- Solid State Reduction
- Production of Alloy Powders
- Production of Flake Powders
- Production of Copper Compounds
- Properties of Copper Powder
- Bulk Properties
Granular copper powder can be produced by a number of methods, the most important being atomization, electrolysis, hydrometallurgy and solid state reduction. Each method yields a powder having certain inherent characteristics.
Typically, copper is melted and the liquid metal flows through an orifice where it is struck by a high velocity stream of gas or liquid, usually water, thus breaking the molten metal into particles which solidify rapidly. Particle size and shape are influenced particularly by the atomizing medium, the pressure and the flow rate. Controlled small additions of deoxidizing elements, such as phosphorus, also influence the particle size and shape. After atomization and annealing in a reducing atmosphere to decrease any surface oxide formed during atomization, the product is milled, classified and blended to achieve the particle size distribution required.
The purity of the product depends on that of the raw material since refining of the melt prior to atomization is generally not practiced. Purity is generally over 99%. The powder can be made either spherical or irregular in shape. Particle size and shape, apparent density, (See Appendix A for definitions of technical terms) flow and green strength are influenced not only by atomization variables but also by controlling oxidation during atomization, subsequent reduction during annealing, and by final processing. Typical particle shapes are shown in Figure 1.
from a liquid
Electrolytic copper powder is produced by following principles used in electroplating with the conditions changed to produce a loose powdery deposit rather than a smooth adherently solid layer. The formation of powder deposits that adhere loosely to the cathode is favored by low copper ion concentration in the electrolyte, high acid concentration and high cathode current density. The addition of colloids, such as glucose, results in the formation of a uniform copper deposit. The starting material is pure cathode copper. Properties of the powder depend on a number of variables including the concentration of sulfuric acid and copper sulfate, type and quantity of the addition agent, temperature of the electrolyte, the current density and the frequency of brush-down. After deposition, the powder is washed to remove all traces of the electrolyte, annealed in a reducing atmosphere, fed to high velocity impact mills to break up clusters, screened, classified and blended to the desired particle size distribution. The properties are influenced also by the temperature used in reducing the powder.
The copper powder obtained by electrolysis is high purity material, averaging more than 99% copper. The powder is dendritic in shape as indicated in Figure 1. A wide range of powders having different apparent densities and high green strengths can be obtained by this method.Back to Top
The hydrometallurgy process can be used to produce copper powder from cement copper, concentrates or scrap copper. The copper is leached from these materials with sulfuric acid or ammoniacal solutions and the pregnant solution is separated from the residue by filtration. The copper is precipitated from solution by reduction with hydrogen under pressure. In one process, for example, reduction is accomplished in an autoclave at 225-280F (107-138C) in one hour under a partial pressure of hydrogen of 400 psig (total pressure 425 psig) with a thickening agent added to minimize plating and control the particle size. During reduction, 90-95% of the copper is precipitated as powder. The powder is pumped as a slurry to a centrifuge where the powder is separated from the liquid and washed. The wet copper powder is dried in a reducing atmosphere, milled, classified and blended to achieve the particle size distribution desired. The physical characteristics of the powder can be varied over a considerable range. Temperature and time of reduction and the quantity of acrysol addition have a marked influence on the powder properties.
The process yields a high purity powder, averaging more than 99% copper. Generally, the powder obtained has fine particle sizes with relatively low apparent ensities and high green strength. The particle shape is indicated in Figure 1.Back to Top
Solid State Reduction
In this method, oxides including mill scale are first ground to control particle size and then reduced by a gas, usually carbon monoxide, hydrogen or cracked natural gas at temperatures below the melting point of copper. Particle size and shape can be controlled within rather wide limits by varying the particle size and shape of the oxides, the reducing temperature, pressure and flow of the gas. The resulting powder is milled, classified and blended to the desired specifications.
The purity of the product depends on the purity of the oxide since there is no refining during the reduction process. Generally, the powders produced by this method tend to be porous and have high apparent densities and green strength. An irregular particle shape is obtained as is indicated in Figure 1.Back to Top
Production of Alloy Powders
Most alloy powders are produced by atomization. Pre-blended powders are mixtures of the desired composition, with or without lubricant, which will form the alloy during sintering. Pre-alloyed powders are produced by atomization of the alloy composition by the methods mentioned for the production of copper powder. Pre-alloyed powder can also be produced by sintering a blend and grinding to obtain powder with desired characteristics.
Alloy powders are available commercially in various materials. They include brasses ranging from 95Cu-5Zn to 60Cu-40Zn (and leaded versions of these alloys), nickel silvers, tin bronzes, aluminum bronzes and beryllium bronzes. As mentioned previously, any copper alloy can be produced in powder form.Back to Top
Production of Flake Powders
The powders discussed previously have been granular in form and are used primarily for the production of P/M parts. Flake powders are used for other purposes. Although pure copper powder is produced in flake form, most flake powder, the so-called " gold bronze" powders, is produced from alloys of copper with zinc and aluminum. Special colors are produced by modifying the base alloys with tin or nickel.
The alloy is powdered by atomization or is melted to produce spatter and the particles are charged into ball mills with a lubricant such as stearic acid and reduced to the desired fineness. Alternately, the Hall paste process involving ball milling in mineral spirits or the Hametag modification of ball milling can be employed. After milling, additional lubricant is added and the powder is polished in drums and stored to develop suitable leafing properties.Back to Top
Production of Copper Compounds
Cuprous oxide (Cu2O), cupric oxide (CuO) and cuprous sulfide (Cu2S) are produced as powders by the controlled reaction of oxygen with copper powder. The products are used in antifouling paints (Cu2O), reagents in chemical reactions, catalysts in the production of silicone compounds and in foundries for hydrogen degassing of non-ferrous melts.Back to Top
Properties of Copper Powder
The properties of the granular copper powders produced by the methods described are indicated in Table 1. As has been noted, the purity is influenced by the purity of the raw material and the method of preparation. Electrolytic powder is produced from high purity cathode copper and the powder is consistently more than 99% pure. Powder produced by the hydrometallurgical process, in which copper is dissolved preferentially from the raw material, also is a high purity product consistently greater than 99% copper. No refining occurs during atomization or solid state reduction and the purity of the powder depends on that of the raw material used as feed, which is selected to produce powder with 99% purity.
|Weight Loss in H2, %||0.1-0.75||0.1-0.75||0.1-0.75||0.1-0.75|
|Acid Insoluble, %||0.5-0.1 max||0.03 max||0.03 max||0.3 max|
|Apparent Density, g/cm3||2-4||1.5-4||1.5-2.5||2-4|
|Flow, sec/50 g||20-35||30-40||none||20-35|
|Green Strength, psi||nil-2500||400-6000||nil-10,000||nil-2500|
|-325 mesh, %||25-80||5-90||60-95||25-50|
|Source: P.W.Taubenblat, "Importance of Copper in Powder Metallurgy," Int. J. Powder Met. & Powder Technology 10:169 (July 1974).|
In addition to analysis for trace elements, other chemical characteristics are indicated by loss of weight in hydrogen and "acid insolubles." Loss of weight in hydrogen, indicated in the table, is a measure of the oxygen content of the material—the finer the powder, the greater the oxygen content because of the greater surface area. The acid insolubles value is a measure of the amount of material insoluble in mineral acid. In copper, a large part of this material is found to be complex compounds of copper with other elements.
Particle shape depends on the production method. Copper powders, produced by the methods discussed, can be spherical, irregular or dendritic. The shape influences the density, surface area, permeability and flow characteristics.
Porosity also varies with the production method and influences the density. Internal pores reduce the density but make no contribution to the activity of the particle. Pores connected with the surface reduce the density but also increase the effective surface and the activity.
The surface area depends on the size, shape and surface conditions of the particles and the particle size distribution. The finer the particles the greater the specific surface. An irregular shaped particle will have a greater surface area than a spherical powder of the same size. Surface roughness and surface connected porosity can increase the specific surface many times more than the area associated only with size and shape factors. The activity of a particle generally increases with increasing surface area. Specific surface is significant because reactions, such as sintering, begin at the surface. Activity influences chemical properties and diffusion.Back to Top
The properties are also influenced by the characteristics of a mass of powder. The particle size can be varied over wide ranges and the average particle size is the statistical average of all particles in the mass. The particle size distribution is influential in determining the flow and packing of powders.
The apparent density is the weight of a unit volume of the powder under specified conditions. It is a function of the size, shape and particle size distribution and is also influenced by the relative surface area and the packing properties of the powders. Apparent density is important in pressing operations because the die is generally filled by volume.
Flow is a measure of the time required for a specified quantity of powder to flow through an orifice of specified dimensions. It is a function of particle size distribution and shape but is also influenced by friction and other variables. Flow determines the time required to fill a die and thus determines the production rate that can be achieved.
Green strength is determined by compacting a mass of powder under specified conditions and breaking the compact. The strength is calculated from the dimensions of the compact and the breaking load. It is a measure of the strength of the compact before sintering.
As indicated in Table 1, considerable variation in properties can be obtained with various types of powders. Although various types can be used interchangeably, individual characteristics give certain powders distinct advantages in some applications.
For example, atomized copper powder is suitable for most P/M applications because it has a high flow rate and good strength. It can be used in electronic and electrical applications requiring high conductivity provided high purity copper powder is specified.
Electrolytic copper powder, because of its high purity, is particularly suited for P/M components in the electronic and electrical industries where high electrical and thermal conductivities are required. However, it is suitable for most other P/M applications as well.
Hydrometallurgical processing generally yields a powder having fine particle sizes, low apparent density and high strength. With these properties, it is particularly suited for use in friction materials.
Powders produced by solid state reduction have characteristics similar to those of atomized powders and are suitable for the same applications.Back to Top