variety of analytical techniques can be used to verify piezoceramic material
composition and properties, depending on the property to be measured and the
detection accuracy needed.
XRF Instruments can detect elements at the parts per million range.
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
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.
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
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
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.
arc instruments vaporize the sample in a 5000°C corona and detect elements with
a solid state detector.
Particle Size and Shape
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
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
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 firstname.lastname@example.org; or visit www.nslanalytical.com.
to Joseph Fielding of Morgan Electro Ceramics, Bedford,
Ohio, for his
assistance in preparing this article.