A. PURPOSE
The driving force for the expansion of materials science is the design and synthesis of new forms of solid-state matter. However, the synthesis of new materials is largely meaningless unless the structures and properties of those new materials are also investigated. Only by understanding the relationship between structure and properties can new materials be designed in a rational way. Thus, in this chapter we examine techniques that play a major role in materials characterization. Although some of these methods will probably be familiar to the reader because of their widespread use in small-molecule chemistry, others are more specialized and will be given greater attention. The following techniques are divided into those that give information about the interior structure of solids and those that probe the nature of the surface. The relevance of solution-based analytical methods is considered briefly at the end of this chapter.
1. Elemental Microanalysis
Despite the existence of more sophisticated techniques, elemental microanalysis to determine the ratios of different elements continues to be the first step required for the characterization of a new material. This can be accomplished by classical “wet chemistry” or combustion analytical techniques of the type that are sometimes taught in inorganic or analytical chemistry courses, or it may involve physical analyses such as X-ray photoelectron spectroscopy. Whatever the techniques employed, the primary purposes are to determine which elements are present and their ratios. Elements such as C, H, N, O, Si, P, S, and the halogens are analyzed routinely by wet chemistry and combustion methods. Some elements such as P or Si are difficult to analyze in the presence of oxygen.
2. Infrared–Raman Spectroscopy
Infrared and Raman spectroscopy (vibrational–rotational spectroscopy) provide indications of the types of bonds that are present in a solid. Thus, bonds with different lengths, strengths, bending, and torsional characteristics absorb different wavelengths of infrared radiation, and the absorption maxima may be characteristic of the types of linkages present. These absorptions generally occur within the 4000–400cm−1 region of the spectrum. Only those vibrations that result in a rhythmic change in dipole moment are detected in the infared. Thus, bonds such as O—H and N—H bonds absorb infrared radiation between 3000 and 3700 cm−1. Ionic sulfate absorbs near 1100 and near 500 cm−1, and the ammonium ion is detected from absorbances near 3100 and 1400 cm−1. Characteristic frequencies can be used to distinguish between aliphatic and aromatic C—C bonds, different types of C—O bonds, and a wide range of different inorganic structures. The actual absorbances may result from bond-stretching motions or vibration–rotation modes, both of which depend on bond strength and length.
Vibrations that involve no dipole moment change may be detected by Raman spectroscopy, which is a technique that analyzes the visible light scattered by the material and detects the wavelength shifts that occur during scattering.
A particular advantage of infrared and Raman spectroscopy for materials analysis is that the substance may be studied as a powdered solid or thin film. A powder can be mixed to a paste (a mull) in mineral oil or ground with potassium bromide and pressed into a pellet. It does not have to be soluble in a suitable solvent. Soluble, film-forming materials (particularly polymers) are cast on a sodium chloride plate by allowing the solvent to evaporate from a solution.
3. Solid-State Nuclear Magnetic Resonance Spectroscopy
Solution-state NMR is perhaps the most widely used characterization technique in small-molecule chemistry. It is employed extensively for studies of the structure of compounds that contain C, H, D, F, Si, P, and many other elements. Materials that dissolve in suitable solvents (particularly polymers) can be examined readily by this technique. However, the analysis of solids by NMR techniques is a more challenging problem. The NMR method depends on the relaxation of nuclear spin states that are induced by radiofrequences. When a small molecule is dissolved in a solvent, these spin relaxations depend on the atoms to which the target atom is attached—with different intramolecular environments affecting the relaxation process. The different environments are denoted by the “chemical shifts.” Thus, the environments of hydrogen or carbon atoms in an organic compound will be recognizable from the chemical shift.
However, the atoms in solid materials are connected to a more extended framework of neighbors, and the nuclear spin relaxations can become coupled to those of a large number of other atoms. This leads to NMR line broadening, a phenomenon that disguises many of the near-neighbor interactions that are of interest to the scientist. The magic-angle spinning technique is used to help resolve some of these problems. Two situations are commonly encountered. In the first, if the material has a random network amorphous structure, there may be so many different local magnetic field environments around an atom, that the lines remain broad, though not so broad as those without magic angle spinning. Moreover, if the atoms of interest are part of a regular crystalline structure, in which the nuclei are surrounded by a uniform regular environment of other atoms, then the resolution and narrowness of the lines can be impressive. Solid-state NMR analysis is now feasible for a wide range of nuclei, including H, D, C, Si, B, N, P, F, Li, Na, Cs, Al, and V. Oxides can also be studied in special cases.
4. Thermal Analysis
a. Differential Scanning Calorimetry (DSC).
DSC analysis is a method that detects thermal transitions in a material. It is particularly effective for the identification of glass transition temperatures (Tg), crystalline melting temperatures (Tm), or liquid crystalline transitions (Tlc). If the material decomposes as the temperature is raised, DSC analysis can also identify the temperature at which this process begins (Td) and the various stages in the thermal decomposition. The technique depends on a measurement of the heat evolved or absorbed when a material undergoes a phase transition. The apparatus is shown schematically in Figure 4.1.
The sample to be examined is placed in one of the two small pans. The second pan is either empty or contains a control material that has no transitions in the temperature range of interest. Both samples are then heated by individual electric heaters, with the temperature being raised typically at a rate of 10°C per minute. The current flow to the heaters is monitored continuously. When an endothermic transition occurs in the test material, more electrical current will be needed to maintain the temperature rise, and the increased current is recorded and plotted on a graph. An exothermic transition will require less background heat and current, and this, too, will be recorded and translated onto the graph. Glass transitions are exothermic. Conversely, the melting of a crystalline material is endothermic because energy must be supplied to separate the component molecules. Figure 4.2 illustrates the profile generated by a differential scanning calorimetry (DSC) experiment. Decomposition temperatures may be exothermic or endothermic depending on the chemistry of decomposition.
Figure 4.2. Top: DSC scans showing idealized profiles for glass transitions and melting temperatures. Bottom: Experimental DSC curves for (a) an amorphous polymer and (b) a polymer with both a glass transition and a melting transition.
A related technique is differential thermal analysis (DTA), which uses similar principles except that, instead of current flow, the instrument measures the difference in temperature between the two samples as the temperature is raised. DTA gives information similar to that from DSC, but is perhaps less sensitive to hard-to-detect transitions.
b. Thermogravimetric Analysis (TGA).
Thermogravimetric analysis measures the weight lost by a sample as the temperature is raised. Decomposition usually results in the formation of volatile small molecules; hence the onset of decomposition is detected by the onset of weight loss. TGA is commonly used to assess the thermal stability of polymers and the progress of reactions that turn preceramic materials into nonoxide ceramics.
The apparatus is quite simple in principle. A small sample pan is weighed continuously on a microbalance inside an oven. The temperature is raised automatically according to a predetermined program (typically 10°C per minute), and the weight of the sample is monitored continuously (Figure 4.3). A plot of weight versus temperature is then obtained.
Figure 4.3. Schematic diagram of thermogravimetric analysis (TGA) apparatus. The weight of the sample is measured as the temperature of the sample is raised. The upper gas outlet allows volatile decomposition products to be collected and analyzed.
The dramatic weight loss shown in the curve in Figure 4.4a is caused by the polymer depolymerizing catastrophically to the monomer, which volatilizes from the system. The curve in Figure 4.4b illustrates the less precipitous weight decline that occurs when a preceramic polymer is heated to convert it to a ceramic.
TGA results yield values such as the initial decomposition temperature (Td) or the temperature at which 10% or 50% of the weight has been lost (T10 or T50). In the case of a preceramic pyrolysis, an important result is the percentage of residue material left after heating is complete (the “char yield”). However, if the decomposition products have a sufficiently high molecular weight that they are nonvolatile, TGA experiments may seriously over-estimate the thermal stability of a material.
c. Thermomechanical Analysis (TMA).
The evaluation of a structural material from an engineering viewpoint requires that its mechanical properties be studied as a function of temperature. This is accomplished in several different ways. First, measurements of the coefficient of thermal expansion are important for a variety of applications, and specialized equipment is available to simplify these studies. Also, the hardness or softness of the material as the temperature is raised or lowered can be assessed by an indentation technique. A probe (vaguely resembling the ones used to play old phonograph records) is placed under a load, and its penetration into the surface of the sample is measured as the temperature is raised. At the Tg the material changes from a glass to a more flexible material that is indented more easily. This provides a measurement of the Tg.
Figure 4.4. Weight loss profiles for (a) a material that decomposes or depolymerizes catastrophically at one specific temperature and (b) a substance that decomposes progressively as the temperature is raised. Note that profile (a) may also be characteristic of a material that decomposes or depolymerizes at a temperature below the weight falloff, but the decomposition products do not volatilize until a higher temperature is reached.
Another device for thermomechanical analysis uses disks that rotate and vibrate to measure the flexibility of a material at different temperatures. Here again, glass transition temperatures may be detected. Elastic modulus and compliance are other properties that are measured. Variable-temperature stress–strain analysis is also used to assess engineering properties, as described in the following section.
5. Stress–Strain and Impact Analysis
The stress–strain behavior of a material can be measured by means of an Instron machine. The sample, in the shape of a “dogbone” has its ends clamped between the jaws of a machine that attempts to tear the material apart (Figure 4.5).
Brittle materials do not elongate when stress is applied, but break suddenly (Figure 4.6). Viscoelastic materials stretch under the same conditions and eventually undergo viscous flow. If these experiments can be carried out with the samples at different temperatures, the investigator can assess how the engineering properties change with temperature.
Impact resistance is measured by allowing a pendulum weight to swing down to hit a sheet or block of notched material to measure the force required to induce fracture. The initial height of the weight above the sample can be varied to change the force of impact.
Figure 4.5. “Dogbone”—type evaluation of a material in an Instron machine. Two jaws connected to a robust mechanic device attempt to stretch the material. Depending on its physical properties, the material may elongate before failing, or it may snap in a brittle fracture.
Figure 4.6. Different profiles in the stress–strain curves are characteristic of various materials. These curves are a vital component in decisions about the engineering uses of materials.
Engineering-type testing also involves measurements of compression strength and torsional stress and strain.
6. X-Ray Diffraction
a. Powder X-Ray Diffraction.
Powder X-ray diffraction is a characterization technique that is widely used in inorganic materials science. The powdered material is packed into a borate glass capilliary tube and a narrow beam of X rays is passed through it. The main X-ray beam passes straight through the sample, but diffracted beams emerge as widening cones of diffraction around the path of the main beam. The theory of X-ray diffraction is beyond the scope of this text, but, in summary, crystalline substances are characterized by layers of atoms or molecules arrayed on a three-dimensional lattice. A large number of different planes can be drawn through the lattice, each set of planes having a different spacing from its neighbors. Diffraction occurs as the incident X-ray beam is “reflected” (actually diffracted) from all of these layer planes. Because the crystallites in the powder are randomly oriented, all possible planes will be sampled by the beam. Hence, the pattern emerging from the crystal will be a series of cones of X rays around the emergent main beam. The number of cones will in principle equal the number of different layer spacings in the sample. The diffraction pattern is sampled in one of three ways: (1) a flat piece of photographic film may be placed in the path of the emergent rays, and the film developed reveals the circles that represent the intersection of the film with the diffraction cones (Figure 4.7); (2) an electronic area detector may replace the piece of film, to provide a quicker means for recording the data; or (3) a narrow strip of photographic film may be curved into a cylinder that encircles the sample. The advantage of this arrangement is that it removes distortions that are inherent in the use of a flat detector.
The next step is to use the recorded data to work backward to calculate the diffraction angles θ in the crystals that gave rise to the recorded pattern, and from those angles to determine the layer spacings d that were responsible for the diffraction. This is accomplished with use of the Bragg equation, nλ = 2d sin θ, where λ is the radiation wavelength. Thus, the dataset consists of a set of layer spacings, and the investigator then attempts to match that set to the values for known materials. A very large compilation of these powder datasets is available. Although powder diffraction data can occasionally be used to determine new molecular and materials structures, their main use is to identify substances in terms of previously studied materials.
Figure 4.7. Powder X-ray diffraction involves passing a thin beam of monochromatic X rays through a powdered crystalline sample contained in a capilliary tube. Each set of identical layer spacings gives rise to one diffraction angle. Because the crystals are randomly oriented, all angles of incidence and diffraction are sampled, generating a cone of X rays. Concentric cones (from different layer spacings) intersect a piece of photographic film or an electronic detector and are recorded as concentric circles. Measurement of the diffraction angles from the recorded image and the distance from the sample yields the layer spacings from the Bragg equation, nλ = 2d sin θ.
Figure 4.8. Fiber X-ray diffraction uses a sample stretching device (a) to hold the fiber in the X-ray beam. A typical polymer fiber X-ray pattern is shown in (b). Each layer of short arcs gives information about the repeating distance along the polymer chain, while individual arcs on a particular layer give clues to the separation between adjacent parallel chains and the groups attached to those chains.
A variation of the powder diffraction technique is used for polymers. If a polymer fiber is stretched so that the chains are aligned along the fiber axis, the fiber can replace the capilliary tube of powdered crystals described above (Figure 4.8a). The diffraction pattern from an oriented crystalline fiber will then consist of concentric cones, but cones that coalesce into spots rather than arcs as the material is stretched. The pattern can be used to calculate the repeating distance along the polymer chains, the chain conformation, and the separation between chains. However, the resolution of the structural features is hardly ever as good as can be obtained for small molecules in crystals studied by single-crystal methods.
Figure 4.9. A single-crystal X-ray diffractometer allows a narrow beam of monochromatic X rays to strike a crystal that is mounted on a device (a goniometer head) that allows it to be oriented in any direction. As the crystal is moved, diffracted X rays flash out as each set of planes within the crystal come into the diffracting position (determined by the Bragg angle). The intensity of each diffracted beam depends on destructive interference between interlayer features. Both the diffraction angle and the intensity of each reflection are captured by an array detector (a flat-panel electronic device) that transmits the data to a computer where the overall crystal structure and the location of individual atoms in the structure are calculated.
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