Excessively fine particles in a powder can be a disadvantage because they
Answer: A
Surface forces dominate as size falls, so submicron powders clump together and resist wetting. The sintering advantage of fineness is only realised if the dispersion problem is solved first. Fine and badly dispersed is worse than moderately fine and well dispersed.
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Consider the following statements about particle size distributions.
1. The d50 is the median of the distribution.
2. Volume and number bases give different apparent averages.
3. A single mean value fully characterises a real powder.
Answer: B
The first two are correct. The third is wrong, since the width and the shape of the distribution affect packing and sintering as much as the average value does. Two powders of identical d50 can behave quite differently.
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Sieve analysis remains useful in traditional ceramics chiefly for
Answer: C
Correct answer: checking that no oversize material is present
A sieve reliably answers one question: whether anything larger than the aperture is present. That is exactly what matters for a body in which a single coarse grit particle can cause a defect, and it is quick, cheap and needs no calibration. What sieving does not do well is resolve the fine end, where the mesh blinds and the particles agglomerate.
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A sieve analysis retains 5 g on a 250 micrometre sieve from a 500 g sample. What percentage is oversize?
Answer: D
Five grams in 500 is one hundredth, or one per cent. Specifications for traditional ceramic bodies commonly set a limit of this order on the coarse residue. A single oversize grain can produce a visible defect in thin ware.
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Ultrasonic treatment of a suspension before sizing is applied in order to
Answer: A
Without it the instrument reports agglomerate sizes rather than particle sizes, which flatters the coarse end considerably. The duration must be controlled, since prolonged treatment can begin to fracture the particles themselves.
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Assertion (A): The reported size of a powder can change with how long it was sonicated before measurement.
Reason (R): Sonication progressively breaks up agglomerates and can eventually fracture particles.
Answer: B
Both are true and the reason gives the difficulty: there is no single correct duration, so the procedure has to be fixed and reported if two measurements are to be compared meaningfully. Sonication conditions belong in the method rather than in the operator's habit.
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State whether the following is true or false.
Sample preparation has little effect on a measured particle size distribution.
Answer: C
Correct answer: False
Preparation frequently matters more than the instrument. Agglomerates that are not broken down are measured as single large particles, so the choice of dispersant, the ultrasonic treatment and its duration all move the result - sometimes by a factor of several. This is why a size specification must state the preparation method, and why comparing figures obtained by different procedures is meaningless.
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Representative sampling of a powder for size analysis is difficult chiefly because
Answer: D
Pouring a heap sends coarse particles rolling to the outside, so a scoop from one place is not the batch. A spinning riffler or a thief sampler is used to obtain a genuinely representative portion. Sampling error frequently exceeds instrument error by a wide margin.
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State whether the following is true or false.
A scoop taken from the top of a drum gives a representative powder sample.
Answer: A
Segregation during filling and handling means the top is systematically different from the bottom. Proper sample dividers exist precisely because this intuitive approach is so unreliable. Coarse particles migrate to the outside of any poured heap.
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A laser diffraction result reports d10 of 0.8, d50 of 3.2 and d90 of 12.8 micrometres. What is the ratio of d90 to d10?
Answer: B
Correct answer: 16
\[ \frac{d_{90}}{d_{10}} = \frac{12.8}{0.8} = 16. \] The ratio is a measure of how wide the distribution is: a value near 2 or 3 is narrow, and 16 is broad. It is often preferred to a standard deviation because it uses points from the cumulative curve directly and does not assume the distribution has any particular shape.
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