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bchl85
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My physics teacher taught me that, however, when I asked my chemistry teacher, he said NO. And now ... I'm confused. Plz anyone can help me to make me clear about this with explanation.?
PerennialII said:We'd have to be sure whether we're talking about "crystalline" materials or crystal structures, i.e. link it to bond type. Sure, if the latter the above is correct, structure itself does not turn it one way or the other in this case. Might actually be the reason for the two opposite interpretations of ductility.
PerennialII said:Thats why the quotations, not too clear I got to admit... however, material scientists, in terms of fracture mechanics in particular, many times tend to affiliate the word crystalline to materials where the description applies best, like ceramics e.g. with full covalent bonding the model example being diamond... in which case the analogy to brittleness becomes quite straightforward. When talking about structure of metallic materials I'd rather use the expression "lattice" to characterize atomic structure and also give an implication of the underlaying properties and deformation behavior.
However, if we go by your definition of "crystalline", then the question about something having a "perfect" crystal doesn't make any sense.
I agree with Gokul's answer. A "perfect crystal" here would tend to imply a material with only a single crystal orientation, rather than a polycrystal or a single crystal with major impurities and other defects. If we go by this, then it is incorrect to deduce that a perfect crystal is more brittle due to the fact that this properties is not a function of the "perfectness" of the crystal allignment, but rather the nature of the crystal structure itself. Some of the most brittle materials are amorphous which don't even have a well-defined crystal structure, like chalk.
Integral said:Having broken my share of Si wafers, I can testify that a Si cyrstal is brittle.
PerennialII said:... however, material scientists, in terms of fracture mechanics in particular, many times tend to affiliate the word crystalline to materials where the description applies best, like ceramics e.g. with full covalent bonding the model example being diamond... in which case the analogy to brittleness becomes quite straightforward. When talking about structure of metallic materials I'd rather use the expression "lattice" to characterize atomic structure and also give an implication of the underlaying properties and deformation behavior.
I am a materials scientist, and we use the word 'crystalline' to describe all materials that have a periodic arrangement of atoms, irrespective of whether they are covalent (diamond) or ionic (NaCl), ceramic (Al2O3) or metallic (Cu), semiconductor (Si) or superconductor (YBa2Cu3O7) - that just doesn't matter.
A lattice is simply a periodic array of points. When you put atoms in these points, you get a crystal (or a crystal lattice). All crystalline materials have a lattice. The arrangement of these lattice points (along with the basis) is called the lattice structure or crystal structure.
What gives you the impression that the term 'crystalline' is used only to refer to ceramics or covalently bonded materials ? I hope they don't teach that in school ?
Pereniall, I'm not sure I follow your argument. Yes, it is uncertain what the OP means by a perfect crystal. Typically, however, it means a defect-free single-crystal : no impurities, no dislocations, no grain boundaries, and infinitely large (no edge effects).
Thank you all. I'm studying solid material. And it's quite new things with me.PerennialII said:The issue was trying to answer the question on how different people infer different terms inherently different. I do agree very much with how you're using the term crystal and what a typical 100% perfect crystal were to be. In this case it just doesn't lead to any sort of information on brittleness, which to me suggests the person is thinking about something else and using a different branch of termilogy / definition.
Thank you all. I'm studying solid material. And it's quite new things with me.
In my notes, I wrote: Almost perfect crystals of material contain few dislocations to cause plastic flow. Therefore, they are strong but brittle.
That's what physics teacher taught.
No, a perfect crystal is not always brittle. The brittleness of a crystal depends on various factors, such as its chemical composition, crystal structure, and external conditions like temperature and pressure.
The brittleness of a crystal is determined by its ability to resist deformation or fracture under stress. This property is measured using techniques such as tensile testing or impact testing.
Some crystals are more brittle than others due to their crystal structure. Crystals with a simple and regular structure, such as diamond, tend to be more brittle because they have fewer defects and dislocations that can absorb energy and prevent fracture.
Yes, a perfect crystal can become brittle over time due to external factors like exposure to radiation, chemical reactions, or physical stress. These factors can introduce defects and dislocations in the crystal, making it more prone to fracture.
The brittleness of a crystal can be reduced by introducing imperfections or defects in its structure, such as doping with impurities or creating grain boundaries. These imperfections can absorb energy and prevent cracks from propagating, making the crystal less brittle. Additionally, controlling external conditions like temperature and pressure can also reduce the brittleness of a crystal.