The importance of surface-cleaned micropowders for specialized uses in the ceramic industry
06/01/2005
Controlling the surface chemistry of micron and submicron particles is critical during the manufacturing process
Ron Abramshe, PhD., Warren/Amplex Superabrasives.
Engineers, designers, production managers and purchasing professionals should be informed of the processes used to manufacture the materials used in their respective applications. These applications include but are not limited to:
• Micron diamond used in slicing and dicing wheels for silicon wafer production
• Submicron diamond used in the lapping and polishing of GMR read-write heads in computer hard drives
• Micron and submicron diamond used in polishing laser and LED caps
• Calcium bifluoride crystals for detectors
• And micron/submicron diamond used in polishing aluminum oxide and zirconium oxide hip-replacement joints and dental prostheses
This article is also appropriate for the application of high-performance ceramic materials such as alpha aluminum oxide for slip casting of ceramic parts as hip-replacement joints; titanium dioxide used in paints; or wherever other micron and submicron particles are needed in a state where their surfaces represent the material and not a conglomerate of base material and surface impurities.
It is critical to control the surface chemistry of the particles during the manufacturing process of micron and submicron material. Commonly, feed materials have been processed using a number of aggressive chemical techniques. The precipitation of alumina requires acidic conditions. Synthetic diamond also requires extensive treatment with both alkalis and acids. All of these processes leave residual anions and cations on the surface of the particles. Surface cleaning during processing of the initial material is done to meet effluent standards or post-processing treatment of the water, not necessarily for cleanliness of the product.
When producing high-quality new products, it is vital to control the surface chemistry during processing of micron and submicron materials. Customers can be concerned about agglomeration or cross-contamination in their end product. Hence, reducing or eliminating this concern within their specific chemistry process is a desirable characteristic.
When considering the size separation of micron and submicron particles, it is essential to control the incoming feed.
Elutriation
Elutriation is the process of grading micron and submicron sizes (usually less than 60 μm) into equal distributions that are normally distributed (see Fig. 1). Normal distribution exists when there is a mean, mode and median to the number of particles in the population of material being graded. The population of particles will follow the Gaussian form of:
f(x)=1/σ * √ 2 π exp-[(X-μ)2/2 σ2]
Since the population follows a normal distribution, the control (or standardization) of micron material lends itself very well to Statistical Process Control.
 Figure 1. This particle size distribution (PSD) machine measures diamond particles ranging from 4 to 90 ??m in size via the fluid displacement principle. |
For proper elutriation, it is important to have the following in control:
1. Complete dispersion of the material in the fluid
2. Constant fluid velocities
3. Short treatment times for a given weight of material to be separated
4. Sharp separation as measured by a minimum overlap between grades
5. Production of grades that are standardized with customers requirements
6. Minimum attrition of the material being graded
Critical Factors
Loading: Feed Size in weight and microns
Distribution: Is the overall aspect ratio wide or narrow in shape?
Fluid properties: Temperature, pH, pressure, filtration and ionic activity are all important.
Processing: Flow rates, vessel size, output, quality control, and product management.
If we look at item number one in the list above, we note that complete dispersion is important. The dispersion and stability of these dispersed particles in liquids is of paramount importance in processing of elutriated particles after grading. Purity, grain size and chemical heterogeneity are of paramount importance in both a macro and/or micro sense.
In some instances, agglomerates tend to give false results during the grading process by tricking laser-measuring devices into believing the particles are larger than they actually are (see Fig. 2). For instance, in slip casting of ceramic parts they tend to create voids in the final fired part. Well-dispersed powders that have high solids ratios with well-defined rheological properties are wanted in ceramics. These are the type of ceramic powders that can be green-slip casted into intricate geometries without void defects.
We should look at the systems we need for both producing these types of powders and using powders in other systems.
Controlling the particle size distribution (PSD) and balancing the interparticle forces can achieve these results. Controlling interparticle forces is usually accomplished by Zeta Potential measurements. Zeta potentials can be measured to ascertain the correct amount of dispersant or dispersants to be used to keep particles stable.
Before we can say Zeta Potential is all we need, we should explore the background of putting particles into suspension and the need to keep the surfaces as clean as possible, thus preventing the overuse of dispersants and possible downstream problems in manufacturing.
Particles can be considered as discrete units once the type of powder and what process it came from is determined. These units, based on surface attraction forces from upstream processing, can easily adhere to one another, forming clusters that can be connected by a network of interconnected pores or surface ions.
Agglomerates will depend on their individual initial size, shape, microstructure, spatial relationships and number of primary particles. Some agglomerates may be soft due to van der Walls forces; others may be hard due to chemical bonding.
This phenomenon is increased in submicron powders because of the relationship between surface area, particle size and surface energy. As the surface area to volume area increases there are a large number of molecules at the interfacial region. These can affect the stability of the system as a result of the larger surface area adsorbing larger quantities of chemicals.
In the case of surface energy, crystalline powders have a higher surface energy than amorphous powders. For example, diamond is 5.4 J/m2 at the 111 plane, as opposed to graphite at 1.1 to 1.3 J/m2.
Putting a powder into a liquid system involves a few different steps. The simple act of pouring, slaking or casting a powder into a liquid involves a process of deaeration of the powder as it descends into the liquid. There is a proportional amount of air surrounding the particle that must be displaced. It takes a certain amount of time-which is dependant upon agitation and/or the length the particles have to travel-to displace the air. Larger particle size due to unwanted surface chemicals will increase the time.
After removing the air from the surface, the next step is the actual wetting of the surface. Time can vary since hydration at the surface will be dependant on the amount of chemical and residual surface ions present. If there are only a few parts per million (ppm) of anion or cation, this hydration step can be quick. If, on the other hand, there are a few wt% of these cations or anions present, it will take a bit longer as these ions need to dissolve and remove themselves from the surface of the particles.
It is at this stage that problems can occur. For example, the overall ionic activity of the system is affected if the surface contains too many complexing ions. Generally, overcompensation is made either positively or negatively through the use of dispersants. Over a period of time, as the final few ions dissolve, stabilization may take place with the system breaking down either as a floc or as hard-packed sediment.
Comparatively, if the surface is clean and ionic activity is minimal, we can then reach the final phase of the process; specifically, having all the particles immersed in the liquid at an equilibrium point where they are in suspension or in a dispersive state. At this stage, these particles can be graded using the elutriation technique noted earlier or mixed in a binder for further processing (e.g., forming shapes or being green-machined after pressing).
This discussion stresses the need for each researcher or engineer to be aware of the positive and negative effects of surface cleanliness. When grading micron and submicron particles, it is vital to determine what is on the particle surface and in what quantities. This is essential in order to obtain sharp Gaussian distributions as well as to know what the rheology of the mix is like in the high-solids (> 60 vol.%) suspensions being formulated for the next hip-replacement joint. III
Ron Abramshe, PhD, is the Product Manager/Technical Sales Manager for Warren Amplex Superabrasives. He has a PhD in Engineering Management from Kennedy-Western University, a Master of Science in Engineering from Polytechnic University of New York and a BS in Industrial Engineering from the University of Dayton. He can be reached at [email protected]