Ceramic Technology and Processing, King
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that is similar to DTA, thermal conductivity, specific heat, thermal expansion, and loss on ignition (LOI).
Thermal Gravimetric (TGA)/Differential Thermal (DTA)
TGA measures the change in weight as the sample is heated in an atmosphere of choice. Equipment is capable of reaching above 1500 °C and usually has an air, nitrogen, or an argon atmosphere. TGA is useful in studies such as binder burnout, desorption, dehydration, and thermal decomposition. DTA measures the temperature difference between a sample and an inert material as they are heated. This is useful for locating the temperature where reactions occur such as for melting, phase changes, and oxidation. Instruments can measure both the change in weight and the change in temperature. Figure 11.8 shows one kind of TGA/DTA apparatus.
Figure 11.8: TGA/DTA Apparatus. TGA measures weight loss on heating. DTA measures temperature changes relative to an inert standard, as the sample is heated. (Courtesy of TA Instruments)
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Thermal conductivity
Measuring thermal conductivity requires both thermal diffusivity and specific heat. Laser pulse method easily measures diffusivity while calorimetry determines specific heat. An instrument used to measure thermal conductivity by the laser pulse method is shown in Figure 11.9.
Figure 11.9: Thermal Conductivity Instrument. Laser flash measures thermal diffusivity. (Courtesy of Holometrics)
The measurement is made by pulsing a known amount of energy from a laser onto the sample and then measuring the temperature rise. There is a complicated ASTM method for refractories. Barring the need to make many thermal conductivity measurements, it is prudent to farm the work out
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to a commercial laboratory. Because anisotropic materials often align in the microstructure, they result in different direction-dependent thermal conductivities. For example, thermal conductivity in graphite is much greater across the platelet than along the c-axis. When the platelets are aligned parallel to the refractory wall, the refractory is insulating. When the platelets are aligned perpendicular to the refractory wall, the refractory is thermally conductive. Either can be done, depending on the forming method.
Sometimes, thermal conductivity determines the material of choice for a given application. For example, MgO has a high thermal conductivity and is used as electrical insulation in Calrod heaters. Zirconia has a low thermal conductivity and is used for thermal insulation at high temperatures. Also, materials with a high conductivity are better in thermal shock than insulators.
Porosity has a pronounced effect on conductivity, which is one reason why fibers are used for kiln insulation. Closed, small bubble foams are very good insulators, but are not structurally stable at high temperatures since they shrink and warp. Figure 11.10 presents some data sketched from the reference for alumina in dense, foamed, and fibrous forms.1
Figure 11.10: Thermal Conductivity Alumina. Thermal conductivity is highly dependent on form of the structure.
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Specific Heat
Generally, specific heat is measured when thermal conductivity is determined. In applications, specific heat contributes towards how quickly a material warms and cools. Specific heat is measured with a calorimeter.
Thermal Expansion
Thermal expansion is measured with a dilatometer where a sample of determined length (commonly 1") is heated and the change in length measured. Thermal expansion can also be measured with a high temperature X-ray diffractometer where the change in lattice spacing along different lattice planes is measured. Ceramics show a wide range of thermal expansions, over better than a decade. Alkaline earth oxides have high expansion coefficients. The coefficient is the expansion % over one degree of temperature rise. Fused silica glass has a very low coefficient that explains why it is used in dilatometers as the sample holder and push rod. Single crystal sapphire is often used for higher temperatures.
Thermal expansion of zirconia was previously discussed and varies depending on the amount of stabilizer. The transformation of the monoclinic phase to tetragonal is a contraction and reduces expansion as its concentration increases in the cubic matrix.
Figure 11.11 is of a dilatometer that can be routinely used for thermal expansion measurements.
Loss on Ignition (LOI)
LOI is commonly reported by chemists as part of an analysis. It is simply the amount of weight loss after the sample is heated to a red heat. The temperature is sufficient to dehydrate and burn off organics.
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Figure 11.11: Dilatometer. Measures thermal expansion and glass transition temperature for glasses. (Courtesy Theta Instruments)
Check List, Thermal Analysis
• TGA
Weight loss on heating Atmosphere Binder burnout Desorption Decomposition
• DTA
Phase changes Decomposition
High temperature reactions
• Thermal Conductivity
Farm measurements out.
Materials can have different conductivities in different directions
Some materials are selected for their conductivity Porosity reduces conductivity
Fibers are useful as thermal insulation
• Specific Heat
Not usually measured
Need for thermal conductivity measurements
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• Thermal Expansion
Usually measured with a dilatometer
Low expansion improves resistance to thermal shock
• LOI
Routine with chemical analysis Often part of materials specifications
Particle Size
Measurement of particle size distribution is key for characterizing a ceramic powder. This data is often presented as a plot of % Finer Than, with the logarithm of Particle Diameter. A typical plot is shown in Figure 11.12.
Figure 11.12: Particle Size Distribution Plot. Data is usually presented in this form.
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When the curve is steep, the distribution is tight. When the curve is shallow, the distribution is broad. These results are often summarized by three values: d10,d50,and d90. The two end values are sometimes instead selected as d20 and d80. These are marked on the preceding figure, where d10 indicates that 10% of the material is below the size indicated, which in this case is about 0.38 μm.
Light Diffraction/Dynamic Light Scattering (DLS)
Coarse particles are best measured by screening, as previously described. Fine particles are usually measured by light diffraction or DLS. Other techniques are based on settling either with or without a centrifuge. Doppler shift measures very fine (to 0.003 μm) particles due to Brownian motion. A number of good general purpose instruments exist, one of which is shown in Figure 11.13.
Figure 11.13: Particle Size Analyzer. Several types of instruments are available. This one uses light diffraction. (Courtesy of Horiba)
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Analysis is performed by taking an aliquot of the material, adding surfactant, and dispersing ultrasonically with stirring. A magnetic stirrer is used, with a top surface having an angle that lifts the suspension vertically. The sample, in a beaker on a lab jack, is encased in a soundproof chamber. The sample is sonified at a set amplitude for a specified period of time, determined experimentally. After sonification, a few drops of the suspension are removed using a pipette. Using throw-away polyethylene pipettes is convenient. Drops are added to the instruments reservoir until the desired concentration is attained, as described in the operating instructions. Measurements are then made and printed out.
While flat-ended horns are best, they abrade and must be remachined. Only a minimum of material should be remachined as machining renders the horn out-of-tune. Since the metal is titanium and difficult to machine, cutting is best done using a sharp-cemented carbide cutter at a moderate cutting speed with coolant.
When engaged in process research, there exists the need for realtime data. Measurements might be needed in a few minutes rather than the few days that is typical of an analytical lab. Such time constraints necessitate an on-site instrument.
Since dust fogs up the optics, install a blower and air filter on the instrument to keep the optics clean. Ensure that the air flow is from the lab through the filter into the instrument, and not the other way around.
Microscopic Particle Size Measurements
An alternative to light diffraction is microscopy. Colloidal-sized particles are better visualized with a TEM than by light diffraction. Additionally, particle shape can be determined. Light diffraction simply provides an "Equivalent Particle Diameter." The principle disadvantages are that microscopy is much slower and more laborious. However, microscopy is direct and precise. Microscopy should be intermittently used to check for foreign contaminants and particle shape. SEM is a good tool here as is optical microscopy when the particle size is larger than a few micrometers.
Preparing a sample of powder for SEM is tricky because particles agglomerate. Settling and drying onto the stub of a low molecular weight
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organic, such as methanol, helps to retard agglomeration. The suspension should be diluted in order for individual particles to be visible. Good and poor dispersions are illustrated in an SEM photo of alumina particles. See Figure 11.14.
With poor dispersion, individual particles are not separated to the point of observation or measurement.
When using an optical microscope for larger (3 μm+) particles, prepare a slide by placing a small amount of a ceramic powder onto a glass slide, adding a drop of an index of refraction liquid, and placing a cover glass over the sample. Press down the cover glass and smear it around to disperse the powder using a pencil eraser. When there is a wide variety of particle sizes in the sample, the material can segregate with coarse particles moving toward the edges. Watch for this. Excess liquid produces a mess. Excess powder renders the slide opaque, making visualization of particles difficult. With water's low viscosity, micrometer-sized particles will jump around by Brownian motion. Using a more viscous oil eliminates this problem. Sets of index of refraction liquids are available for immersion of powdered samples. View the powder with plane or polarized light at the appropriate magnification, examining for size, shape, color, transparency, contrast with the index liquid, and birefringence.
Most ceramic systems have only a few phases present. They are more easily characterized by SEM compositional analysis, particle size analysis instruments, and X-ray diffraction structural analysis. Ceramic microscopy of powders is thereby simplified. A quick glance at a slide is by far the easiest way to obtain an impression of the size, shape, color, transparency, contrast, and birefringence of the ceramic powder.
Contrast for colorless ceramics is determined by the difference between the index of refraction of the ceramic and the index liquid in which it is immersed. Unlike the story of the jewel thief who hides diamonds by dropping them into a glass of water, the ceramic particles become invisible in plane light when the two indices are identical. The jewel thief is doing time since the index of water is 1.333 while the index of diamond is 2.417 a difference of 1.084. The mineral cryolite has an index of 1.338, close to that of water, and can be concealed in a glass of water, though such information might prove worthless to a precious-jewel-seeking thief.
Figure 11.15 shows alumina in two different index of refraction
liquids.
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Figure 11.14: Quality of Powder Dispersions. The two figures show a good
and poor dispersion of alumina powder. SEM Photographs. Scale bar 1.0 μm.