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Horizons: diamonds--adding luster to plastics.

Diamonds and diamond-like materials could be the new breakthrough in plastics. Besides the aesthetic appeal of diamonds, their most dramatic properties are hardness and thermal conductivity.

A diamond consists of carbon atoms bonded by tetrahedral carbon--carbon single bonds. In contrast, plastics usually consist of polymer chains with carbon-carbon backbones. Some polymers have other atoms such as nitrogen or oxygen in the backbones, but many, like polyolefins, have only carbon-carbon bonds. Crosslinking usually connects chain backbones with covalent bonds. Often this involves converting one of the side groups to a carbon-carbon bond. Thus, in a sense, crosslinking is changing the polymer structure closer to that of diamond. Diamond is, therefore, the ultimate crosslinked polymer.

I used to tell my students that polymeric materials range from soft gels to hard diamond. My metallurgist colleagues objected vigorously, claiming that diamond is a ceramic and not a polymer Perhaps by convention this is but from a bonding or basic structure point of view, diamond is a polymer.

Polymers are very big molecules, with molecular weights from thousands to millions and beyond. Since the atoms in diamonds are connected by single carbon-carbon bonds, strictly speaking, each separate diamond piece is a molecule. That means a five-carat diamond weighing 1 gram would have a molecular weight of 1 x 6 x [10.sup.23], a ridiculously large number.

Diamond under conventional conditions is not plastic and is difficult to process. It is the hardest of materials, with a hardness of 10,000 kg/[mm.sup.2], elastic modulus of 1.22 GPa, density of 3.52 g/[cm.sup.3], and tensile strength of 1.2 GPa. The hardness of diamonds is very useful, especially for cutting. Thus we use diamond-filled plastics as abrasives.

Thermal conductivity is usually associated with electrical conductivity. Copper and nickel are good thermal conductors and have very high electrical conductivity. Diamond is an electrical insulator but still shows extremely high thermal conductivity--1000 to 2600 W/mK. This unexpected effect has something to do with the way in which thermal vibrations or phonons can move through a tightly coupled structure. Single tetrahedral bonds seem particularly good for this process. Copper, by contrast, has a thermal conductivity of only 390 W/mK, and that of silver is 430 W/mK. The value for glass is 1 W/mK, and that of expanded PS is 0.026 W/mK.

Natural diamonds are forged in the depths of the earth under extreme heat and sure. Strangely enough, they are also formed in certain meteorites as they smash into the earth because of the enormous heat and pressure. It was quite an achievement when General Electric and others synthesized diamond under heat and pressure in the laboratory.

Because cost is such an important part of the plastics business, it is difficult to imagine using diamonds as fillers except for expensive abrasives. However, never underestimate the creativity of engineers. It turns out that chemical vapor deposition can be used to deposit or form diamond-like materials, crystalline or amorphous, on surfaces. Even though the bonding is the same, many are hesitant to call these materials "diamonds," so the term "diamond-like" is often used. And, because of their properties, these diamond-like materials have many uses.

The Maro patent-link collection lists 233 U.S. patents dealing with diamond. Some are about applications; 52 deal with chemical vapor deposition (CVD is the basic way of forming these modern versions of diamonds); and 48 are about the formation of diamond-like coatings. All sorts of materials can be coated with a diamond-like material, including metals, glass, and plastics. Even medicine gets into the act, with five patents on implants with diamond surfaces. These are mainly used in artificial joints for lubrication and wear resistance.

Diamond and silicon are in the same periodic table group with similar electronic structures. Of course, the electron energy gap is considerably bigger for diamond than silicon. Nevertheless, diamond can be doped to form a whole new class of semiconductors. Thus, 18 patents are about electronic applications, and four of these are about diamond-like sensors.

Only two patents deal with thermal-conductivity applications. Maybe the prospect of such a high thermal conductivity for an electrical insulator is so outlandish that such applications are not taken seriously.

What are the most interesting applications for diamonds and plastics? Coatings with hardness and thermal conductivity are probably the most interesting. Enhancing thermal conductivity of tooling with diamond-like coatings and composites is an attractive possibility. The most peculiar application in this patent list is the formation of diamond-foam material: a diamond-like material is deposited into an open-cell foam material; see #6,749,931, June 15, 2004, by John M. Pinneo and Howard Davidson. These materials are designed for high-temperature filtration such as molten metals. Compared with the conventional filter of compacted ceramic powders, these filters show exceptional heat transfer capability and superior mechanical capability due to a strong, contiguous framework for diamond deposition. They also have excellent corrosion resistance owing to diamond's superior chemical properties. Perhaps a useful application could be polymer melt filtration.

Diamond has been used as a hard, wear-resistant material at least as far back as Pliny the Elder in ancient Rome, who used diamond shards as tools.

Send an email request to cornelrd@bee.net for 100 of the recent patent links on diamond-like materials.

Roger D. Corneliussen is Professor Emeritus of Materials Engineering, Drexel University, in Philadelphia. He is editor of Maro Polymer Alerts and the Maro Polymer website (www.maropolymeronline.com). He has been active in SPE since 1962 and has served on the board of the Philadelphia Section and as National Councilor. For his Maro Patent Alerts he reviews all U.S. Patents weekly, makes links to the polymer-related patents, and sends the links daily to subscribers. These comments in Horizons are based on the weekly selection process. Maro email Alerts are available to SPE members without charge--simply email a request to cornelrd@bee.net.
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Author:Corneliussen, Roger D.
Publication:Plastics Engineering
Geographic Code:1USA
Date:May 1, 2005
Words:988
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