A good understanding of material structure and composition is essential to any industry that uses a wide range of materials in a variety of applications, such as the ceramic industry. In conjunction with the physical testing of material properties, obtaining detailed and reliable information on structural parameters and composition can be of extreme importance during materials development and applications testing.
The many analytical instruments available today for obtaining information about materials properties and characteristics are as diverse as the materials used and the applications for ceramics themselves. The latest advances in instrumentation allow the observation and measurement of smaller features, achieve better detection limits and determine more accurate chemical and structural parameters than ever before.
The use of advanced ceramic materials has increased dramatically during the past few decades. For example, ceramic-based materials have become widely accepted for use in medical applications, particularly for orthopedic and dental implants. These bioceramics represent a diverse class of materials divided into the following three major categories: bio-inert ceramics such as alumina- and zirconia-based high-strength ceramics; bioactive glass or glass/ceramic composites that can form direct chemical bonds with living tissue; and ceramics that mimic natural bone, such as calcium phosphate-based ceramics.
Although most synthetic bioceramics offer physical and/or chemical properties that meet or even exceed their natural counterparts, they can result in a number of adverse physiological reactions. Thus, the biocompatibility and mechanical strength of implant materials is becoming an important focus area. Examples include the development of composite materials and the use of specialty coated base materials.
Figure 1. Bubble chart illustrates analytical dimension (X-axis) against detection range (Y-axis).
Choosing the Right Technique
The right analytical tool must be used for the cost-efficient problem solving and characterization of ceramic materials, (i.e., one that has the correct measurement capabilities and characteristics to address the problem at hand). Measurement-related factors include depth of analysis, detection range and analytical spot size. The type of information desired (or available from a given measurement) should also be considered. Information of interest can include bulk elemental composition, the presence of trace contaminants or dopants, chemical bonding information, crystallographic information (e.g., grain size and orientation), and roughness.
Figure 2. Depth of analysis chart.
Figure 1 lists the detection range (Y-axis) and analytical spot size (X-axis) for a range of techniques. This method of presentation provides an effective method for visualizing many techniques that sometimes have overlapping characteristics. Techniques with blue bubbles provide elemental information while those in red can also provide chemical bonding or even molecular information (if of interest). Techniques above the chart are imaging-only techniques and provide no compositional information; only visual information is provided, in some cases down to the angstrom scale. Techniques to the right of the chart are bulk-only techniques and do not provide any spatially resolved data, only composition information. In some cases, the detection limits are in the parts-per-billion (ppb) to parts-per-trillion (ppt) range.
Figure 2 shows the typical depths of analysis for various analytical techniques. The depths of analysis range from only the top few atomic layers to many micrometers into the sample. Matching the problem or characterization need with a technique that has an appropriate depth of analysis is critical to providing results that are relevant, useful and meaningful. If a tool with a large (i.e., deep) analysis depth is used to characterize a very thin film or contaminant, the chances are very low that the contaminant or thin film will actually be identified, correctly characterized or even detected. Similarly, trying to characterize the bulk of a material with a surface sensitive technique may be misleading, as outer surfaces are often substantially different than the inner bulk of a material due to segregation, contamination and other processes.
Figure 3. SEM images of various ceramic nanoparticles. (3a)
Various types of analyses can be of importance to ceramic materials developers and users, and can be organized as follows:
- Visualizing a sample (e.g., microscopy)
- Determining the composition (major elements) of a sample
- Determining trace elements (< 1%) present in a sample
- Determining the crystallographic properties of a sample
- Determining the thermal stability of a sample (e.g., weight loss or phase changes)
- Investigating the surface of a sample or a sample that has a thin film on its surface
Several of these categories include multiple techniques, such as optical microscopy or electron microscopy. Even within electron microscopy, the user might have a choice between scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
The following sections discuss some of the strengths and weaknesses of different approaches. (Note that the materials used in these examples are for illustrative purposes only and that the techniques discussed can typically be applied to a wide range of ceramic materials.)
Microscopy techniques provide high- magnification images of samples to observe topography and features of interest. Microscopy can also be performed on cross-sectioned samples to image buried features, layers, interfaces and crystalline structures.
The previously mentioned SEM and TEM are essential for applications where resolutions greater than those provided by optical microscopy are required. SEM is one of the most popular analytical tools because of its ability to provide high- resolution images with excellent depth of field. Figure 3 shows SEM images of various ceramic nanoparticles at different magnifications. Detailed information about the surface morphology and particle size can also be obtained.
For additional information about a sample with even greater magnification, TEM and scanning TEM (STEM) are options. In this case, samples must be thin enough that electrons can pass through them (typically < 150 nm). Thus, the analysis is restricted to small particles unless thin slices of the sample can be prepared, either through physical polishing or by cutting the sample with a focused ion beam (FIB). Figure 4 shows TEM images of ceramic nanoparticles obtained without sample preparation. In this case, the different crystallites within each particle can be seen due to their different orientation relative to the transmitted electron beam passing through the sample.
Figure 4. TEM images of various ceramic nanoparticles. (4a)
A range of analytical options is available for both SEM and TEM. The most common is energy dispersive X-ray spectroscopy (EDS), which permits qualitative or semi-quantitative elemental analysis from micron or sub-micron features. Available on TEM or STEM only, electron energy loss spectroscopy (EELS) can be used for compositional information, especially for lighter elements.
Quantitative Elemental Survey Analysis of Bulk Materials
Beyond particle size and morphology, another important aspect of materials analysis is elemental composition and the potential presence of trace elemental contaminants. An accurate survey analysis of all elements present may be a requirement to identify all of the constituents present, from matrix elements to trace levels. Unwanted trace level impurities can be introduced into ceramics from various sources, such as raw materials, reactors, catalysts, transferring pipelines, molding components and other equipment used in manufacturing. Techniques available for multi-element analyses with trace level sensitivities include inductively coupled plasma optical emission spectroscopy (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and glow discharge mass spectrometry (GDMS).
GDMS is one of the most sensitive analytical methods available for the quantitative trace elemental analysis of solid materials. It combines the stability of a glow-discharge atomization/ionization source with the high sensitivity and specificity of an analytical mass spectrometer. GDMS can detect and quantify the trace elemental contents in the broadest range of solid materials (e.g., from conductive metals to ceramic insulators and from large pieces to powders). It is a direct solid sampling technique that eliminates the need for dissolution of the sample prior to analysis.
Figure 5. TGA (left axis) and DTA (right axis) of bulk and nanoparticle calcium phosphate. 10°C/min, nitrogen atmosphere.
Due to the simple calibration requirements and matrix insensitivity, one of the unique advantages of GDMS is that it can analyze multi-compositional and multi-layered samples directly. In addition to quantitative bulk measurements, it can provide both layer and substrate information over broad concentration ranges in coatings or composites. Detection limits for GDMS are in the ultra-trace range for most elements. Table 1 illustrates the use of GDMS to analyze three different ceramic samples. In all cases, the achievable detection limits are in the sub-ppm (by weight) range.
Figure 6. XPS survey scan of nano-hydroxyapatite particles (left). High-resolution XPS scan (right) of carbon spectrum showing the presence of calcium carbonate functionality.
Because of the increased use of bioceramic material in surgical implants, a standardized test method has been developed by ASTM International (ASTM F 1581-08E1, "Standard Specification for Composition of Inorganic Bone for Surgical Implant"). This method covers the material requirements for the chemical composition of bone (apatite) ceramics intended for surgical implants. Limits are prescribed for: arsenic, cadmium, mercury, and lead and other heavy metals, which can be determined by ICP-MS or graphite furnace atomic absorption spectrometry (GF-AAS).
Another aspect of this test method is the determination of the Ca:P ratio using ICP-OES. Pure hydroxyapatite has a Ca:P ratio of 1.67. Table 2 illustrates the compositional data of major elements (P, Ca, O) and trace elemental impurities of As, Cd, Hg and Pb for several raw materials, as determined by ICP-MS and instrumental gas analysis (IGA) for light elements (carbon and oxygen, in this case).
X-ray diffraction (XRD) is a powerful nondestructive technique for characterizing crystalline, polycrystalline and microcrystalline materials. It provides information on crystalline structure and phase; preferred crystal orientation (also known as texture); and other structural parameters such as average grain size, degree of crystallinity, strain, and crystal defect density.
XRD peaks are produced by the constructive interference of a monochromatic beam of X-rays scattered at specific angles from the lattice planes in a sample. XRD is an extremely valuable tool for the analysis of ceramics, providing basic phase identification to more complex analyses such as determining percent crystallinity and crystallite size, and monitoring phase change processes.Thermal Analysis
Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are essential tools to examine the thermal behavior and stability of ceramic materials. These techniques find particular application in the field of nanomaterials, where the properties of nanoscale materials are not necessarily the same as those of bulk materials.
Hydroxyapatite is often applied as a coating on an inert metallic implant such as Ti-6Al-4V. A typical deposition method is to use thermal spraying to coat the amorphous hydroxyapatite nanopowder precursors onto the metallic implant. TGA/DTA is vital in investigating the morphological stability of the precursors and in helping to develop the heat treatment protocols for forming hydroxyapatite coatings of the required density or porosity.
Figure 5 shows the TGA (left axis) and DTA (right axis) of bulk and nanostructured calcium phosphate samples. Nanopowder calcium phosphate exhibits a slight weight loss (~ 1.6 wt%) up to 800øC, likely due to surface desorption. The DTA data of this nanopowder form sample reveals an intense crystallization peak around 680°C, compared to the bulk material.
The modern use of ceramics and nanomaterials may involve the construction of layered laminated materials or the creation of a coating. In both of these scenarios, the surface chemistry of the particle or material may play a considerable role in its behavior. In order to address this possibility, tools must be used that have a shallow information depth (see the left-hand side of Figure 2).
Two tools that fit this requirement well and are able to analyze ceramic materials include time of flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). These techniques have information depths within the 10-100A range, meaning that they are not suitable for bulk analysis without some means of penetrating deeper into the sample (such as with an ion beam), but they do give highly specific surface information. TOF-SIMS generally provides excellent sensitivity for species adsorbed on surfaces and can obtain elemental, organic and some inorganic information. XPS has the ability to determine the chemical states of the various elements present on the surface. These characteristics make these techniques excellent choices for the analysis of residues on a surface, such as on ceramic circuit boards.
In Table 2, it can be seen that the hydroxyapatite nanopowder has a substantial carbon content (7500 ppm by weight), as determined by IGA. IGA is a bulk analysis technique that provides the total carbon concentration only. It does not provide any chemical information or depth-specific information. In order to obtain additional information about the carbon, XPS analysis was used, as shown in Figure 6. The XPS survey spectrum shows the presence of the expected O, Ca and P, along with almost 12 atomic% carbon at the surface (i.e., much higher than the bulk amount measured by IGA).
Some hydrocarbon contamination would be expected due to atmospheric exposure; however the high-resolution XPS carbon spectrum shows that a significant percentage of the carbon was present as a carbonate within the upper 100A of the sample. This observation also explains the higher-than-expected Ca:P ratio observed on this sample (1.86 compared to 1.67, see Table 2).
These results strongly suggest that a thin CaCO3
surface layer was covering the hydroxyapatite. Small amounts of carbonate are sometimes substituted in hydroxyapatite to make them more "bone like," but this particular sample was designed to be pure hydroxyapatite with no calcium carbonate. From a biocompatibility point of view, the bone cells exposed to this sample would see CaCO3
only and not hydroxyapatite. Only a surface-sensitive technique such as XPS would be able to detect such a thin surface layer and characterize it.
While it is important to understand the physical and mechanical properties of ceramic materials used in all types of demanding environments, only a full understanding of the surface and bulk compositions of those materials allows one to understand why they may behave in a particular manner in a given test or environment. A full understanding of surface and bulk composition also enables better prospects for modifying a material to achieve even better performance.
The examples presented in this article only partially demonstrate the current breadth and depth of modern techniques available for ceramic characterization. Increasing our knowledge and understanding of new and previously established materials can help improve performance and enhance the product development cycle. For additional information, contact Evans Analytical Group at 810 Kifer Rd., Sunnyvale, CA 94086; (408) 530-3500; fax (408) 530-3501; e-mail firstname.lastname@example.org; or visit www.eaglabs.com.