Volume 1 Number 1 Dennis R. Dinger 1 November 2002
Goals of this E-zine
First things first: Ceramic Processing E-zine is a free monthly publication which will be distributed to all interested ceramists. Please forward this issue to friends and associates who may be interested in receiving Ceramic Processing E-zine.
Each issue of Ceramic Processing E-zine will contain one or two brief articles that touch on fundamental topics in ceramic processing, or on simple computer applications that can help with ceramic processing calculations. Readers are encouraged to submit suggested questions they'd like to see addressed in articles of subsequent issues.
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Dennis R. Dinger
Obviously, since I will be writing the articles contained in this E-zine, I need to tell you briefly about myself. I am a Professor Emeritus of Ceramic and Materials Engineering at Clemson University. I have worked in the field of ceramic processing, with particular interests in computer applications to fine particle processing and in whitewares processing, since the early 1980s. I teamed with the late Professor James E. Funk from Alfred University for more than 20 years. My research throughout my years in academia and my recent consulting have been mainly in the field of traditional whiteware ceramics. More details are available at my web site.
The ceramic processing book, "Predictive Process Control of Crowded Particulate Suspensions applied to Ceramic Manufacturing," which Jim Funk and I co-authored, was published in 1994 and is currently out of print. I recently wrote and published "Particle Calculations for Ceramists." This is available for purchase in both hard copy (paperback) and e-book versions at my website. Click HERE to jump to that web page. I am currently writing the second volume in this series, which will be "Rheology for Ceramists." As soon as it is finished, it will also be available in e-book form for purchase at my web site. A third book in this series, covering the fundamentals of combustion, is planned. Other suggestions are welcome. In addition to books and e-books, I intend to make appropriate software (discussed in the next section) and a variety of small pamphlets and articles available for download from that same web site. Of course, I remain available as a consultant, so if you need specific help in any of these areas, please contact me.
Excel® Add-In Functions Available
A set of add-in functions that work in the Microsoft Excel® spreadsheet program have recently been completed and are ready for sale and distribution. These functions perform a variety of calculations that allow one to calculate the packing potential, surface area, number of particles, histogram, etc., for powders. To perform these calculations, particle size distributions of the powders must have been measured and the data must be available in the form of CPFT vs size. Throughout the 1990s, the late Professor James E. Funk and I distributed several programs written and compiled for use in MS-DOS® which calculated mixtures of particle size distributions and optimum packing compositions for measured and theoretical particle size data. Many have requested that we make versions of those routines available that will work in the Windows® environment and in Excel®. Although complete stand-alone routines are not available, the add-in functions that allow these same calculations to be performed in Excel® are now available for $100 per copy. These routines can be purchased and downloaded from the web site. Click HERE to jump to the Excel® Add-Ins page of the web site.
Dilatancy
Many who know me associate the term 'dilatancy' with me. It therefore seems only fitting that I should include a brief discussion of the time-independent form of rheology known as Dilatancy in the first issue of this e-zine.
There are two phenomena that cause major problems during ceramic processing: syneresis and dilatancy. Syneresis, which will be the subject of a later article, is a chemistry problem. It is produced by the presence of excessive concentrations of flocculants and flocculating cations in a suspension. Dilatancy, however, is a particle physics problem. Specifically, dilatancy results when excessive numbers of violent collisions occur between particles sheared at high shear rates. Instead of flowing smoothly around one another, the particles collide, attempt to climb up and over one another, and open the structure produced by the pile-up of particles. When this occurs, the measured viscosity increases. When extreme levels of dilatancy exist, high shear not only causes measured viscosities to increase, but it causes the pile-up of particles to bridge the complete cross-sectional area of a pipe or flow channel (creating a dilatant blockage) and all flow then ceases.
Many lab and plant rotational viscometers operate at relatively low rpms and low shear rates. To measure dilatancy, one must measure the viscosity of suspensions at more than one shear rate. If a viscometer is only ever run at one constant speed, such as 100rpm, one cannot and will not see dilatancy. But when apparent viscosities are measured at a minimum of two rpm settings, for example, at both 10 and 100 rpms, one can then determine if the suspension is dilatant or not. Better yet, one should measure apparent viscosities at a variety of shear rates from the lowest to the highest possible on any given viscometer. When a fully automatic viscometer is available, it can be set, for example, to measure the apparent viscosity for a 1 minute duration at each of a variety of different rpm values. RPM values should be chosen in a geometric progression and the results should be plotted on log-log axes showing apparent viscosity versus shear rate.
Figure 1, below, shows an example of such a dilatancy program. When apparent viscosities decrease as shear rates increase, the rheology is known as shear-thinning rheology. This is the desirable rheology for most ceramic processes. But when the apparent viscosity goes through a minimum and then increases again as shear rates increase, this rheology is known as dilatant rheology.
In this figure, the dilatant rheogram goes through a minimum apparent viscosity at ~70rpm. This shear rate (defined using viscometer rpm in this example) is known as the 'onset of dilatancy.' At all shear rates higher than ~70 rpm, this suspension is in the dilatant regime. The second rheogram is shear-thinning with no dilatancy evident within the measurement range of the rheogram. Admittedly, the measurements only covered the range from 1 rpm to 400 rpm, but that range should cover the shear rates present in most pipes. The onset of dilatancy in the dilatant rheogram, however, occurs at a relatively a low shear rate. Dilatancy could cause problems in quite a few places if that suspension is pumped around, and used, on the process floor.
Two questions are important when a suspension is known (or thought) to be dilatant: Before this suspension is sent to the plant floor, can it be tuned to decrease the severity of the dilatancy by pushing the onset of dilatancy to higher shear rates? If this suspension is already being circulated for processing throughout the plant, are process shear rates higher, the same, or lower in magnitude than those under which the dilatancy was measured?
The short-range solution to a dilatancy problem is to tune the suspension to decrease the magnitude of the dilatancy problem. To accomplish this, one must generally lower the specific gravity of the suspension, and then flocculate it. Tuning the suspension is a chemical fix to the problem, and as mentioned above, dilatancy is a particle physics problem. The long-range solution to a dilatancy problem is to reduce the magnitude of particle/particle collisions by changing the particle size distribution of the suspended powder to one that packs better.
To live with a dilatant suspension on the process floor, one must decrease the shear rates of all high shear operations. The temporary fix to allow use of a dilatant suspension is to SLOW DOWN!
To fully explain each of these potential fixes would take a relatively long article for each. More on each of these subjects will follow in later issues of this e-zine.
Don't forget to check out the web site to purchase hard copies or electronic copies of 'Particle Calculations for Ceramists,' the Excel® add-in functions, and other downloadable articles and explanations. Click HERE to visit that web page.
Thanks. See you next time.
Copyright © 2002 Dennis R Dinger
103 Augusta Rd, Clemson, SC 29631 (864) 654-3155
All Rights Reserved.
All Rights Reserved.