Ceramic Technology and Processing, King

404 Ceramic Technology and Processing

The picture on the bottom is in a liquid of 1.610 refractive index. Alumina has an index of about 1.76. Due to this large difference, the particles are readily visible. The picture on the top is in an index of 1.76. Although the concentration of particles is about the same, they are almost invisible. Therefore, a ruby in a glass of this index liquid has virtually no contrast. However, the ruby is red and the jewel thief finds himself once again in the slammer. Color is another way to discern the differences between phases.

It is very easy to take a quick look at a ceramic powder on a slide and discern between phases by index, shape, size, or color. With two phases present, when the mixture in an index liquid matches one phase, the other is visible by itself due to contrast. The technique has ppm sensitivity and is easy to do.

Astrophysicists are engaged in a conspiracy against petrographers with their claim that the speed of light is a universal constant, in all directions and regardless of the motion of the light source. What they choose to hide is that the speed of light depends upon the index of refraction of the medium through which it is passing. High index equals low speed. And, the speed of light is not the same in all directions through anisotropic crystals. Those include the multitude of all crystals except cubic. Differences in light speed in the crystal throw the light out-of-phase in polarized light with interference. This is called birefringence and is visible in a powder or thin section when the polaroids are crossed. Figure 11.16 is of two powders where one has a much higher birefringence than the other. This causes it to appear much brighter in the field of view.

The bright particles are zircon where the index in one crystal direction is around 1.923 and in the other 1.968, with a difference of 0.045. The other phase is alpha alumina (corundum) where the indices are 1.767 and 1.759, with a difference of 0.008. Zircon is said to have a high birefringence while alumina is said to have a low birefringence. Zircon is easily spotted in the slide because it is much brighter in polarized light than alumina. The test between two materials of different birefringence is very sensitive, and can easily be in the ppm range just with a glance.

When the index of refraction of the liquid is close to that of the powder, the particles disappear unless they are colored or contain porosity. That is a good way to determine the index of the ceramic, which is one of its diagnostic criteria. Alumina and mullite have an index of refraction of about

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1.74 to 1.76. For good visualization, the index of the liquid should be about 1.6 to provide some contrast. When the indices are far apart, the contrast will be so high as to render the particles almost opaque. It is advisable to use a liquid with an index closer to that of the ceramic. Always use a cover glass to prevent damage to the objective lens.

Figure 11.16: Contrast by Birefringence. When two or more materials are present, the birefringence can distinguish between them. Transmitted light slide. Scale bar 100 μm.

Check List, Particle Size

•Characteristics of the distribution curve

•Light diffraction

Ultrasonic dispersion

Stirring

Prevalent technique

Fast

406 Ceramic Technology and Processing

• SEM

Particle shape Foreign materials Stub preparation

Magnification, 20,000X ± max. Particle count labor intensive

• TEM

Great detail Labor intensive 200,000X ± max.

Particle count labor intensive

• Optical Microscope Slide preparation Particle shape Optical properties

Particle count can be labor intensive 1000X, max.

Density Measurements

The density or specific gravity of a sample is an essential measurement. The two cases of permeable and impervious materials are discussed below.

Permeable Materials

Measurement can be performed in two ways. The surface can be sealed and the specific gravity determined. Or, the sample can be saturated with water with the bulk density and apparent porosity measured.

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Density Measurement. For small samples, the balance is a standard analytical balance with a way of connecting a sample holder to the weighing mechanism through the bottom of the balance. The sample holder screws into a connection to the balance pan. The balance is supported on a stand that has a transparent breeze shield and a door. The lab jack is used to raise and lower the sample holder, which, in this case, is a spiral of stainless wire basket with a connector for attachment. The procedure is as follows.

•Attach the empty sample holder to the balance and submerge it in a beaker of water containing a drop of a wetting agent up to the mark on the connecting rod and tare the balance.

•Disconnect the sample holder from the balance pan.

•Then quickly place the sample on the submerging sample holder and place it in a beaker of water containing a drop of a wetting agent.

•Tip or swish it to remove bubbles.

•Raise the beaker with the labjack and connect the sample holder to the balance keeping the sample submerged.

•Raise the beaker again to where the surface of the water is at the mark on the connecting rod.

•Now weigh the submerged sample.

The following equations are used for the calculations.

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Impervious Materials

Since there is no need for saturating the sample, those steps can all be eliminated. When unsure, the saturation procedure provides the answer when calculations are made.

The procedure is essentially the same, expect the infiltration steps are avoided. As these samples are often small, the procedure has some additional steps to impart more precision.

•Handle the sample with tweezers after it has been cleaned.

•Wait until the weight swing settles down before recording it.

•Measure the temperature of the water and make a correction for the water density. Handbooks on chemistry have these tables.

•Specific gravity is simply a / (a-c).

Mercury densitometers are available. Mercury is poisonous and must be handled with great care. The poison is cumulative over many years and has devastating effects on brain function. For these reasons, I do not recommend a mercury densitometer but instead advocate finding alternatives. For example, one can machine the part into a geometrical shape and weigh it.

Check List, Specific Gravity

•Permeable Materials Paraffin procedure Saturation apparatus Saturation procedure

Use of vacuum/pressure Measurement procedure Calculations.

•Impervious Materials

Saturation is not necessary.

Measure the water temperature and correct for its specific gravity.

• Measuring Procedure

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3.0 SLIP PROPERTIES

Some slip properties were discussed in Chapter IV on slip preparation. This section provides additional detail on viscosity measurements. Three types of viscometers are described.

Strain Rate Controlled Viscometers

This type of viscometer usually has a rotating element submerged in the slip where the speed of rotation is controlled and the viscosity or stress is measured. Viscometers measure viscosity. Rheometers measure strain rate and shear stress where a stress/strain curve can be obtained. The principle difference is in the software and peripherals. Figure 11.19 is a picture of a common viscometer.

The instrument is digital, with controls for selecting shear rate and rotor speed (which is related). The spindle type is selected to put the measurements on scale. Slip temperature is also measured.

As with most modern instruments, software, a computer, printer, and accessories are available. Accessories include a spiral adapter that can be used for pastes and thixotropic materials, a helipath stand that is even better for thixotropes, temperature controls, and means to prevent evaporation of the sample. Some data taken with an instrument is shown in Figure 11.20.

Measurements are first made by increasing the spindle rpm in increments and then decreasing it with the same settings. The data in the figure shows pronounced shear thinning where the up curve follows the relationship: