David HitchcockMaterial Science Report11-23-10

Introduction

Many people have heard the expression that diamonds are a woman’s best friend. Men have

been giving diamonds to the women they love ever since the diamond was first crafted into the

beautiful specimen that we have come to know. The majority of the money spent on diamonds today is

for the purpose of jewelry, but the use of diamonds for industrial applications is a very important one.

Man has been searching for a method to make diamonds for centuries. This goal became more central

as the utility of diamonds in industry increased. Today synthetic diamonds have become very important.

While most women would prefer to receive a natural diamond most manufacturers would prefer to buy

synthetic diamonds today. They have found their way into many tools and products that need to

maintain strength or durability. This paper covers some brief history of natural and synthetic diamonds

as well as the production of synthetic diamonds and how they are tested to ensure their quality. Some

of the various applications of diamonds are briefly discussed.

Brief History of Diamonds

India was the only source of natural diamonds until the 1860’s. During the time of the American

civil war, diamonds were discovered in Africa. 1 Now there are diamond mines in America, Canada,

Australia, and Russia as well. The diamonds used in jewelry are crafted by extracting the gems from

natural diamond deposits. The remaining portion of the diamond that is not useful as gemstones are

then used as industrial diamonds. They have been used in saw blades, grinding wheels, core drills, and

many other abrasive and cutting tools.1

In 1770 Lavoisier proved that diamonds were made out of carbon, and with this information

many scientists began to attempt to create diamonds. The typical method attempted was to reproduce

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the natural method by which diamonds are created. This meant subjecting carbon based materials to

extreme temperatures and pressures. Figure 1 shows a simplified phase diagram of carbon which

indicates the pressure and temperature range at which diamond is a stable form of carbon. Over the

next couple of centuries many scientists failed to make any diamonds. In 1950 General Electric

assembled a team of scientists to create the first synthetic diamonds. They developed a new method

and apparatus for producing and controlling extreme temperatures and pressures. Four years after the

team formed they had produced what they had thought was diamond. After intensive testing they had

come to the conclusion that they had indeed made a diamond. General Electric went on the produce

many types of diamonds to fit the different applications of diamonds. Now that man could make

diamonds they could craft diamonds with characteristics specific to the job they were needed for.1

Figure 1: The carbon phase diagram shows that graphite is typically the stable form of carbon and diamond is only stable at very high pressures and temperatures.2

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Diamond Structure and Characteristics

Diamonds are made of a crystalline structure of carbon atoms and some impurities. The crystal

structure consists of planes of puckered hexagonal rings. The planes are stacked in a pattern so that

every fourth plane is aligned. The spacing between all carbon atoms is 1.54 Å, and the atoms are bonded

at the tetrahedral angle, 109.28°. The cleavage plane is halfway between two planes of atoms. 3 Figure 1

shows how the carbon atoms are arranged in a diamond.

Figure 1: Projections of the diamond crystal lattice. In the top view plane a is drawn with solid lines, plane b is drawn with dashed lines, and plane c is drawn with dotted lines.3

Diamond has long been regarded as the hardest substance known to man. The Mohs scale of

hardness shows diamond at 10 on a scale of 1-10. Another test of hardness uses a Knoop indenter, a

pyramidal point with long and short axes in the ratio of 7:1. The values obtained from this test a 5700-

10,400 kg/mm2. This means that it takes 5700-10,400 kg of load to make an indentation with a surface

area of 1 mm2. The young’s modulus of diamond is 1050 GPa and its tensile strength is approximately

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400 kg/mm2.2 Impurities within a diamond, usually substitution or interstitial defects, can change some

of its properties. For example, Nitrogen impurities will increase the hardness of diamonds and the

smoothness of their surface. 4 The nitrogen content of a natural diamond determines its classification,

type I if it contains nitrogen and type II if it has negligible amounts of nitrogen. Type II diamonds have a

thermal conductivity of about 20 W/cm-K while the nitrogen content of a type I diamond inhibits its

thermal conductivity by about half. The synthetic diamonds are often produced with no nitrogen so they

have a thermal conductivity about 50% greater than that of Type II diamonds. Diamonds are naturally

electrical insulators. They have an approximate dielectric constant of 5.7 which is similar to that of glass.

Typical resistivity values for pure diamond range from 1014-1016 Ω-cm, but with higher levels of boron

impurities this value can be as low as 0.1 Ω-cm which is similar in value to seawater. 2It has been found

that diamond can be a super conductor in certain circumstances if it contains Boron impurities. This is

because boron has one less electron so it can carry electrons through the diamond structure. 5

Diamonds can be synthesized into different forms of diamonds. The one that most people are

familiar with is the single crystal diamonds where the diamond produced is one large continuous crystal.

All of the previously mentioned properties are measured using single crystal diamonds. Polycrystalline

diamonds can also be produced. These diamonds are made of many smaller crystals that shape into the

appearance of one crystal. Polycrystalline diamonds, aside from a few exceptions, have the same

structural properties as single crystal diamonds. Polycrystalline diamond has a much lower fracture

strength than single crystals do. This is attributed to the higher amount and frequency of flaws in the

overall crystal structure. Since polycrystalline diamonds are made of many diamonds they do not have

the well-defined cleavage planes that make natural diamonds ideal for use in jewelry. 2They are not able

to be cut into the perfect shapes that most people know diamonds for. Diamonds films can be

synthesized using many different methods. These are usually polycrystalline sheets of diamond that

bond to a substrate to give it a diamond surface. Nanodiamonds are one of the most recently developed

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diamonds that can be synthesized. These are very small diamonds that are useful for coatings in many

applications because they retain all of the properties of diamonds.

Synthetic Diamond Production

High Pressure High Temperature Method

The first method of creating diamonds employs high pressure and high temperatures. This is

commonly referred to as the HPHT method. One of the first of these methods was designed by H. Tracy

Hall of Brigham Young University. This method was then reproduced by General Electric, making it the

first known successful reproduction of another man’s experimental design to form diamonds. The belt

apparatus was used to do so. This apparatus is designed to produce very high pressures and

temperatures in the tube where diamonds are to be made. This goes along with the quest to replicate

the natural conditions under which diamonds are formed. The features of the apparatus are shown in

figures 2-4. These figures display the experimental setup of the first successful synthesis of diamonds by

H. Tracy Hall. 3

Figure 2: A cross-sectional view of the tube in which diamonds are formed using the belt aparatus3

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The graphite plugs and the graphite tube serve as a source of carbon to be used in the formation

of diamonds. The atomic structures of graphite and carbon are very similar. The planes of graphite are

less puckered and the same plane alignment as in diamond is present to a significant extent. In

diamonds the atoms are all spaced 1.54 Å apart, but in graphite the atoms within a plane are 1.42 Å

apart and the planes themselves are spaced 3.37 Å apart. 3 The similarities in structure made graphite a

reasonable choice for a carbon source for the first experiments. Typically diamonds are found in ferro-

magnesium silicates, but diamonds have been found in FeS portions of meteorites. This made FeS a

reasonable candidate for the solvent in which diamond formation could occur. A solvent was used so

that the carbon atoms from the graphite could be removed and then reassembled piece by piece into a

diamond structure. Pyrophyllite is used on the sides of the tube because it is very good at transferring

pressure. The diamond seed did not change during the experiment and subsequent experiments proved

that it is unnecessary.

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Figure 3: An exploded view of the belt apparatus used by H. Tracy Hall at General Electric Research Laboratories in Schenectady, New York.3

Figure 4: A cross-sectional view of the belt apparatus.3

The two conical pistons of this apparatus allow for tremendous pressures to be achieved. The

readings for the first experiment put the pressure at 95,000 atm. The copper conductor ring allows a

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current to be passed through the apparatus that increases the heat in the tube. The first experiment ran

at approximately 1650°C. During the first experiment the maximum temperature was held for three

minutes, dropped to room temperature using the cooling water in about five minutes, and then the

pressure was reduced to atmospheric pressure in about eighteen minutes.3 This allowed for a safe

reduction of pressure and temperature while still allowing the content to cool and depressurize rapidly

enough for diamonds to form. This is because graphite is a more stable compound. If the contents were

given more time to cool and depressurize graphite would have formed instead of diamond.

This process is widely used because it is quick and also relatively inexpensive to purchase and

operate. The apparatus has been useful for many other areas of research that require such high

pressures and temperatures. The belt that is used by industry is much larger than the one first used by

H. Tracy Hall, but it still operates on the same principles. The solvent that is used for the belt varies

because many others have been found that produce desirable results. Some add impurities that change

the diamonds characteristics and some are just more inexpensive. This has made synthetic diamonds

much more affordable for purchase by manufacturers.

Detonation Synthesis

A newer method of diamond synthesis is detonation synthesis. The detonation synthesis of

diamonds, also called dynamic synthesis, results in the formation of nanodiamonds. The basic setup of a

detonation synthesis is the detonation of an explosive charge in a preservative material that will prevent

the carbon from oxidizing or forming graphite while it expands. The products are then washed with a

strong oxidant such as perchloric acid, nitric acid, and ozone. Typically explosive charges are a mixture of

TNT and RDX. Preservatives can include inert gas, water, and pyroltic salt. The results of Q. Chen and S.

Yun’s experiments show that using water as a preservative results in the least graphite formed and the

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most diamond. 6 Research is still being done to design methods to add impurities to these

nanodiamonds.

Chemical Vapor Deposition

Chemical vapor deposition, typically referred to as CVD, is a commonly used technique for

depositing a film of a substance on the surface of a substrate. The substance is synthesized from a gas

that contains the necessary elements for the substance to form. For Diamond CVD the temperature of

the substrate is typically 700-1200°C, the gas is in a pressure range of 20-150 torr, and the gas is typically

about 1-5% methane and the balance hydrogen. The high substrate temperature limits the possible

substrates to ceramics, refractory metals such as W and Mo, metal carbides and nitrides, and other

metals with a very high melting point. 7 Combustion synthesis is one of the simplest methods of CVD

diamond synthesis. The first experimental setup of this method used an acetylene torch and a high

temperature substrate such as Mo, Si, or another diamond. Figure 5 shows how the torch is setup.

Figure 5: A basic setup for combustion synthesis of diamond film. The acetylene torch is positioned so that the acetylene feather touches the substrate. This provides the greatest amount of the

components necessary for diamond growth.2

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This method produces a polycrystalline diamond film. The structure of the diamonds is

controlled by the oxygen to acetylene ratio in the flame. In most cases a ratio higher than one will not

produce any diamonds and a ratio lower than 0.7 will produce graphite. The diamonds grow in a circular

pattern with this method, but many studies have been done to control the shape of the film. Using a flat

flame gives a more uniform distribution of the diamond film. 2

Ultrasonic Cavitation

Ultrasonic cavitation is the newest method of synthesizing diamonds. This method employs

ultrasonic waves to convert graphite into diamonds. The primary product of this is nanodiamonds. The

waves have to be very strong in order for this process to work. As the waves pas through the liquid

medium they will create area of negative pressure and positive pressure by altering the densities of each

area. With strong waves, the negative pressure areas form into cavities that can be seen as bubbles. Any

gas that is present in the medium will diffuse through the cavity wall and contribute to the vapor inside

the bubble. When the cavities burst due to pressure and surface tension forces they undergo a strong

adiabatic compression very quickly. This can result in tremendous pressures and temperatures

approaching 1,000,000atm and 1000K for an instant. This has a small effect on the bulk temperature of

the fluid. These pressure and temperature conditions are within the range of the pressure and

temperature required to transform graphite into diamond.8

A.Kh. Khachatryan et al. describe the experimental setup that they used to create diamonds

from graphite using ultrasonic cavitation. The apparatus consisted of a tube made of stainless steel or

heat-resistant glass with an ultrasonic emitter attached to each end. They had a small hole through

which to add or extract the medium and they used a water cooling system to stabilize the reactor’s

temperature. The medium that they used consisted of an isomeric mixture of five- and six-ring

polyphenyl esther with an 11% mass fraction of the six-ring polyphenyl esters and powdered graphite of

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100-200μm. The solid to liquid ratio of the medium was 1:6. They degassed the medium before

beginning the diamond synthesis, bringing the medium temperature to 80°C. They began the synthesis

by increasing the ultrasound power to maximum and terminated the experiment when the bulk

temperature of the medium reached 120°C. The product was separated with a centrifuge. The solid

portion was washed with acetone and the non-diamond constituents were oxidized using sulfochromic

acid, a mixture of H2SO4 and K2Cr2O7. After testing the product it was concluded that it was diamond.

They were able to convert ~2% of the original mass of graphite into diamonds.8 This experiment proved

that ultrasonic cavitation is a possible method of synthesizing diamonds. The ability to produce

diamonds at atmospheric pressure and relatively low temperatures makes this process one that is of

interest to diamond manufacturers because it does not involve the purchase of a special apparatus

designed to withstand extreme temperatures and pressures. It also greatly reduces the safety risk since

these conditions are not uncommon even in a normal kitchen. A.Kh. Khachatryan et al. also make a

comparison of this method to the HTHP and CVD methods that is worth noting. They conclude that this

method requires 600 kJ of energy per gram of diamond produced. This is compared to 2000 MJ g-1 of

diamond using CVD and 400 kJ g-1 of diamond using traditional HTHP methods. 8 This shows that it is

almost as energy efficient as the HTHP method, making it a potentially plausible process to be used

commercially for the manufacture of nanodiamonds.

Testing

One method of determining if the product is a diamond is to use X-Ray diffraction. Diamonds will

exhibit constructive interference with X-rays at the (111) and (220) planes. Figure 6 shows an example of

three X-Ray diffractograms for three diamond films of different thickness. Raman Spectroscopy is

another commonly used method of analysis. In Raman spectroscopy, a photon hits the material and a

photon is then emitted. When the material is hit by a photon it experiences a change in its energy state.

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It then emits a photon with a wavelength of the difference between its original state and its final state.

The inverse of the wavelength is called the wave number, units of cm-1, and this is what is used to

measure the change in energy state of the material. On a graph produced by Raman spectroscopy the

results are plotted as intensity against wavenumber. A peak occurs at the wavenumber range of 1331-

1335cm-1. This is shown in Figure 7. Scanning electron microscopes also provide some useful qualitative

information about the products. This can be used to see the surface morphology of the diamonds. 2

Figure 6: The X-Ray Difractogram for diamond films of 3 different thicknesses. The peaks of the (111) and (220) planes are characteristic of diamond.2

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Figure 7: Typical Raman Spectroscopy results for diamond, graphite, and microcrystalline graphite. The characteristic peaks are useful for determining what carbon species are present in a sample.2

Applications

One well known use for synthetic diamonds is jewelry. Diamonds have long been used in cutting

and grinding tools. This is because of their extreme hardness. This is what led to the discovery of how to

make synthetic diamonds in the first place. They are useful as coatings for joints of almost any kind. This

is due both to the durability of diamonds and their very low coefficient of friction. Diamonds have begun

to find their way into electronics since the discovery of how to make them conductive. Since diamonds

have a high heat conductivity they are useful in small scale heat exchangers.

Since nanodiamonds are biocompatible they have great potential for use in medicine. They can be used

as coatings for implants. They have also been found to be able to bind to essential molecules produced

by the human body. This means that they can be used to carry these molecules to patients that need

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them. 9 Nanodiamonds have even found their way into pencil lead recently. Uni has recently developed

a nanodiamond based pencil lead. It is much stronger and longer lasting than any of their previous

polymer leads.

In recent years there have been a lot of studies done with diamonds in the polymers field as

well. There are studies to try to graft polymers onto diamonds, grow polymers on the surface of

diamonds, and to use diamonds to link polymers together.

References

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[1] General Electric, The Story of Man-Made Diamond., 1966.

[2] Jes Asmussen and D. K. Reinhard, Eds., Diamond Films Handbook. New York, United States: Marcel Dekker, Inc., 2002.

[3] H. Tracy Hall, "The Synthesis of Diamond," Journal of Chemical Education, vol. 38, no. 10, pp. 484-489, October 1961.

[4] Shane A. Catledge and Yogesh K. Vohra, "Effect of nitrogen addition on the microstructure and mechanical properties of diamon films grown using high-methane concentrations," Journal of Applied Physics, vol. 86, no. 1, pp. 698-700, July 1999.

[5] E. A. Ekimov et al., "Superconductivity in diamond," Nature, vol. 428, pp. 542-545, April 2004.

[6] Quan Chen and Yun Shourong, "Nano-sized diamond obtained from explosive detonation and its application," Materials Research Bulletin, vol. 35, pp. 1915-1919, 2000.

[7] Koji Kobashi,. Oxford, UK: ELSEVIER Ltd, 2005, ch. 2.2, pp. 9-10.

[8] A. Kh. Khachatryan et al., "Graphite-to-diamond transformation induced by ultrasound cavitation," Diamond & Related Materials, vol. 17, pp. 931-936, 2008.

[9] Amanda M. Schrand, Suzanne A. Ciftan Hens, and Olga A. Shenderova, "Nanodiamond Particles: Properties and Perspectives for Bioapplications," Critical Reviews in Solid State and Materials Sciences, vol. 34, pp. 18-74, 2009.

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