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Improved powder processing creates better materials.

Improved Powder Processing Creates Better Materials MATERIALS SCIENCE researchers are finding that by developing new processes for making powders, they can improve a powder's quality and performance and create innovative new materials.

Three factors have spurred these process development efforts:

* more knowledge about powder characterization and its relationship to performance

* a preference for powder materials over liquid materials because of fewer environmental and pollution problems

* the capability of ultrafine powders to extend the utility of scarce or precious materials.

In many applications, a powder-making process needs to be tailored to satisfy specific product requirements. For example, to make a new superconductor material, the process fuses two or more powders in a single operation. For photocopier toner, several individual powders, each with its own distinct particle size distribution, must be precisely controlled and combined to maintain product quality and performance. With magnetic metal applications, a reactive powder's ultra-high purity must be retained to maintain product quality and performance.

Making better superconductors

One of these powder processes uses Hosokawa Micron Group's surface modification equipment to create and control the composite particles of yttrium, barium, and copper oxides that are used to make high-temperature superconductive powders. This equipment also is used to make a Pb-Bi series of superconductors.

The surface modification-based process lets researchers eliminate repeated sintering and pulverizing of the ceramic materials. Less material handling means better quality control, fewer sintering problems, more uniformity in the distribution of the constituent atoms throughout the powder, and higher critical temperatures.

You also can use surface modification systems to process electronic materials, pure metals, synthetic ceramics, and mixed powders for specialty applications, such as those in thermal spray technologies. In one ceramic application, for example, fine zirconia particles are fused onto the surface of larger stainless steel particles. After sintering the fused mixture, acid is used to dissolve the stainless steel, leaving a sponge-like structure of zirconia behind.

Our surface modification equipment consists of a shallow cylindrical chamber that holds the powder materials (see accompanying figure). A shaft projects from the chamber's center toward the curved wall. A curved material contact surface, or "shoe," is attached to the end of the shaft near the curved chamber wall.

The shoe has a radius of curvature that is smaller than that of the chamber. The clearance between the shoe and the chamber wall is adjustable.

During operation, the shoe remains fixed while the chamber rotates. As the wall spins, the powder is compacted by centrifugal force onto the inside chamber wall and compressed into the clearance with the shoe. The resultant powder is subject to various complex forces, including compression, attrition, shearing, and rolling.

Research on the influences of these forces, acting singly and in concert, on various materials reveals that you can enhance the process by applying a controlled amount of heat to the chamber, by isolating the chamber to permit operation under inert gas or vacuum conditions, and by continuously introducing and removing process materials from the chamber.

Our current surface modification systems have chamber volumes up to 100 l and can contain components made from hard, wear-resistant ceramic materials. As a general rule, the process is two materials, each with a different hardness.

This surface modification process has several advantages:

* Powders of pure metals, such as aluminum and nickel, can be thoroughly mixed. The resulting composite particles will not separate, thus yielding an alloy with special physical properties after sintering or thermal spray processes.

* Powders of precious metals can be coated on microspheres, which function as extenders. These processes also can be used to extend pharmaceutical active ingredients, food additives, and pigments.

* Mixed an unmixed products can be processed to achieve uniform particle sizes and shapes. Xerographic toner particles, for example, can be made with more uniform surface characteristics.

* By influencing a powder's particle size, shape, and electrical surface characteristics, you can influence the material's flow properties.

* You can create new materials. Our system can impart a similar structure to each particle, which then behaves in a specific way. Particles, for example, can be made of concentric shells of different metals.

Mixing and controlling powders

Other powder applications can require processes aimed at controlling the individual characteristics of multiple materials.

In the photocopier toner example, mixtures of polymers, carbon black, light-sensitive chemicals, flow agents, and various other additives must be controlled to minimize the amount of both small particles (fines) and large particles. Particle sphericity and electrostatic charge also must be controlled to produce dense, reliable images on a copier.

This complex process consists of an intensive mixing of precrushed materials, "kneading" of the powder, size reduction, air classification, and fines removal, and additive blending.

In a photocopying operation, the dry toner adheres to paper by electrostatic attraction. This also is the bonding method used with powder coatings, an attractive, durable substitute for conventional paints.

With both photocopier and coatings, fines must be removed to assure top performance. This is because the smaller particles will satisfy the charge differential without fully covering the target surface, thereby causing defects in a coating operation.

It is important to realize that this process's success depends on the combined knowledge and effort of the equipment developer and the toner manufacturer. This interdependency is encountered more often as materials require complex processing development.

Maintaining high purity

To process rare earth powders, such as those used to make magnets, a process that maintains the high purity of the powder has been developed.

These materials are highly reactive with oxygen and radicals containing oxygen. To maintain product performance, it is essential that the rare earth be processed as near to elemental purity as possible. Consequently, it must be isolated from air to avoid oxygen contamination.

To prevent contamination, powders, such as neodymium-iron-boron, are pulverized in a fluidized bed jet milling system before being formed into shapes and magnetized.

This system reduces the particle size of the rare earth alloy by using pressurized jets of nitrogen gas to generate collisions between particles in a fluidized bed. The fine powder product then is vented out of the bed through a turbine air classifier.

After milling, the powdered rare-earth compound sometimes is mixed with plastic or rubber to make a formable magnetic composite. This process can produce NdFeB magnets with magnet strength that are 12 times greater than those of conventional Alnico alloys.

This system has been specifically developed to process rare earth alloys to d99 , 3.8 [micrometer]. However, it can be adapted to other powder metallurgy applications that require isolation from an oxygen environment.

Shooting powders

Plasma spraying is a process that introduces metal and ceramic powders into a gun consisting of two electrodes attached to a nozzle. An inert gas flows between the electrodes and is ionized to form a hot plasma that approaches 15000 c.

Powders are injected into this plasma flame, where they are entrained and projected through the nozzle. The particles are melted by the high temperatures and propelled onto the target surface, where they solidify and accumulate. The result is a strongly bonded cermet coating. Heavy layers can be built up to make three-dimensional objects.

Prepare powders for this process presents many unsolved problems for researchers.

* The mixed powder must feed into the gun without gaps or surges. This might require surface modifications to the particles to improve flowability.

* The powder blends must present uniform proportions as they are introduced into the gun.

* Each constituent in the powder must be pure. This is a bigger problem with ceramics reduced in mechanical processes than with metal powders that are made by spray atomization.

* Commensurate particle size distributions of the powder constituentsd likely will need to be achieved to improve coating performance.

* Oversize particles might not melt completely in the plasma, thereby causing flaws in the composite coating.

Researchers in powder technology include the following:

* Thomas Meloy, Particle Analysis Center, West Virginia Univ., Morgantown, is researching methods to prevent compositional variation in multiphase compacted powders through analysis of locked particles.

* Herbert Herman, Thermal Spray Lab, State Univ. of New York, Stony Brook, is developing surface modification technologies for preparing plasma spray powders.

* Joel Clark, Massachusetts Institute of Technology, Cambridge, is developing process models, for injection molding of metal and ceramic powders, and CVD powder synthesis.

* Richard Riman, Center for Engineered Materials, Rutgers Univ., New Brunswick, NJ, is investigating particle shapes in powders made by controlled precipitation.
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Title Annotation:making better superconductors, magnets, and electrostatic toners
Author:Hixon, Larry; Van Cleef, Jabez
Publication:R & D
Date:Apr 1, 1991
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