The importance of surface cleaned micro powders for specialized uses in the ceramic industry
The purpose of this article is to inform engineers, designers, production managers and purchasing professionals 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,
• Sub-micron diamond used in the lapping and polishing of GMR read-write heads in computer hard drives,
• Micron and sub-micron diamond used in polishing laser and LED caps,
• Calcium bi-fluoride crystals for detectors,
• And micron-sub-micron diamond used in polishing aluminum oxide and zirconium oxide hip replacement joints and dental prosthesis.
This article is also appropriate for the application of high-performance ceramic materials as alpha aluminum oxide for slip casting of ceramic parts as hip replacement joints, titanium dioxide used in paints or where other particles of micron and sub-micron 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 sub-micron 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 sub-micron 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 sub-micron particles, it is essential to control the incoming feed.
Elutriation is the process of grading micron and sub-micron sizes (usually less than 60 microns) into equal distributions that are normally distributed. 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/s * Ö 2 p exp-[(X-µ)2/2 s 2]. Since the population follows a normal distribution, the control (or standardization) of micron material lends itself very well to Statistical Process Control.
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.
Elutriation — critical factors
Loading > Feed Size in weight and microns. Depending on micron size of the material to be elutriated, weight in terms of vessel loading is important in that there needs to be a balance between the fluid density and the apparent density of the feed charge. Too much material will not provide enough spaces for the material to separate properly resulting in coarse and fine particle carry over into the body of the elutriated product.
Temperature is important in maintaining fluid density for accurate particle separation. pH is important to keep van de Walls forces in check. Pressure control of the system is needed to keep fluid flow constant. Filtration is needed to prevent unwanted particles from being introduced into the elutriation system. Ionic activity must be monitored to prevent the overloading the particles with too many cations or anions.
Flow rates need to be monitored and charted to prevent any fluid surges. Vessel size selection is important for proper grading and productivity. Output in terms of the number of vessels needed to fill a particular order quantity is also important, as is quality control.
If we look at item 1 under elutriation, we note 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, during the grading process agglomerates tend to give false results by tricking laser-measuring devices into believing the particles are larger than they actually are.
For instance, in slip casting of ceramic parts they tend to create voids in the final fired part. Well-dispersed powders are wanted in ceramics that have high solids ratios with well-defined rheological properties. These are the type of ceramic powders that can be green-slip casted into intricate geometries with out void defects.
We should look at the systems we need to work in both producing these types of powders and using powders in other systems.
Controlling the particle size distribution and balancing the interparticle forces can achieve these results. Controlling interparticle forces are usually done 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. Which can form 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 sub-micron 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 because 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 1.3J/ 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 de-aeration 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 to displace the air, which are dependant upon agitation and/or the length the particles have to travel. Additional 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 ppms of anion or cation this hydration step can be quick. If on the other hand there are a few wt% of these cation 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 sub-micron 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.
About the author
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.