4 ms·

Transport properties all "happen" at any temperature. The rate is exponential with T, though, so at some point you dismiss it. Kinetics usually (always? Who rem

Transport properties all "happen" at any temperature. The rate is exponential with T, though, so at some point you dismiss it. Kinetics usually (always? Who remembers? ) is continuous with T. Phase changes, on the other hand can happen abruptly wrt T (first order, melting/boiling ice), or gradually (second order, curie transition, boiling mixtures). Of course, first order transitions require a pure substance that doesn't actually exist since the chemical potential is infinite at infinite dilution. But that's entirely academic. EDIT: I forgot to mention that creep has many proposed mechanisms and their relative contribution is not fully understood; however most depend on a transport phenomena, typically diffusion.

WJW 3y ago
Diffusion of atoms through solid materials is one of those things I intellectually know happens, but still don't really grok. Sintering is one of the weirdest things. Why does squishing things together AND warming them up cause the particles to stick together more than when you just squish them together or just warming them up?
- imchillyb 3y ago
  > Why does squishing things together AND warming them up cause the particles to stick together more than when you just squish them together or just warming them up? >> The equations describing these laws are special cases of the ideal gas law, PV = nRT, where P is the pressure of the gas, V is its volume, n is the number of moles of the gas, T is its kelvin temperature, and R is the ideal (universal) gas constant. >> https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/relating-pressure-volume-amount-and-temperature-the-ideal-gas-law/ https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/rela... The laws of thermodynamics describe how gases work, those laws cause the effects you’re asking about. The above is a bit much for a primer on the topic, but provide enough key words for a Google-dive into the subject.
  - WJW 3y ago
    Did you miss the part where I was talking about sintering and diffusion of atoms in materials in their solid phase? A quick search in the article you linked shows zero results for either "sintering", "diffusion" or related terms. The ideal gas law is very nice but hardly applicable to describe things like creep in metals under stress, let alone things like the grain boundary diffusion mechanism for sintering metal particles together.
    - salawat 3y ago
      What is a solid but a, relatively speaking, very, very cold gas? Put two solids together with no intervening medium, and heat everything up, and you'll see some mixin' and mingling. Often, the reason you don't see more of that sort of thing is because everything is immersed in an oxygen rich fluid with a tendency to create large, relatively reluctant to mix and mingle oxide coatings before anything fun can happen. Hence why another word for cold welding is vacuum welding. Materials, when you really look at em' are not as 'solid' and 'stable' as you may think. Hence why I have a mechanics of materials book I peruse from time to time.
      - thsksbd 3y ago
        "What is a solid but a, relatively speaking, very, very cold gas?" Well there are two first degree phase changes to consider. Then there are those pesky mtls that dont even follow the general trend of gases, like water that expands when cooled.
- hansvm 3y ago
  Squishing is important, else the forces involved are negligible. Warming them up increases their movement and thus the likelihood of a number of things (including random interactions with the thing they're squished against). You might also be interested in cold welding. https://en.m.wikipedia.org/wiki/Cold_welding https://en.m.wikipedia.org/wiki/Cold_welding
- thsksbd 3y ago
  Diffusion: Its not hard to understand once you see the mechanism in full (vacancies, interstials, etc) but its hard to write as txt. Suffice to say, there's plenty of room to squeeze. "Squeezing" When you squeeze you get rid of air bubbles, right? Well, when you squeeze you get rid of boundaries which are really volumes at small scales. Well nature hates surfaces, so if you squeeze surfaces close enough together and hot enough you'll give the material an opportunity to rearrange atoms to get rid of boundaries. "Ostwald ripening" If you thought sintering and diffusion are hard, I had a respected mechanical engineering professor working on a refractory materials project claim, when he first encountered it, that Ostwald ripening cannot occur. The mtls Eng guys rolled our eyes...

WJW 3y ago

Diffusion of atoms through solid materials is one of those things I intellectually know happens, but still don't really grok. Sintering is one of the weirdest things. Why does squishing things together AND warming them up cause the particles to stick together more than when you just squish them together or just warming them up?

imchillyb 3y ago
> Why does squishing things together AND warming them up cause the particles to stick together more than when you just squish them together or just warming them up? >> The equations describing these laws are special cases of the ideal gas law, PV = nRT, where P is the pressure of the gas, V is its volume, n is the number of moles of the gas, T is its kelvin temperature, and R is the ideal (universal) gas constant. >> https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/relating-pressure-volume-amount-and-temperature-the-ideal-gas-law/ https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/rela... The laws of thermodynamics describe how gases work, those laws cause the effects you’re asking about. The above is a bit much for a primer on the topic, but provide enough key words for a Google-dive into the subject.
- WJW 3y ago
  Did you miss the part where I was talking about sintering and diffusion of atoms in materials in their solid phase? A quick search in the article you linked shows zero results for either "sintering", "diffusion" or related terms. The ideal gas law is very nice but hardly applicable to describe things like creep in metals under stress, let alone things like the grain boundary diffusion mechanism for sintering metal particles together.
  - salawat 3y ago
    What is a solid but a, relatively speaking, very, very cold gas? Put two solids together with no intervening medium, and heat everything up, and you'll see some mixin' and mingling. Often, the reason you don't see more of that sort of thing is because everything is immersed in an oxygen rich fluid with a tendency to create large, relatively reluctant to mix and mingle oxide coatings before anything fun can happen. Hence why another word for cold welding is vacuum welding. Materials, when you really look at em' are not as 'solid' and 'stable' as you may think. Hence why I have a mechanics of materials book I peruse from time to time.
    - thsksbd 3y ago
      "What is a solid but a, relatively speaking, very, very cold gas?" Well there are two first degree phase changes to consider. Then there are those pesky mtls that dont even follow the general trend of gases, like water that expands when cooled.
hansvm 3y ago
Squishing is important, else the forces involved are negligible. Warming them up increases their movement and thus the likelihood of a number of things (including random interactions with the thing they're squished against). You might also be interested in cold welding. https://en.m.wikipedia.org/wiki/Cold_welding https://en.m.wikipedia.org/wiki/Cold_welding
thsksbd 3y ago
Diffusion: Its not hard to understand once you see the mechanism in full (vacancies, interstials, etc) but its hard to write as txt. Suffice to say, there's plenty of room to squeeze. "Squeezing" When you squeeze you get rid of air bubbles, right? Well, when you squeeze you get rid of boundaries which are really volumes at small scales. Well nature hates surfaces, so if you squeeze surfaces close enough together and hot enough you'll give the material an opportunity to rearrange atoms to get rid of boundaries. "Ostwald ripening" If you thought sintering and diffusion are hard, I had a respected mechanical engineering professor working on a refractory materials project claim, when he first encountered it, that Ostwald ripening cannot occur. The mtls Eng guys rolled our eyes...