Laboratory

Ceramic powder can be heated and processed into a large variety of forms, but for the standard shape for testing of these materials is the disc or pellet. This shape is preferred for a number of reasons, the principle one being that by making it cylindrical there are only two edges to act as stress concentrators¹. This is important, as if there is no binder present in the initial powder the resulting green compact can be incredibly fragile and may break before further processing. This is true also of the heat processed form, where an increased number of edges can increase the risk of fracture during densification².

The other reason for choosing the pellet as the shape to test these materials is that it can be cut to suit the majority of the tests that will be performed upon it. Some typical tests that are performed on engineering ceramics include infrared (IR) and X-ray fluorescence (XRF) spectroscopy³. These tests typically use a flat cylindrical sample, and the ability to test the pellets without further cutting or grinding is a huge help. Additional processing can introduce problems in the form of inconsistencies in the dimensions of the final sample which may then cause errors in the measurement.

With the reasoning behind the shape explained, there are then some important features of the pellets themselves. A pressed disc should fulfil the following quality criteria: homogeneity across the composition, solidity to avoid loose particles, stability, and finally storability⁴. In the case of spectroscopy, it is the content of the pellet that is being investigated and it should be made sure that any binder does not interact with the main powder. It is much preferred to have a pellet that has been produced without the aid of a binder.

There are other tests that benefit from a flat pellet. X-ray diffraction is better with a solid pellet as the concentration of the elements over a certain area is higher in density than that of a loose powder. This causes a greater amount of counts for the sample testing time. In addition, a smooth surface is preferable to a rough one from an optical point of view, with any contrasts much easier to view under optical microscopy.

The equipment used to press pellets for investigation must therefore be engineered with precision in order to produce pellets that conform to the aforementioned criteria. Carver Inc. has been designing and producing laboratory standard presses for over 100 years and is proud to offer a complete range of products to suit all manner of testing⁵.

Every researcher wants to ensure that their pellets are produced in a consistent and uniform manner, but not all research environments can accommodate the exact same press. This is why Carver Inc. provides a variety of presses that are suited for research or industry, whether it’s for the production of single pellets one at a time, or a large run of the same powder.

For labs where space may be limited, a bench top press should be considered. Although smaller in foot print, they offer the same high quality and customization options as other, larger presses. Carver’s benchtop press range has both hand operated and automatic models, with a number of optional features for greater specificity of task.

On the manual side the standard model is available in 12, 25 and 30 ton capacities, and can be used for tasks such as pelletizing, destructive testing, fluid extraction, pressure forming, to name a few. For producing pellets, especially useful in the case of IR or XRF spectroscopy, Carver offers a set of dies in 12, 13, 25, 31 and 40 mm sample sizes. For heat related processing like molding and laminating, models with temperature controlled heated platens are also available.

Some testing requires a large number of similarly shaped samples. This can be tiring and time consuming when done by hand, so automation can be a better option. Carver also offers a solution in this case, with both the standard Auto Series Plus and AutoPellet automatic laboratory presses. These examples will sit comfortably on a lab benchtop, although scaled up versions can be found too.

The Auto Series Plus General Purpose Presses feature a large 7” color touch screen for operator input. If it is a large number of pellets that needs to be produced then the AutoPellet is the superior option. Similar to the manual model it can be outfitted with dies 6, 12, 13, 25, 31 and 40mm, and comes in both 25 & 40 ton varieties, with an electrically powered hydraulic system driving the 5" diameter platen. In order to maintain precise alignment the slab side construction is exceptionally rigid, with provision for a vacuum line connection to the pellet die. The AutoPellet presses have a programmable control system with 10 recipe storage capacity and programmable decompression rates. This can control the platen temperature, as well as auto-tuning, ramp heating capability, and output of force and temperature in real-time. The system can also store up to 250 recipes, and be augmented with options such as a seven day timer for auto platen heat-up and remote access to data through Wi-Fi. There are a number of Auto Series Plus press models available with clamping force of 15 to 48 tons platen sizes of 6" x 6" to 19” x 19”.

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References and Further Reading: 1. Barsoum, M. . Fundamentals of Ceramics. (2002) 2. Johnson, D. L. Fundamentals of the Sintering of Ceramics. Mater Sci Res 11, 137–149 (1978) 3. Dent, G. Preparation of Samples for IR Spectroscopy as KBr Disk. Available at: http://www.irdg.org/the-infrared-and-raman-discussion-group/ijvs/ijvs-volume-1edition-1/dent/  4. Eads, A. & Miller, G. Pressing Ceramic Sample Discs. 34–35 (2019) 5. Carver Inc. Products. Available at: https://www.carverpress.com/products