Diamond HPHT

Towards the end of the 18 century a British chemist, Smithson Tennant, showed that diamonds are composed of nothing but carbon a discovery that gave a more scientific direction to synthesis efforts. By the beginning of the 19 century, it was known that carbonaceous materials, heat and pressure are required for diamond formation. Finally, success in artificial synthesis was achieved in the middle of the 20 century by two routes the High Pressure High Temperature (HPHT) route leading to the formation of diamond grit and the Low Pressure... 

Several researchers have reported successful deposition of homoepitaxial diamond films on natural as well as HPHT diamond substrates (e.g., 101-104). 

Prospects for inexpensive and large-scale production of diamond increased tremendously in the mid 1970s when researchers discovered that diamond can be grown as thin coatings at low deposition pressures (10 -10 Pa) from hydrocarbon/hydrogen mixtures by chemical vapor deposition (CVD). Since then, there has been an explosion in diamond and related- material research with the expectation that CVD will allow faster, cheaper, and easier production of diamond. CVD diamond technology was proved to be more versatile in coating intricate shapes and could be less expensive than HPHT. " ... 

C. Cagniard de la Tour claimed to have synthesized diamond from a solution. His crystals turned out to be aluminum and magnesium oxide. Many other claims and attempts were made but until the discovery of diamond in kimberlite, the role of high pressure and high temperature (HPHT) in the formation of diamond was not known. Diamond can be formed at great depths in the earth because of HPHT it is the dominant phase of carbon while at low pressures like those attained in a typical laboratory experiment, graphite is the dominant phase (Fig. 4). 

In the last 40 yr, the development of synthetic diamond in various forms has fueled a revolution in the use of diamond as an engineering material. The process of HPHT diamond synthesis was responsible for stunning growth in the abrasives market. During that time, the world s consumption of diamond abrasive materials increased from 5 to over lOOtons/yr. 

High pressure and high temperature is a method used to grow diamond under an HPHT environment. The process is performed in the diamond stable region at pressures ranging from 7 to 10 GPa and temperatures ranging from 1700 to 2000 K. Graphite or other carbons are dissolved in molten transition metals and then precipitated in diamond form. 

Today there are several other ways of diamond synthesis besides the HPHT method. For example, it is possible to utilize the pressure of a shock-wave generated in an explosion. This process mostly yields powdery products with particle sizes in the range of micrometers (1 mm at max.) that may be employed for industrial purposes as well. Moreover, very small diamonds (5-20 nm) can be made by reacting explosives in confined containers. Diamond films are produced on various substrates by chemical vapor deposition (CVD method using methane as a carbon source. Detonation synthesis and vapor deposition will be described in detail in Chapters 5 and 6. 

TliC first brcaktiirougii CmnC in 1953 wlicn H. Li ndcr st Allcrn nns Svenska Elektriska A. B. (ASEA) in Sweden developed a HPHT processf using a liquid metal solvent-catalyst at pressures and temperatures where diamond is thermodynamically stable. Independently, the General Electric team, F. P. Bundy and co-workers, synthesized diamond using the HPHT teclmique in 1954,Fil followed by H. B. Dyer and co-workers at De Beers Adamant Research Laboratory in South Africa. Through surmounting the... 

The potential for economic scale-up of diamond CVD techniques qualifies it as a viable processing alternative to the HPHT methods for... 

It has been known for decades that transition metals such as Fe, Co, Ni, Cr, Pt and Pd can be used as solvent-catalysts for diamond synthesis under HPHT conditions.t lP" The most common and effective catalysts are Fe-... 

Natural diamonds are supposed to have grown in the depth of the mantle [2] under high-pressure high-temperature (HPHT) conditions above the Berman-Simon line [1], shown in Fig. 1. Although possible mechanisms of natural diamond formation are still debated, C-H-O fluids seem to play an important role [3]. 

Reports on man-made diamond obtained by HPHT synthesis were first published in 1955 by General Electric [4]. Usually, metals able to dissolve carbon under HPHT conditions are used as catalysts and increase growth rates. Diamond crystals of several millimeters in size can be obtained in this way, but usually small grains for abrasives are produced. Direct conversion of graphite to diamond without catalyst in HPHT apparatus is possible, but uneconomical for industrial application. Direct transformation can be done by the detonation method and produces nanosized powders of diamond and diamond-like carbon [5]. 

While HPHT, shock wave, and CVD synthesis of diamond have been commercialized, these methods have serious limitations. None of them allows economical growth of large single crystals and the diamond quality does not satisfy the requirements for electronic or gem applications. Since the quality and size of the best natural diamonds have not yet teen reproduced in the lab, the search for a tetter method of diamond synthesis continues. Recent work demonstrates that diamond can be formed metastably under a variety of conditions. 

Theoretical calculations and experimental studies [28] suggested that smaller, colloidal diamond particles of about lOnm may be stable at lower pressures and higher temperatures than macroscopic diamond. A so called crystallite size effect [29] might account for metastable diamond nucleation and growth under hydro-thermal conditions. Thus, the HPHT graphite-diamond equilibriiun should only be applied to crystallites >100nm in diameter [28]. 

The first industrialized process for the synthesis of diamond employed the HPHT method, which mimics nature in that thermodynamically stable conditions are used for growth [19]. In HPHT, typically a synthesis capsule containing graphite, seed crystals, and a metal solvent is compressed to tens of thousands... 

Exceptional process control is required to produce large (>1 carat) sc HPHT diamond relatively free from impurities [21]. Small heavily boron-doped HPHT diamond crystals <10 mm have been fabricated using ultrahigh pressures, 8-20GPa, and temperatures of >2500K, and diamond superconductivity at 4K has been reported. However, such synthesis conditions require highly specialized HPHT capabilities and to date these materials remain a research curiosity [22]. 

The most commonly used method for growing BDD both in the laboratory and commercially is CVD [11], due the efficiency of the process in controlling dopant incorporation and the ability to grow over large areas and on structured substrates [23]. CVD is the method used to produce the vast majority of diamond electrodes for the electrochemical work discussed later. Where alternative growth techniques such as HPHT have been employed, discussed in Section 5.1.2.1, this will be highlighted. 

Pro-Aqua, http //www.proaqua.cc/en/products/pro-aqua- diamond-electrode/ HPHT BDD grit on a flexible fluoropolymer substrate Electrodes 260 mm X 270 mm X 0.5 mm aimed at water treatment applications... 

Figure 5.15 (a) Boron-doped film with its HPHT substrate before cutting and polishing, (b) Freestanding boron-doped diamond film. (Taken from Ref. [33] with permission.)... 

The diagram illustrates that graphite converts to diamond under high-pressure high-temperature (HPHT) conditions. 

Lindblom J, Hosla J, Papunen H et al (2003) Differentiation of natural and synthetic gem-quality diamonds by luminescence properties. Opt Mater 24 243-253 Lindblom J, Holsa J, Papunen H (2005) Luminescence study of defects in synthetic as-grown and HPHT diamonds compared to natural diamonds. Am Miner 90 428-440 Mayer I, Layani J, Givan A et al (1999) La ions in precipitated hydroapatites. J Inorg Biochem 73 221-226... 

Diamond Nanocrystals Obtained by Milling of HPHT Diamonds.253... 

Diamond nanocrystals obtained by milling of large crystals synthesized from graphite at high static pressures and temperatures in the presence of catalyst metals, the so-called high-pressure-high-temperature synthesis (HPHT). 

---