conductivity. Some diamonds have super thermal conductivity.
	Thermal expansion	Diamond’s thermal expansion coefficient is very small at low temperature, but increases rapidly with the rising of temperature.
	Heat resistance	Diamond’s ignition point is 720 – 800 °C in oxygen and 850 – 1000 °C in air. The flame is blue when diamond is burning.
Electromagnetic Properties	Magnetism	Pure diamond is not magnetic. But as containing impurities of catalyst alloy, e.g., Ni, Co, Fe, Mn, Cr, synthetic diamond is magnetic. The more catalyst impurities, the more magnetic the synthetic diamond is. Low magnetism indicates the diamond’s good crystal and its good quality.
	Electrical conductivity	Normally, diamond is a poor electrical conductor. Its electrical conductivity increases when temperature rises. Some diamonds are good semiconductor.
	Photoconductivity	When diamond is irradiated by ultraviolet of 2.1 x 10^-4 – 3.0 x 10^-4 mm wavelength, there are photocurrents generated in the diamond.
	Triboelectricity	When diamond rubs on the surface of glass, hard rubber or plexiglass, positive charge can be produced on the diamond.
Surface Properties	Lipophilicity & Hydrophobicity	Diamond cannot be wet by water, but can be easily wet and stuck by oil. This property can be utilized to extract diamonds using oil when making synthetic diamonds.
Chemical Properties	Chemical stability	Diamond has very good chemical stability. It does not react with acid and alkali under room temperature. Its surface can only be oxidized a little by just a few oxidants under high temperature (below 1000 °C). So acid and alkali can be used to refine synthetic diamonds.

The Manufacture Methods of Diamonds

Hanjiang Wang — Wed, 14 Oct 2009 04:11:57 +0000

There are many methods to manufacture diamonds in the world, but the most common method is static high-pressure high-temperature synthesis with catalysts. According to the characteristics of the synthesis technology and the crystal growth mechanism, these methods can be categorized to the following types:

1) Static high-pressure high-temperature: This method also has two sub-types – with or without catalysts. The one without catalysts is to directly change graphite to diamonds under super high pressure and temperature. Its requirement for pressure is about 10 GPa, and requirement for temperature is above 3000 K. These requirements are higher than the requirements of the method with catalysts by about 1 time.

2) Dynamic high-pressure high-temperature: This method is also called explosive method, which changes graphite to diamond grains of micron sizes under instantaneous super high pressure and temperature which are generated and disappear very quickly. The pressure can reach 1000 GPa, and the time is only a few hundred microseconds.

3) Chemical Vapor Deposition (CVD): This method grows diamonds at low pressures (133 Pa – 106 kPa) and non-high temperatures (500 – 800°C). By this method, a hydrocarbon gas mixture is ionized into chemically active radicals to form diamonds. The advantages of CVD diamond growth include the ability to grow diamond over large areas and on various substrates, and the fine control over the chemical impurities and thus properties of the diamonds produced.

The Mechanical Properties of Diamond

Hanjiang Wang — Wed, 07 Oct 2009 10:00:27 +0000

Perhaps diamond is the hardest material on the earth (it is said that there are still some harder materials). It has high thermal conductivity, high insulation and high chemical stability. It can be used to process non-ferrous metals such as aluminum and copper both efficiently and precisely, and is especially suitable to process hard and brittle non-metallic materials.

1) High hardness, high wear resistance: Diamond has the highest hardness value. Its abrasive hardness is 150 times of corundum, 1000 times of quartz and 6 times of carbide alloy.

2) Low friction coefficient: The friction coefficient between diamond and metal in the air is 0.1. Therefore, diamond has a very high frictional resistance. Its friction resistance is 90 times of corundum, 100 times of carbide alloy and 9000 times of steel.

3) High strength: Diamond’s crystal structure, its Impurity’s composition and content determine the strength of a diamond. Natural diamond’s compressive strength is about 8000 kg/cm², which is about 3.5 times of corundum, 1.5 times of carbide alloy and 9 times of steel.

4) Though diamond has high hardness and high strength, it also has brittleness. When a diamond receives a certain degree of impact force, it may generate cracks or even be broken into regular-shape small pieces. This is the biggest weakness of diamond.

5) High thermal conductivity, bad thermal stability: The change of temperature has a fairly big influence on the strength of diamond. With the rise of temperature, diamond’s strength declines.

Your Diamonds May Lose Their Values

Hanjiang Wang — Tue, 29 Sep 2009 03:09:43 +0000

On September 5, 2009, Henan Polytechnic University (China) successfully synthesized an 8.2mm, 2 carats weight high quality yellow diamond.

8.2 mm, 2 carats synthetic diamond made by Henan Polytechnic University (China).

Diamonds can be synthesized or be so called “cultured” in man-made environments, and the synthetic diamonds can be the same with or even better than natural diamonds. Gemesis, the world’s leading gem quality synthetic diamonds producer in Florida, USA, says: “Gemesis cultured diamonds are at the forefront of a revolution in the diamond jewelry industry. A Gemesis cultured diamond is a diamond, grown under the same conditions as diamonds grow beneath the earth’s surface. By applying tremendous heat and pressure, a diamond begins to grow, atom by atom, layer by layer, just as it does in nature. A Gemesis cultured diamond possesses the same physical, optical and chemical properties as earth-grown diamonds.”

Yellow synthetic diamond gemstones made by Gemesis, a company in Florida, USA.

It was said that the production cost of a Gemesis cultured diamond is less than 1% of the price of a same natural diamond. Gemesis is producing big diamond gemstones in huge quantity. Also, its diamonds’ color can be yellow, pink, blue, green, or even red, which are rare in the nature and are known as “fancy color”.

Cultured yellow diamonds surround a pink diamond. Made by Gemesis.

It was reported that one of the leading synthetic diamond manufacture companies in China is also researching gem quality synthetic diamonds and already can produce 6mm ones. This company used to produce small synthetic diamonds for diamond tools.

The market’s responses to synthetic diamond gemstones are active. A jeweler in USA said: “If you go into a florist and buy a beautiful orchid, it’s not grown in some steamy hot jungle in Central America; it’s grown in a hothouse somewhere in California. But that doesn’t change the fact that it’s a beautiful orchid.” “Nobody cares if it’s from De Beers. My clients just want a nice diamond.”

With the same or even better but much cheaper synthetic diamond gemstones’ emerging on the jewelry market, it can be anticipated that the natural diamond gemstones will have no advantages and their values will decrease dramatically. If you have natural diamond gemstones or have investments on them, you will have to be prepared.

Colorless gemstone cut from diamond grown via another method called “CVD” (Chemical Vapor Deposition) by Apollo Diamond, a company in Boston, USA.