Domestic

Tungsten carbide is sometimes used to make the rotating ball in the tips of ballpoint pens that disperse ink during writing.

A tungsten carbide ring

Tungsten carbide can now be found in the inventory of some jewelers, most notably as a primary material in men's wedding rings. When used in this application the bands appear with a lustrous dark hue often buffed to a mirror finish. The color is more similar to that of hematite than to that of platinum. The finish is highly resistant to scratches and scuffs, holding its mirror-like shine for years. Although it is possible to inlay precious metals, woods, and other materials, these are less scratch-resistant than tungsten carbide.

A common misconception concerning tungsten carbide rings is that they cannot be removed in the case of emergency medical treatment, requiring the finger to be removed instead. This is not true. Emergency care providers have specialized tools that allow them to easily break tungsten rings into large pieces. As a result, it is easier and less dangerous to remove tungsten carbide rings than gold, silver, or titanium without injuring the hand or finger. An easier way to remove tungsten carbide rings is to use a tool such as a vise, which can be used to shatter the ring.

Many manufacturers of this emerging jewelry material state that the use of a cobalt binder may cause unwanted reactions between the cobalt and the natural oils on human skin. Skin oils cause the cobalt to leach from the material. This is said to cause possible irritation of the skin and permanent staining of the jewelry itself. Many manufacturers now advertise that their jewelry is "cobalt free". This is achieved by replacing the cobalt with nickel as a binder.

Toxicity. The primary health risks associated with carbide relate to inhalation of dust, leading to fibrosis. Cobalt–Tungsten Carbide is also reasonably anticipated to be a human carcinogen by the National Toxicology Program.

UNIT 7

CEMENTED CARBIDES

A unique material

Cemented Carbide is one of the most successful composite engineering materials ever produced. Cemented Carbide's unique combination of strength, hardness and toughness satisfies the most demanding applications.

A key feature of the Cemented Carbide is the potential to vary its composition so that the resulting physical and chemical properties ensure maximum resistance to wear, deformation, fracture, corrosion, and oxidation. In addition, the wide variety of shapes and sizes that can be produced using modern powder metallurgical processing offers tremendous scope to design cost-effective solutions to many of the problems of component wear and failure encountered in both the engineering and domestic environment.

What are the different types of Cemented Carbide?

The Cemented Carbides are a range of composite materials, which consist of hard carbide particles bonded together by a metallic binder.

The proportion of carbide phase is generally between 70-97% of the total weight of the composite and its grain size averages between 0.4 and 10 μm.

Tungsten carbide (WC), the hard phase, together with cobalt (Co), the binder phase, forms the basic Cemented Carbide structure from which other types of Cemented Carbide have been developed. In addition to the straight tungsten carbide – cobalt compositions – Cemented Carbide may contain varying proportions of titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC). These carbides are mutually soluble and can also dissolve a high proportion of tungsten carbide. Also, Cemented Carbides are produced which have the cobalt binder phase alloyed with, or completely replaced by, other metals such as iron (Fe), chromium (Cr), nickel (Ni), molybdenum (Mo), or alloys of these elements.

There are three individual phases which make up Cemented Carbide. In metallurgical terms, the tungsten carbide phase (WC) is referred to as the a-phase (alpha), the binder phase (i.e. Co, Ni etc.) as the b-phase (beta), and any other single or combination of carbide phases (TiC, Ta/NbC etc) as the g-phase (gamma). Other than for metal cutting applications, there is no internationally accepted classification of Cemented Carbides.

WC-Co grades

T his group of Cemented Carbides contains WC and Co only (i.e. two phases) and a few trace elements. These grades are classified according to their cobalt content and WC grain size. The grades with binder content in the range 10-20% by weight and WC-grain sizes between 1 and 5 μm have high strength and toughness, combined with good wear resistance.

The microstructure of wc-co grade

T he grades with binder contents in the range 3-15% and grain sizes below 1 μm have high hardness and compressive strength, combined with exceptionally high wear resistance.The Sandvik grade program also includes WC-Co grades which utilize a range of ultra-fine WC grain sizes (< 0.5 μm). With such fine, uniform grain sizes, a unique combination of hardness, wear resistance and toughness can be achieved.

Microstructure of γ-phase grade

The grades of Cemented Carbides in this group contain WC and Co as the main elements, although small additions or trace levels of other elements are often added to optimize properties. These grades are classified according to their Cobalt content and WC grain size and are often called the "straight grades". They have the widest range of strength and toughness of all the Cemented Carbide types and this is in combination with excellent wear resistance. This range of Cemented Carbides can be subdivided into its major application areas as follows:

How are Cemented Carbides made?

The manufacturing process begins with the composition of a specific tungsten carbide powder mixture - tailored for the application.

The tungsten carbide powder is compacted into a form.

In a high-temperature sintering furnace, the tungsten carbide structure of the blank is shaped at precise temperatures for strictly defined periods. During this heat treatment, the tungsten carbide blank undergoes shrinkage of some 50% in volume.

The sintered Cemented Carbide component gains its final finish by additional grinding, lapping and/or polishing processes.

The main use of tungsten (in the form of tungsten carbide) is in the manufacture of cemented carbides. After Scheele’s discovery of "Tungsten" in 1781, it took an additional 150 years before his successors’ efforts led to the application of tungsten carbide in the industry.

Cemented carbides, or hardmetals as they are often called, are materials made by "cementing" very hard tungsten monocarbide (WC) grains in a binder matrix of tough cobalt metal by liquid phase sintering.

Microstructure of a Wc-Co Cemented Carbide

The combination of WC and metallic cobalt as a binder is a well-adjusted system not only with regard to its properties, but also to its sintering behaviour.

The high solubility of WC in cobalt at high temperatures and a very good wetting of WC by the liquid cobalt binder result in an excellent densification during liquid phase sintering and in a pore-free structure. As a result of this, a material is obtained which combines high strength, toughness and high hardness.

The beginning of tungsten carbide production may be traced to the early 1920’s, when the German electrical bulb company, Osram, looked for alternatives to the expensive diamond drawing dies used in the production of tungsten wire.

These attempts led to the invention of cemented carbide, which was soon produced and marketed by several companies for various applications where its high wear resistance was particularly important. The first tungsten carbide-cobalt grades were soon successfully applied in the turning and milling of cast iron and, in the early 1930’s, the pioneering cemented carbide companies launched the first steel-milling grades which, in addition to tungsten carbide and cobalt, also contained carbides of titanium and tantalum.

By the addition of titanium carbide and tantalum carbide, the high temperature wear resistance, the hot hardness and the oxidation stability of hardmetals have been considerably improved, and the WC-TiC-(Ta,Nb)C-Co hardmetals are excellent cutting tools for the machining of steel. Compared to high speed steel, the cutting speed increased from 25 to 50 m/min to 250 m/min for turning and milling of steel, which revolutionized productivity in many industries.

Shortly afterwards, the revolution in mining tools began. The first mining tools with cemented carbide tips increased the lifetime of rock drills by a factor of at least ten compared to a steel-based drilling tool.

In all these applications, there has been a continuous expansion in the consumption of cemented carbide from an annual world total of 10 tons in 1930; to 100 tons around 1935; 1,000 tons in the early 1940’s; through 10,000 tons in the early 1960’s and up to nearly 30,000 tons at present.

The development of metal cutting tools has been very rapid over the last four decades, having been greatly stimulated by much improved design and manufacturing techniques, e.g. the introduction of indexable inserts in the 1950’s and the invention of coated grades around 1970.

The first coating was with a thin layer (~5 µm thick) of titanium carbide made by a Chemical Vapour Deposition (CVD) process. It improved the lifetime of tools by a factor of 2 to 5.

This technique has since been improved by multilayer coatings, where layers of alumina, titanium nitride, titanium aluminium nitride and other materials have been added which have further improved the lifetimes by 5 to 10 times.

However, coating and improved design are only one side of the coin. Continuous improvement of intermediates and manufacturing techniques led to improved performance of hardmetals and opened new areas of applications. The introduction of solvent extraction in tungsten chemistry, new techniques in hydrogen reduction and carburization improved the purity and uniformity of tungsten and tungsten carbide powder.

In parallel, new powder milling, spray drying and sintering techniques resulted in improved hardmetal properties and performance. Notably, the continuous improvement of vacuum sintering technology and, starting from the late 1980’s, hot isostatic pressure sintering (SinterHIP) led to new standards in hardmetal quality.

The history of tungsten powder metallurgy, and especially that of the hardmetal industry, is characterized by a steadily widening range of available grain sizes for processing in the industry; while, at the same time, the grain size distribution for each grade of WC powder became narrower and narrower.

The most important reason for this widening of the spectrum of available WC grades is that, besides those variations achieved by cobalt contents and some carbide additives, the properties of WC-Co hardmetals such as hardness, toughness, strength, modulus of elasticity, abrasion resistance and thermal conductivity can be widely varied by means of the WC grain size. While the spectrum of available WC grain sizes ranged from 2.0 to 5.0 µm in the early days of the hardmetal industry in the mid 1920’s, the grain sizes of WC powders now used in hardmetals range from 0.15 µm to 50 µm, or even 150 µm for some very special applications.

Based on the wide range of grain sizes now available, not only very hard and abrasion resistant, but also very tough, hardmetals can be produced for widespread applications in high tech tools, wear parts and mining tools as well as for many sectors of the engineering industry.

There has been a rapid development in mining and stone cutting tools, with improved performance which has led to the increasing substitution of steel tools by cemented carbide tools, in particular in the oil industry. Notably, the use of very coarse grained hardmetals is growing in this application area.

A large portion of the tungsten volume in cemented carbide is today used in wear part applications, where there is a wide range of products from the very small (such as balls for ball-point pens) to large and heavy products, such as punches, dies or hot rolls for rolling mills in the steel industry.

Most of these wear parts and the mining tools are made of straight WC-Co hardmetals without any addition of other carbides.

Fine and ultrafine grained WC hardmetals have become more and more important today in the field of wear parts, tools for chipless forming and cutting tools for cast iron, non ferrous alloys and wood.

Application range of straight grade cemented carbides

The first submicron hardmetals were launched on the market in the late 1970s and, since this time, the microstructures of such hardmetals have become finer and finer. The main interest in hardmetals with such finer grain sizes derives from the understanding that hardness and wear resistance increase with decreasing WC grain size.

A special application for these fine or ultrafine WC hardmetals, involving large quantities of cemented carbide, is in drills for the drilling of the very fine holes in printed circuit boards for the computer and electronic industries. For this purpose, new cemented carbide compositions, based on extremely fine-grained carbide, have been introduced.

Basic data for different WC-Co and WC-(W,Ti,Ta,Nb)C-Co hardmetal grades
Grade (wt%)	Hardness HV30	Compressive strength (N × mm -2)	Transverse rupture strength (N × mm -2)	Young’s modulus (kN × mm -2)	Fracture toughness (MPa × m -1/2)	Mean thermal expansion coefficient (10 -6 × K -1)
WC-4Co	2000	7100	2000	665	8.5	5.0
WC-6Co/S*	1800	6000	3000	630	10.8	6.2
WC-6Co/M**	1580	5400	2000	630	9.6	5.5
WC-6Co/C***	1400	5000	2500	620	12.8	5.5
WC-25Co/M	780	3100	2900	470	14.5	7.5
WC-6Co-9.5 (Ti,Ta,Nb)C	1700	5950	1750	580	9.0	6.0
WC-9Co-31 (Ti,Ta,Nb)C	1560	4500	1700	520	8.1	7.2

S* = submicron; M** = fine/medium; C*** = coarse

UNIT 8

METAL CLAY

Metals such as silver, bronze, copper, gold, and steel have been made into materials known as metal clay. These materials are used by artists and jewelers to make art objects in a home or studio setting. When fired the organic binder burns off and the microscopic metal powder in the clay is sintered. Silver and gold metal clays can be fired in a normal kiln environment while most base metal clays must be fired in a reduced atmosphere using activated carbon to prevent oxidation from inhibiting proper sintering.

What is Metal Clay?

Metal clay first came out in Japan in 1990^[1] to allow craft jewelry makers to make sophisticated looking jewelry without the years of study needed to make fine jewelry.

Metal clay is a crafting medium consisting of very small particles of metal such as silver, gold, platinum, or copper mixed with an organic binder and water for use in making jewelry, beads and small sculptures. Originating in Japan in 1990, metal clay can be shaped just like any soft clay, by hand or using molds. After drying, the clay can be fired in a variety of ways such as in a kiln, with a handheld gas torch, or on a gas stove. The binder burns away, leaving the pure sintered metal. Shrinkage of between 8% and 30% occurs (depending on the product used). Alloys such as bronze or steel also are available.

Silver and gold metal clays have been available to the public since the mid-1990's. Mitsubishi and Aida Chemicals make Precious Metal Clay (PMC) and Art Clay Silver respectively. The ingredients are just fine silver particles, an organic binder and water. They can be worked in the same way as ceramic clays - rolled, formed, textured and cut. When the water is allowed to evaporate and the dry piece is fired, the result is a 99.9% fine silver item which is hallmark quality. There is also a gold variety of metal clay which works the same way. To find out about the history of PMC, click here.

Bill Struve was the first person to develop bronze and copper clays which come as ready mixed lumps of clay and have the brand name Bronzclay™ and Copprclay™. These were released in mid 2008.

Hadar Jacobson has produced bronze and copper clays that come as powders which you mix with water. Both these products work in a similar way to the silver and gold clays although the firing is more complicated. The original bronze and copper clays require an oxygen free environment to fire so the dried pieces are buried in activated carbon in a stainless steel pan and fired in a kiln. They cannot be torch fired and require a very long firing in the kiln.

The makers of Art Clay, Aida Chemicals launched their own copper clay, called Art Clay Copper, in September 2009. This has a much simpler firing schedule than the other copper clays available and can be torch fired or kiln fired. Find out more about Art Clay Copper here.

Prometheus Copper Clay is also now available. Prometheus Copper Clay fires in 30 minutes at 850 degrees C and requires no special firing pan. And it comes as syringe clay as well as lump clay.

Hadar Jacobson launched a copper clay powder and a bronze clay powder with a simpler firing schedule in December 2009. They are called Quick-Fire Copper and Quick-Fire Bronze.

Prometheus™ bronze clay is also available. Developed in Europe, this bronze clay has a shorter, lower temperature firing schedule without the need for activated carbon. It still needs a kiln to fire it. Available as lump clay and in syringe form.

Hadar Jacobson has also developed Stainless Steel Clay which became available in December 2009. It comes in two forms, traditional and Quick-Fire. Hadar launched White Bronze Clay in early 2010. In June 2010, Pearl Grey Steel clay powder became available. In July 2011, Hadar launched Rose Bronze Clay.

Bill Struve launched Fast Fire Bronzclay™ in July 2010. Mitsubishi launched PMC Pro in July 2010. Both these products were launched at the PMC Conference.

In December 2010, Meteor bronze clay was launched by a French developer and then Meteor copper clay was released in January 2011. Meteor white bronze clay was launched in July 2011.

Metal Clay Mania Clay has developed bronze, copper and brass clays that come in powder form.

In early 2011, Lisa Cain developed hallmark quality Sterling silver metal clay by combining the existing commercial clays. She shared her method of mixing and firing this clay with the metal clay community through the March 2011 edition of Metal Clay Artist Magazine.

Check back often to find out what's new in the world of metal clay, send us your questions and we'll add them to the FAQ page.

If you are a complete beginner, check out our metal clay beginners page. You'll find basic information, links to more resources and a useful article about how to work with metal clay without spending lots of money on tools and equipment.

Silver Metal Clay

Silver metal clay results in objects containing 99.9% pure silver, which is suitable for enameling. Although gold metal clay is more expensive, it provides richer color. Lump metal clay is sold in sealed packets to keep it moist and workable. The silver versions are also available as a softer paste in a pre-filled syringe which can be used to produce extruded forms, in small jars of slip and as paper-like sheets, from which most of the moisture has been removed. Common brands of silver metal clay include Precious Metal Clay (PMC) and Art Clay Silver (ACS).

Precious Metal Clay (PMC)

PMC was developed in the early 1990s in Japan by metallurgist Masaki Morikawa.^[3] As a solid-phase sintered product of a precious metal powder used to form a precious metal article,^[1] the material consists of microscopic particles of pure silver or fine gold powder and a water-soluble, non-toxic, organic binder that burns off during firing. Success was first achieved with gold and later duplicated with silver.

The PMC brand includes the following products:

The original formula of PMC, now called "standard": fired at 900 °C (1,650 °F) for 2 hours, shrinks by 30% during firing.

PMC+: fired at 900 °C (1,650 °F) for 10 minutes or 800 °C (1,470 °F) for 30 minutes; shrinks 15%, due to a particle size reduction. PMC+ is also available in sheet form which can be worked like paper; for example, for origami.

PMC3: fired at 599 °C (1,110 °F) for 45 minutes or 699 °C (1,290 °F) for 10 minutes; shrinks by 10%. It can also be fired using a butane torch by heating it to orange heat for at least 2 minutes. It has a longer working life than the older formulations. It is also available in slip and paste forms which can be painted onto the surface of an object to be used as a mould.

Aura 22: a 22 k gilding material, a gold paste intended to be painted onto the surface of silver PMC pieces, or ready-made silver objects.

PMC Pro: a harder product which is only 0.900 silver, hence it canot be hallmarked as sterling silver. It also requires kiln firing in a tub of activated carbon for 1 hours at 760 °C (1,400 °F).

Art Clay Silver (ACS)

ACS was developed by AIDA Chemical Industries, also a Japanese company. ACS followed PMC Standard with their Art Clay Original clay (more like PMC+ than PMC Standard), which allows the user to fire with a handheld torch or on a gas hob. Owing to subtle differences in the binder and suggested firing times, this clay shrinks less than the PMC versions, approximately 8–10%.

Further developments introduced the Art Clay Slow Dry, a clay with a longer working time. Art Clay 650 and Art Clay 650 Slow Dry soon followed; both clays can be fired at 650 °C (1,202 °F), allowing the user to combine the clay with glass and sterling silver, which are affected negatively by the higher temperatures needed to fire the first generation clays. AIDA also manufacturers Oil Paste, a product used only on fired metal clay or milled fine silver, and Overlay Paste, which is designed for drawing designs on glass and porcelain.

In 2006 AIDA introduced the Art Clay Gold Paste, a more economical way to work with gold. The paste is painted onto the fired silver clay, then refired in a kiln, or with a torch or gas stove. When fired, it bonds with the silver, giving a 22ct gold accent. The same year also saw Art Clay Slow Tarnish introduced, a clay that tarnishes less rapidly than the other metal clays.

Lump Metal Clays

Lump metal clay in bronze was introduced in 2008 by Metal Adventures Inc. and in 2009 by Prometheus. Lump metal clays in copper were introduced in 2009 by Metal Adventures Inc. and Aida. Because of the lower cost, the bronze and copper metal clays are used by artists^[5] more often than the gold and silver metal clays in the American market place. Due to Hallmarking requirements laid out in the UK Bronze and Copper are not regarded as highly. Furthermore, due to the complex firing process and the fact that during the firing process acidic vapour is emitted and can result in extreme wear and tear on your kiln eventually leading to the requirements of a replacement kiln, it is often regarded as not as economically logical to use these cheaper metals. The actual creation time of a PMC Bronze or Copper piece is also far greater than that of its PMC3 counterpart.

Base Metal Clays

Base metal clays, such as bronze, copper, and steel metal clays are best fired in the absence of oxygen to eliminate the oxidation of copper by atmospheric oxygen. A simple means to accomplish this (place the pieces in activated carbon inside a container) was developed by Bill Struve.

MIM PRODUCTS

M etal injection moulding (MIM) has over the past decade established itself as a competitive manufacturing process for small precision components which would be costly to produce by alternative methods

It is capable of producing

• in both large and small volumes

• complex shapes

• from almost all types of materials including metals, ceramics, intermetallic compounds, and composites.

Components made by MIM technology are finding new applications in industry sectors such as automotive, chemical, aerospace, business equipment, computer hardware, bio-medical and armaments.

MIM and Powder Metallurgy

Metal injection moulding (MIM) is a development of the traditional powder metallurgy (PM) process and is rightly regarded as a branch of that technology.

The standard PM process is to compact a lubricated powder mix in a rigid die by uniaxial pressure, eject the compact from the die, and sinter it.

Quite complicated shapes can be and are regularly being produced by the million, but there is one significant limitation as regards shape.

After compaction in the die the part must be ejected, i.e. pushed out of the die cavity. It will be obvious, therefore, that parts with undercuts or projections at right angles to the pressing direction cannot be made directly.

That limitation is substantially removed by the metal injection moulding process developed during the last decade and now expanding rapidly.

Plastic Material

The use of injection moulding for the production of quite intricate parts in a number of plastic materials has been known for many years, and most of us come into contact with them in some form or other every day.

One important feature of such parts is that they are relatively cheap.

However, for engineering applications these thermo-plastic materials have quite inadequate mechanical properties.

Metal and Ceramic Material

Some improvement is made possible by the use of solid fillers - ceramic or metal powders – but the real breakthrough occurred when it was found possible to incorporate a very high volume fraction of metal powder in a mix so that, instead of a filled plastic part, a plastic-bonded metal or ceramic part is produced.

Careful removal of the plastic binder leaves a skeleton of metal or ceramic which, although fragile, can be handled safely and sintered in much the same way as traditional die compacted parts.

After sintering densities of 95% or more are reached and the mechanical properties are, for that reason, generally superior to those of traditional PM parts.

UNIT 10`

NANOTECHNOLOGY

Nanotechnology may be defined as the ability to work at the molecular level, atom by atom, to create large structure with fundamentally new molecular organization. Since this technology is developed from the scale range of 10-9 m or 10 A, its interaction by intermolecular and inter-atomic forces will lead to the next industrial revolution. Thus, providing the ability to control and manipulate the physical, biological, and chemical properties of a system. In other words, materials will be constructed from the bottom-up instead of conventional methods, top-down methods. The ability to largely influence the properties and structure of materials has made nanotechnology a quickly developing field that has been gaining interests among the public due in part to the possibilities that this technology provides.

Many scientists suggest that nanotechnology will fuel enormous growth within the areas of Biotechnology and Bio-medical Chemistry, and Atomic Positioning. Here the possibilities are endless. Using nanotechnology will play a major role in the field of Bio-medical Chemistry. Many pharmaceutical companies are performing research to decline the particle size. If drugs were able to have smaller particle size they would be better absorbed by digestive tract lining therefore the amount necessary would be reduced making medicines more affordable. The ability to deliver antibiotics in aerosol form to the lungs would mean easier ways of treating infections like tuberculosis. By decreasing particle size, the bioavailability increases. This would lead to faster-reacting drugs and many other applications in the health industry.

“Nanorobots,” are imagined molecules that could be made to seek out certain cells and perform a task in that particular area of the body, for example, the seeking out of liver cells and facilitating the production of a certain proteins. By developing manmade nano-robots, it would be possible to manipulate and self-assemble cells that cause diseases and cancers and eliminate current health problems. The potential of nano-robots in the field of Biotechnology reaches beyond healing living cells.

Nanotechnology experts contend that human life can be extended by nanorobots’ ability to self-copy and keep human cell replicating in normal ways, which reduce aging. The ability to build materials from the smallest particles means that atomic properties can be adapted for certain purposes for example; the electronics field. The ability to manipulate properties means that these materials can be made to be better conductors leading to circuits that are smaller and faster. The circuits will in turn be able to achieve more complicated functions. The potential for this technology is endless. For example, the National Nanotechnology Initiative have set these challenges for research scientists to accomplish: reducing entire content of the library of Congress to a sugar cube, constructing new plastics and other polymers with the strength of steel, improving computers speed by a factor of millions, and etc. Improving the performance of a catalyst is very important for economical and social aspects of society.

In an article discussing the economic contributions of catalysis noted that “one-third of material gross national product in the U.S. involves a catalytic process somewhere in the production chain”. An example is the petroleum industry. Many of their processes heavenly depend on catalysts to produce oil, gasoline, and other petroleum products. If a catalyst’s efficiency improves by 3 %, this may lead to an increase of millions, possible billions of dollars for the petroleum industry. The social impact would be the price reduction of different petroleum products for the consumer.

The environment also benefits since harmful bi-products will subside as a result of increasing the yield of a process. In order to improve catalyst, scientists are investigating the nanocatalyst as an avenue. The advantages of using a nanocatalyst are profound. By nanocatalyst being very small in size, the particle size creates a very high surface to volume ratio. This increase the performance of the catalyst since there is more surface to react with the reactants. Another advantage is that nanocatalysts are able to place where traditional catalysts will not fit.

Nanopowders

The basis of nanotechnology is the ability to form nano-sized particles, for example nanopowders, which are solid particles that measure on the nanoscale, usually comprised of three to five molecules together. Nanopowders can be used in most of the aforementioned applications; therefore, they have been a field of great interest. Nanopowders have been of extreme interest in the pharmaceutical field. Drug delivery has been impacted in several ways due to the advances in nanopowder technology. Smaller particles are able to be delivered in new ways to patients, through solutions, oral or injected, and aerosol, inhaler or respirator. New production processes allow for encapsulation of pharmaceuticals which allow for drug delivery where needed within the body. Dosing of pharmaceuticals had also improved. Smaller particles mean better absorption by the body therefore less drug is needed. Because of a combination of these, side effects are lessened due to better use of pharmaceuticals.

Production of Nanopowders

Although there are many uses for nanopowders within the pharmaceutical field, the production of these powders has been of great interests. For this reason, at this point in time the research around nanopowders has been focused mostly on methods of production.

Conventional Methods

Conventional methods of particle size reduction include milling, grinding, jetmilling, crushing, and air micronization. There are several drawbacks to these methods. First, they might not accomplish the desired amount of particle size reduction. The second drawback is associated with the physical and chemical properties of the materials undergoing size reduction.

Certain compounds are chemically sensitive or thermo-liable, such as explosives, chemical intermediates, or pharmaceuticals which cannot be processed using conventional methods due to the physical effects of these methods. Other compounds such as, polymers, pigments or dyes, etc. maybe difficult to process by conventional methods due to physical properties such as physical degradation under high pressures or temperatures, “softness”, or waxy texture.

Nanopowder Production

The most common methods of nanopowder production involve the rapid depressurization of saturated solutions. This causes the saturate to precipitate out of the solution in the form of nanopowder.

There are several methods for production of nanopowders. One of them is through a chemical route. Using the point of zero charge concept, single molecules or small packets of molecules may be synthesized, usually on a support.

• Homogeneous nucleation occurs at fast saturation and temperature drops.

• Certain dye, chemical intermediates, pharmaceutical and biological compounds, polymers and explosives, which are chemically sensitive, thermo-labile, waxy or soft, are difficult to powderize by conventional techniques.

• Most new techniques for nanopowder production use CO₂ non-toxic, non-flammable, cheap and easy recyclable.

• beta=(actual concentration/saturation concentration)

• Size, size distribution, morphology, polymorphic nature all depend on evolution of beta

• Where large levels of super saturation are quickly attained small particle size distribution occurs and smaller crystals are formed due to nucleation phenomena being enhanced over crystal growth, meaning smaller particles.

• The efficiency of a crystallization process is highly related to the compound solubility and solute concentration evolution, which determines the super saturation during the crystallization process.

Rapid Expansion of Supercritical Fluids

Rapid Expansion of Supercritical Solutions (RESS) is a crystallization technique that uses the properties of a supercritical fluid, typically CO₂, as a solvent to facilitate nanopowder production.

The RESS process is described in two steps: solubilization and particle formation. The driving force for this process is caused by the rapid depressurization of the supercritical fluid dissolved with the solute of interest through a nozzle to cause fast nucleation and fine particle generation.

RESS process operation As showing in Figure 1, the RESS process begins by subjecting liquid CO2 to high pressure and temperature in order to reach the supercritical region of the fluid. The supercritical fluid is then mixed in the extraction autoclave with the solute. At the extraction step, the flow rate plays a role since the thermodynamic equilibrium may not have been obtained. This may affect the solvating power of the solvent since temperature fluctuations occur outside the region of thermodynamic equilibrium. The next step involves the depressurization of the mixture from high pressure to atmospheric pressure by a nozzle. This rapid decrease in pressure causes nucleation by lowering the solvating power of solvent. Since CO₂ is in the gaseous state at ambient conditions, the solute precipitates and is gather in the collecting device. The CO₂ is then purges out of the container through a valve.

Advantages and Disdvantages

There are many advantages to the RESS process. Although the process takes place at high pressures, the temperatures required are fairly low. This lowers energy costs. Another advantage is the lack of significant environmental hazards. Since CO2 is not a hazardous material when released in the quantities used in this process and it is almost pure CO2, meaning that there is little of the solute left after the process is completed, there are almost no environmental considerations that have to be taken into account for this process. The greatest advantage of all is the ability to make extremely small particles in the micro or nano scale. For all the advantages though there are some disadvantages to this process. The major disadvantage is the cost of operating at the high pressures required for the process. High pressures are costly due to pumping expenses as well as the equipment costs. Besides the high costs of being a high pressure process, the other major disadvantage is that the RESS process only works for solutes that are soluble in CO₂. If the solute is not soluble in CO₂ then another process must be considered.

Supercritical Anti-Solvent The Supercritical Anti-Solvent process (SAS) uses solvent/anti-solvent binary systems to induce the formation of nano and micro-size particles. The supercritical fluid (i.e. CO₂) acts as an anti-solvent that causes the crystallization of the solute. The main driving force for this process is the droplet formation, which is caused by the solvent/anti-solvent interaction. Since many results are influenced by the process arrangement, an investigation of the different SAS-related processes will be discussed.

Batch operation The precipitation vessel or precipitator is loaded with a specific quantity of liquid solution. The supercritical fluid CO₂ is injected into the vessel, typically from the bottom to achieve a better solvent and anti-solvent mixture. Once injected and due to the dissolution of the compressed gas, the expanded solvent has a lower solvating power than the pure solvent. This mixture becomes supersaturated and solute precipitates to form micro-size particles. After a holding time, the expanded solution is drained under isobaric conditions to wash and clean the precipitated particles.

Semicontinuous operation There are two different methods in which particles may be produced by semicontinuous operation. The first type is sometimes referred to as Aerosol Solvent Extraction System (ASES) and the second type is known as Solution Enhanced Dispersion by Supercritical fluids (SEDS). The difference between these types is due to the way the supercritical fluid and solution interacts within the injection device. We shall first discuss the ASES process, followed by the SEDS process. ASES method involves spraying the solution as fine droplets into the supercritical fluid. The dissolution of the supercritical fluid is followed by a large volume expansion, which is called the anti-solvent effect. This cause a reduction in the liquid solvating power and a sharp increase in the supersaturated within the liquid mixture, which leads to small and uniform particles. Liquid CO₂ is elevated to the supercritical region through pumps and heat exchangers. The CO₂ is allowed to enter the mixing device at a constant flow until desired pressure is established. Next, pure organic solvent transverse through the mixing device with the aim of obtaining steady composition conditions during the solute precipitation. After steady composition conditions are acquired, the flow of the pure solvent is stopped and the liquid solution from the solution vessel is delivered to the mixing device. The experiment seize when the flow of liquid solution stops and the CO₂ is purge. Typically, the CO₂ continues to flow after the flow of liquid solution has stop in order to remove the residue on the precipitate.

SEDS method was developed to achieve smaller droplet size and intense mixingof supercritical fluid and solution for increased mass transfer rates. The supercriticalfluid is used for its chemical properties and as a ‘spray enhancer’ by mechanical effects. The nozzle plays an important role in particle formation. Having two coaxial passages allows the supercritical fluid and the solution to mix inside the nozzle. This means that the high velocity of the supercritical fluid assist in the break up of the solution into very small droplets. Designed conditions permit the evaporation of the solvent from the solution into the supercritical fluid. As an effect, the droplet size reduces which leads to the formation of micro and nano-size particles.

Advantages and Disadvantages

There are many advantages using the SAS process over traditional liquid antisolvent processes.

The first advantage is based on the ability to completely remove the supercritical antisolvent by pressure reduction. Traditional liquid antisolvent requires complex post-processing treatments for the complete elimination of liquid residues which translates into additional cost.

The second advantage is due to the fact that the magnitude of the diffusivities for supercritical antisolvents may be two orders higher than liquid antisolvents. Hence, the rate of diffusion into the liquid solvent produces the supersaturation of the solute and its precipitation in the range of micronized particles to the particle diameters, which is not possible by traditional liquid antisolvent or other methods. One disadvantage is finding a solvent that is completely miscible with the supercritical fluid. Also, the crystallization tends to be heterogeneous, which depends on the efficiency of the mixing with in the nozzle. A third disadvantage is that the solute after crystallization must be rinsed to remove residue on the particles.

The SAS technique has been applied to explosives, catalysts, superconductor precursors, polymers and biopolymers, and some pharmaceutical compounds. In 1988, Schmitt produced triamcinolone particles of diameter 5-10 μm from THF. In 1992, Krukonis et al. were able to use the SAS process for crystallization and the separation of two explosives; RDX and HMX. Debenedetti et al. were able to create catalase and insulin particles with 1-5 μm size [10]. Reverchon et al. produced micronic and nanometer particles of Rifampicin by varying the pressure of the process. Those were only a few examples of the applications of the SAS process. Another application of this process is the ability to allow co-precipitation of two different compounds. The advantages acquired from co-precipitation falls into terms of formulation, dissolution rate of drug releases systems. Since SAS process is rapidly changing, new applications are discovered daily and the science of nano-powder is being revolutionized.

Particle From Gas Saturated Solution The Particle from Gas Saturated Solution (PGSS) process uses a SCF, usuallyCO2, as a solute to crystallize a solution. The PGSS process can be used to create microsized particles with the ability to control particle size distribution. PGSS also allows for the production of particles that are solvent-free using a method that is sensitive to the chemical and physical properties of the materials. The most common use of PGSS is for the mirconization or encapsulation of pharmaceuticals.

The driving force of the PGSS is a sudden temperature drop of the solution below the melting point of the solvent. This occurs as the solution is expanded from a working pressure to atmospheric conditions due to the Joule-Thompson effect. The rapid cooling of solution causes the crystallization of the solvent. The cooling is sudden and homogeneous throughout the solution; therefore, homogenous nucleation is the method of particle formation.

Process Operation

The PGSS process is a two-step process. A solvent is created by melting the desired product under a blanket of SCF. These conditions are favorable to high solubility of the SCF into the liquid product, giving rise to a gas saturated solution. The solution is allowed to reach equilibrium and homogenize. After the solution has equilibrated, it is expanded to atmospheric conditions. A filter in the expansion chamber collects the powder produced. The product does not need to be cleansed due to its solvent-free production. The SCF may be recycled if necessary. Rodrigues, M. et al. have shown the most dramatic effects of working pressure changes, is a change in the morphology of the particles. At higher pressures, 16 MPa to18 MPa, the morphology of particles seemed to mostly spherical with some aggregation. As the working pressure was lowered to 14 MPa to 12 MPa the morphology dramatically changed. Particles became flatter and developed protrusions from the surface. These protrusions are “spike” shaped and tend to become larger as pressure drops. This becomes evident in the image of this phenomenon. The image also make apparent that as pressure decreases the amount of aggregation between particles also increases. These differences can be attributed to the difference of in which period nucleation begins. At lower pressures, nucleation begins earlier in the expansion process, where is large amounts of acceleration, giving rise to long thread-like structures because. Spherical particles are evidence of higher pressures, therefore, nucleation that begins at a later, less accelerated stage, of the expansion process. Although pressure has a noticeable change in particle morphology, there is no apparent effect upon the size or size distribution of particles. techniques. The most evident advantage to the PGSS process is the ability to form nanopowder without the need for solvents for the products. This reduces operating costs in two ways. First, the need for expensive chemical solvents is reduced therefore, operating costs are lowered. Second, the lack of solvents means that the product is high in purity and removal of residue or rinsing is not required. Another advantage of the PGSS process is the ability to form micro composites or encapsulated particles. This can be achieved by the dissolution of a SCF into a mixture of two solvents, both of which will be present in the product. The process can then be run at conditions that optimal for encapsulation or for formation of micro composite particles. The disadvantages of the PGSS process are due to the need for a SCF to dissolve into a solvent. Although at higher working pressures dissolution becomes more effective, there are compounds for which a SCF can have low solubility. In these cases a different SCF might be an option. Another related disadvantage is the difficulty in the dissolution of a SCF into several solvents with different SCF solubilities. This becomes most important when encapsulation or micro composite particles are the desired product.

Depressurization of an Expanded Liquid Organic Solution

Unlike any other method Depressurization of an expanded liquid organic solution (DELOS) is a process that uses a supercritical fluid, as a co-solvent for the formation of micro- sized particles. The DELOS process is best for organic solutes in organic solvents and it is particularly useful for pharmaceuticals, dyes, and polymers, where conventional methods of particle size reduction tend to be ineffective due to physical and chemical limitations. The driving force for the DELOS process is a fast and large temperature drop. This occurs when the pressurized solution is expanded from a working pressure to atmospheric pressure. The drop in pressure and temperature is homogeneous throughout the solution due to the fact that the system is allowed to reach equilibrium before being expanded. This fast drop in temperature causes the saturation limit to drop equally as fast causing the crystallization of particles from the solution. This process therefore favors nucleation.

The DELOS process is a simple three-step process. The first step is the dissolution of a solute into an organic solvent in a pressure resistant chamber that is heated to a desired working temperature. Once this is complete, a pre-heated CF is dissolved into the solution and used to achieve the desired working pressure. Sufficient time is provided for the ternary solution to reach equilibrium and the working temperature. Once equilibrium is achieved, the solution is expanded through a one-way valve into a chamber at atmospheric pressure. Pure nitrogen is pumped into the solution chamber to maintain the working pressure during expansion. A filter at the bottom of the expansion chamber collects the solute powder. The formed powder can be cleansed using the pure CF.

The solvents from this process can easily be separated and recycled if deemed necessary. Since crystallization through the DELOS process is dependent on a large temperature drop, the yield can be maximized by maximizing the amount of CF used. However, there is a disadvantage; there is a limit to the amount of CF which can be used. If this limit is surpassed, the DELOS process will not be possible and instead crystallization will occur through the SAS process. The limiting amount of CF can be obtained by finding the intersection of the experimental solubility curve for the system and the working line which depicts the evolution of the solvent concentration as CF is added to the system. As the working concentration of CF reaches the limit concentration, the size of particles and the size distribution are minimized. Therefore it is possible to control the particle size characteristics by controlling the working concentration of CF. This allows for the possibility of both micro- and macro- sized particles. Nano-sized particles are attainable through this procedure.

The DELOS process is not dependent on the pressure change from the working pressure to atmospheric pressure. Ventosa, N. et al. have shown that for a given system yield, particle size, and particle size distribution are dependent on the temperature drop for the working temperature to the final depressurization temperature, therefore the main factors that control yield are the working concentration of CF and the initial solubility ratio. This allows for the process to be carried out at lower temperatures without any effects. This can lead to cost reduction of running the process. Particle characteristics or yield of the products of the DELOS process are not dependent on the flow rate of solution through the one-way expansion valve. Particle characteristics are only dependent upon the progression of the supersaturation ratio through the process. Product yield for the DELOS process is directly proportional to the initial supersaturation ratio of the equilibrated solution. Since the solution is allowed to equilibrate, the saturation profile of the process is homogeneous and therefore, does not depend on the efficiency of mixing.