Redefining Density Measurement in Ceramics
Laser profiling has been pioneered as a non-contact, non-destructive method of measuring density that is particularly suited to fragile samples such as green compact ceramics.
Measuring density is crucial for the manufacture of many of today’s products. Being able to characterize the structure and quality of solid materials quickly and efficiently can not only help guide development and manufacturing processes, it also influences the quality of the end products.
Measuring the density of ceramics has historically been challenging, however, as traditional mercury displacement methods are toxic and therefore undesirable. In fact, they are now illegal in the European Union. Innovation in this area is helping to redefine density measurement; developments in laser scanners, among other techniques, now offer valuable insights into single solid products.
Importance of Density Measurement
In simple terms, density is defined as the mass of an object divided by its volume (g/cm3). Although determining mass can be achieved easily with a balance, finding out the volume presents a challenge. According to ASTM International, there are over 40 definitions of density, while the British Standards Institute has narrowed it down to 14 types. As the method for calculating mass of a solid differs between industries, the definition of density also varies. The problem with measuring the mass of solid materials is that these materials also contain surface irregularities and pores.
Preventing porosity is crucial to maintaining structural integrity, but it is notoriously difficult to measure. To determine the porosity, the number of open and closed cells must first be noted in order to highlight any unwanted voids in the material. By measuring the density of the product, it becomes easier to regulate porosity and adjust formulations accordingly. For example, decreases in density can signal the presence of voids, whereas increases may indicate that a process (e.g., a polymer approaching its crystalline state) is optimized.
In particular, the porosity of the green ceramic body is key in determining end product quality. Density measurements must therefore be as accurate as possible in order to reduce the number of cracks in the material, which could potentially damage its structural integrity. As such, monitoring density could not only help with quality control, it could also reduce waste for manufacturers.
Density Measurement Techniques
Many measurement techniques can be used to determine the density and volume of solid materials. However, most approaches exclude the presence of void spaces, thereby decreasing the accuracy of the testing. It is therefore important to first consider the physical characteristics of the sample material, as well as to ensure that void volumes are included in final calculations.
Traditional techniques for measuring density are based on the displacement method principle. Although this is used primarily to measure volume, density can be calculated by dividing the mass of the sample by its volume—or Boyle’s Law. Gas pycnometry is one example of a laboratory technique that uses a displacement medium to determine ceramic volumes. An inert gas such as helium or nitrogen is introduced into an empty chamber, where the pressure is measured. When the sample is placed in the chamber and resealed, the same quantity of gas is expanded into the area at the same pressure. The difference between the two pressures, combined with the known volume of the empty sample chamber, allows the volume of the sample’s volume to be calculated.
Although modern gas pycnometers have improved accuracy, most instruments are limited to testing small samples, making them unsuitable for many ceramic applications. In addition, this method relies on the sample material and analysis gas being completely free of moisture. The sample must also be pre-treated in a vacuum oven to remove volatile substances. Otherwise, contamination could cause pressure instabilities and potentially result in inaccurate readings. Errors can also occur due to leaks, temperature instabilities or thermal gradients. In addition, when testing sample ceramics with closed pores, it can be more difficult to calculate true density, particularly compared to open pores.
Density measurement can also be calculated manually. For instance, the hydrostatic displacement method determines a product’s volume by comparing the mass of the sample in air to its mass in a liquid of known density. The volume is the difference in the two masses, divided by the density of the liquid. Manufacturers also use this method to measure porous samples, by first sealing the sample with a coating. To exclude pore volume, the pores must also be completely filled with liquid. However, this method brings challenges because the surface tension of (and trapped gases in) small pores prevents them from being filled.
Given the potential inaccuracies of displacement methods in determining porosity in ceramics, manufacturers have been keen to explore other options. Laser profiling has been pioneered as a non-contact, non-destructive method of measuring density that is particularly suited to fragile samples such as green compact ceramics.
Laser scanning equipment removes many of the inaccuracies and limitations of traditional density measurement techniques. For instance, one profiling instrument works by loading a sample onto one of several platform options—one of which is a disposable plate to which the sample can be adhered to support it on its perimeter.* The desired settings can simply be entered into the software before running the test. Most measurements (volume, surface area and density) are automatically produced after each scan. It is also easier to compare samples of the same batch, since the mean, standard deviation and coefficient of variation are automatically calculated.
During the test, the product is weighed on an external balance, measuring to three decimal places. The laser device, which has been declared safe for eyes, is scanned vertically to measure the contours of the product at selectable intervals while it rotates. Each interval collects 400 data points to provide a detailed profile. Density scans can be performed quickly using this method; scans range from a few seconds to several minutes, depending on the chosen interval (0.05-50 mm) and the preferred degree of precision. Manufacturers can test samples up to 300 mm in height and 190 mm in diameter, with a weight of up to 10 kg (using the load cell within the instrument), making the process suitable for tile and tableware, among other ceramic products.
*Ceramscan, developed by Stable Micro Systems.
Future of Density Measurement
Ceramics and other laminate structures need to display minimal porosity in order to maintain their structural integrity and ensure high-quality end products. However, the fragile nature of the material means that it is particularly difficult to measure. While traditional methods of measuring density, such as gas pycnometry, are adequate in gaining a reasonable view of the sample, they are also dependent on stable conditions to make optimum calculations. These techniques also often require the ceramic to be broken, resulting in waste and inefficiency.
Having access to a method of measuring density that avoids damaging fragile ceramics is a lower cost and less time-consuming alternative to more traditional techniques. Laser profiling instruments offer promising replacements for previous methods, allowing manufacturers to make accurate judgements by the safest means possible in order to inform the formulation process and enhance the quality of their end products.
For more information, call (44) 1483-427345 or visit www.stablemicrosystems.com.