- THE MAGAZINE
Ceramic products are designed to meet certain performance criteria based on the needs of the application, and it is the responsibility of manufacturing and quality operations to ensure that the final goods meet those criteria.
Gas pycnometry and powder displacement techniques can help ceramic manufacturers quickly, easily and accurately analyze product quality.
Ceramic products are designed to meet certain performance criteria based on the needs of the application, and it is the responsibility of manufacturing and quality operations to ensure that the final goods meet those criteria. Testing for total performance might be possible in a reasonable (i.e., short) timeframe prior to shipment, but more often than not, extended testing to specification (or failure) is not done. Rather, it is the process that created the product that is qualified by testing a reasonable number of products and/or intermediate items for some characteristic physical properties. This is done on the understanding that a given performance results from some specific set of physical properties that are created and/or modified by the manufacturing processes, since the processes themselves are the result of significant research and development activities.
For this reason, the quality analysis techniques that are implemented must be fast, accurate and easy to use, so that any defects are caught early in the production run. Two of the most important ceramic characteristics that are measured in quality control applications are density and porosity. These properties are very much inter-related, and it would be unusual for a change in one not to affect the other. Nevertheless, one normally does not dare to rely on the measurement of just one parameter, since it would be all too easy to make false inferences.
Defining the VolumeSince all density values are equal to mass per unit volume, the first problem to tackle is how to define the volume to be considered. If the final material composition has undergone any change whatsoever during manufacturing-by reaction sintering, for example-the density of the solid phase(es) will be important. For a crystalline material, X-ray diffraction might be sufficient since the exact crystal phase-and, hence, the crystal density (often referred to as theoretical density)-can be known. But this technique usually requires that the ceramic be finely ground, ideally without causing any pressure-induced phase change. X-ray diffraction also requires a considerable investment both in capital equipment and specialized training. Amorphous materials are not readily amenable to this method, and mixtures of materials present significant difficulties regarding quantitative analysis.
For these reasons, many manufacturers prefer the direct measurement of solid volume by fluid displacement. Gas, rather than liquid, is the preferred medium since it is cleaner, faster and more amenable to automation, and does not require the rigorous thermostatting associated with liquid pycnometry.
Gas PycnometryThe gas pycnometry technique is based on the displacement of a volume of gas by the solid-and all other gas-impenetrable space, such as totally isolated pores or voids. Therefore, it is also a measure of how well theoretical density has been achieved. The measurement is performed by expanding gas from one chamber (the sample holder) to another (the reference volume) (see Figure 1). The volumes of both chambers when empty are determined by a suitable calibration step, and the pressure of the gas is recorded both before and after expansion under isothermal conditions. The relationship between the pressure and volume of a fixed amount of gas is known as Boyle's Law (see Equations 1-3). The calculation of the sample volume is straightforward.
Modern gas pycnometers are equipped with a number of features designed to improve accuracy, reproducibility, sample identification and results archiving, such as different sample chamber sizes with proportionately scaled reference chamber volumes, alphanumeric input and a PC interface. Despite the name "pycnometer," which is derived form the Greek word for "dense," these instruments truly measure volume, not density. The latter is calculated by knowing the mass of the sample, and this is facilitated in state-of-the-art pycnometers by having an electronic interface between the pycnometer and the balance.
The most accurate results are obtained when the gas is moisture-free, nonreactive and behaves as ideally as possible. For this reason, helium is the gas of choice for most applications. Consequently, removing the atmospheric gases is important, and users of automatic pycnometers benefit from an instrument-controlled purge sequence. Normally, the user will be able to choose from timed evacuation, timed flow of purge gas or repetitive pulsing (think multiple rinses) of purge gas.
Repeat measurements are usually required to ensure the complete removal of atmospheric gases and the proper thermal equilibration of the sample with analysis gas. Fortunately, automated multiple determinations on a single sample require no more operator effort than a single measurement. For even greater efficiency, automated pycnometers with multiple sample chambers can be used.
Envelope Density and PorosityWhile some applications might rely solely on true density as a quality parameter, most require that porosity also be evaluated. Porosity is simply the ratio (expressed as a percentage) of internal pore space to bulk sample volume (solid + pores).
Pore volume can be determined directly by a number of different techniques according to pore size (see Table 1), or it can be calculated simply as the difference between true volume (from helium pycnometry) and bulk volume, thereby transferring the measurement of empty space to "space filling." The challenge, then, is how to measure bulk volume.
Simple geometric shapes do not pose a problem, but irregular shapes can be difficult to analyze. The space-filling volume described by an irregular shape is best referred to as envelope volume; hence its density is known as envelope density. The envelope volume might be described by a non-wetting (i.e., non-penetrating) liquid, such as mercury, or by a dry powder whose particle size precludes it from entering surface pores. The mercury displacement technique is readily available to anyone with a mercury intrusion porosimeter, while the dry powder pycnometric technique is readily available to anyone with a standard tap density analyzer.
The tap density method basically involves obtaining the tapped volume of an amount of the pycnometric powder, then adding the piece or pieces under test to the powder, followed by tapping once more (see Figure 2). Since most tap density analyzers use familiar, everyday graduated cylinders, the change in powder volume is easily read directly from the scale. This change in powder volume is the envelope volume of the sample. The cylinder size is chosen according to sample size, available amount of sample (generally more is better) and desired readability. It is possible to use standard cylinders from as little as 10 ml total capacity (sample plus powder), up to 1000 ml with a spacious 55 mm inner diameter. Less voluminous pieces can be accommodated in specially designed capillary cells that measure up to a 3 ml sample volume while still accommodating up to a 25 mm diameter and readable to within 0.0125 ml.
The pycnometric powder used in a tap density analyzer can be any material with an appropriate particle size and reasonable flow characteristics. Materials such as graphite,1 metal powders,2,3 aluminum hydroxide2 and glass beads4,5 have been successfully used. Rapeseed displacement is also a recognized method.6 Well-characterized powders for use in conjunction with a tap density analyzer are commercially available.
Accurate AnalysisA variety of instruments are available to assess the density and pore volume (porosity) of finished ceramic components. However, some methods require complicated and time-consuming preparation steps, making them difficult to implement in a production setting. By using dry techniques such as gas pycnometry and powder displacement combined with modern technology, manufacturers can quickly, easily and accurately analyze product quality. c
For more information about quality analysis techniques, contact Quantachrome Instruments, 1900 Corporate Dr., Boynton Beach, FL 33426; (561) 731-4999; fax (561) 732-9888;