Ceramic piezoelectric sensors are used in a wide range of critical applications, including fuel tanks, aircraft, medical imaging, therapeutic devices, accelerometers, monitors, positioning sensors and CPU hard drives. Because of the critical nature of these applications, manufacturers of piezoceramics must be able to certify the mechanical and electrical performance of their materials.

In addition, designers of electronic devices are moving to thinner structures to improve sensitivity and electrical response, and to reduce size and weight. As a result, suppliers must produce higher-purity, more homogeneous materials that provide more repeatable electrical properties.

Producers of piezoceramic materials also add dopants to produce specific electrical properties. For example, a key property of piezoceramic materials is coupling coefficient, which depends on the use of rare earth elements in trace amounts (100 ppm to about 2%). Other dopants typically are used at 100 to 500 ppm levels. Studies show that any impurities in the base ceramic material can offset dopant effects. In addition to impurities present in the starting ore, ceramics can contain trace amounts of phosphorus and chlorine, which are used in the refining process. Both of these elements are detrimental to the properties of the finished material.

In the past several years, the acceptable level of impurities has dropped by 80% to less than 1000 ppm, which requires that ceramic material suppliers improve their processing methods. Another issue is that, due to cost pressures, large amounts of piezoceramic materials are produced overseas. This requires more vigilance in chemical analysis to verify chemical composition. In addition, material suppliers are always developing new and improved materials that require precise characterization.

Verifying the composition and purity of incoming raw materials requires sophisticated, cost-effective testing. A number of analytical tests can be used to verify the composition and purity of ceramic materials. Each has benefits and limitations that must be considered before selecting the appropriate method.

Bulk Chemical Analysis

Bulk analysis involves determining the concentration of the major chemical constituents of the ceramic material. Analytical instrumentation used for bulk analysis includes X-ray fluorescence (XRF) and inductively coupled plasma optical emission spectroscopy (ICPOES).

XRF instrumentation determines elemental concentration by analyzing the emission of characteristic secondary (or fluorescent) X-rays from a material that has been excited by bombarding it with high-energy X-rays or gamma rays. The fluorescent radiation can be analyzed either by sorting the photon energies emitted (energy-dispersive analysis) or by separating the radiation wavelengths (wavelength-dispersive analysis). Once sorted, the intensity of each characteristic radiation is directly related to the amount of each element in the material. Samples for XRF are made into pressed powders or fusion beads.

Pressed powders consist of the sample material mixed with a small amount of binder and pressed into a pellet form under extreme pressure. Provided the sample has uniform particle size, elements can be detected at the parts per million (ppm) level. One challenge of using pressed powders as a qualitative tool is acquiring matching material with known concentrations to calculate the final concentration. These may not be available for the element of interest and the ceramic material matrix.

Fused beads are created by mixing the sample with a known amount of flux, such as lithium tetraborate. The mixture is then heated in a furnace or fusion equipment and melted into a glass bead. The advantage of using fused beads is that measurement standards can be created synthetically for comparison. The main drawback is that the sample material is diluted by the flux, limiting detection accuracy to about 0.1 to 0.5%, depending on the fusion technique.

ICP instrumentation introduces an aqueous solution into an extremely hot plasma gas. Light emitted by the atoms of an element consumed in the plasma is resolved into its component radiation, and the intensity is measured with a photomultiplier tube or solid state detector. The intensity of the electron signal is compared to previously measured standards of known element concentration, and a concentration is computed.

One advantage of using ICP analysis is the ability to match the sample solution to a synthetically created standard once the sample has been dissolved by various digestion techniques. Also, concentrations from trace to major levels can be quantified using the same sample preparation technique.

Sample preparation usually entails taking 0.1 to 1.0 g of material and digesting it using various procedures. After the sample has been broken down, it is placed into solution by diluting it with acid by a factor of 1000 or more. The use of proper analytical techniques is critical to avoid variances in the final concentration caused by the extreme sample dilution and by the influence of matrix interference. Also, some materials can be difficult to analyze because they are not readily soluble by standard procedures.

Trace Analysis

ICP instruments consume the sample in a plasma flame and detect elements with a photomultiplier, solid state chip or mass spectrometer.

Trace analysis involves determining the presence of elements in trace amounts, particularly impurities and the dopants used to produce specific electrical properties. Test methods used for trace analysis include DC arc and inductively coupled plasma/mass spectrometry (ICP/MS).

DC arc analysis involves vaporizing a 20 to 50-mg solid sample in a high-intensity DC arc that creates a 5000°C corona. As in ICP analysis, a solid state detector measures the light emitted. Then the intensity of the electron signal is compared to standards of known concentration, and a concentration is computed. Measurement accuracy ranges from parts per billion (ppb) to 0.2%.

The main advantage of DC arc analysis is that it does not dilute the sample, thus minimizing the potential for contamination (provided no additional preparation is required). However, because the sample size is so small, material homogeneity is critical for accurate results. In addition, fine particle size is required to ensure accuracy, so the material may have to be crushed and blended prior to preparing a sample. This process can potentially introduce contaminants.

ICP/MS is a highly sensitive type of mass spectrometry that can determine element concentrations below one part per trillion. It is based on coupling an ICP instrument (to produce ions) with a mass spectrometer (to separate and detect the ions).

Coupling mass spectrometry with ICP allows for low-level detection in ceramics, from parts per billion to 0.1%. As in ICP analysis, a photomultiplier tube or chip detector measures the light intensity to determine the presence or absence of specific elements. Advantages of ICP/MS include the ability to identify and quantify a large group of elements, and measurement standards are readily available.

The drawback to ICP/MS is that the ratio of sample to diluent can cause analysis errors and uncertainty. Also, because the sample must undergo digestion to break down the material, contamination is a potential problem.

Particle Size and Shape

DC arc instruments vaporize the sample in a 5000°C corona and detect elements with a solid state detector.

Scanning electron microscopy (SEM) is widely used to evaluate the elemental composition, grain size, distribution, surface roughness, and porosity of conventional ceramic products and  engineering ceramics (metal oxides, nitrides, carbides, borides, high-temperature superconductors, ceramic whiskers and fibers, single crystals, etc.). SEM allows structural and surface defects to be analyzed at the micro or nano scale. Compared to other methods for materials characterization, SEM is faster, uses smaller samples and is nondestructive.

SEM uses an electron beam produced by a filament or field emission gun that accelerates electrons through a high-voltage field (up to 50 kV) in a vacuum. The beam focuses onto a 1 to 2-nm portion of the specimen and then scans the surface. Lateral resolution is 1 to 50 nm.

Source electrons are scattered elastically, produce backscattered electrons, or interact with the solid sample. These interactions can produce X-rays as well as new electrons, called secondary electrons. All the emissions are collected by detectors and processed to characterize the material.

Specifically, backscattered electrons provide topographic information, including surface asperities, grain boundaries, cracks and pores. For composite materials, fibers and whiskers pulled out of the fracture surface can be observed with superior depth of field compared to optical microscopy.

Secondary electrons provide atomic number contrast that can be used to distinguish phases that contain elements with substantially different atomic numbers. This feature can be useful in yielding a good contrast between ZrO₃ and Al₂O₃ grains in ZTA composites, for example.

Many ceramic materials undergo intergranular fracture upon failure, which is a useful feature for analyzing grain structure and size. Small fragments (up to 100 mm) produced by fracturing a ceramic material are valuable for analyzing the fracture surface. Environmental SEM makes possible the imaging of samples in their natural state without coating or sample preparation, allowing processes such as wetting, drying, mixing, crystallization, or curing to be observed.

Information stored digitally can be processed easily by computer for image analysis (i.e., to determine particle size and area), as well as to adjust contrast or eliminate undesirable features. X-ray spectroscopic analysis provides nondestructive elemental analysis to examine overall ceramic composition or individual phases.

For additional information regarding piezoceramic material analysis, contact NSL Analytical Services, 4450 Cranwood Parkway, Cleveland, OH 44128; (216) 438-5200; fax (216) 438-5050; e-mail nsl@nslanalytical.com; or visit www.nslanalytical.com.

Authors' Acknowledgment

Thanks to Joseph Fielding of Morgan Electro Ceramics, Bedford, Ohio, for his assistance in preparing this article.