4 ms·

But metalic Si will quickly grow a layer of passivating SiO2 whose lattice constant just happens to match Si's ensuring it doesn't flake off. Either way metall

But metalic Si will quickly grow a layer of passivating SiO2 whose lattice constant just happens to match Si's ensuring it doesn't flake off. Either way metallic Si is, essentially, a forever "chemical"

ben_w 3y ago
> metalic Si will quickly grow a layer of passivating SiO2 whose lattice constant just happens to match Si's ensuring it doesn't flake off This sounds like the exact opposite of a problem. > Either way metallic Si is, essentially, a forever "chemical" Silicon dioxide: https://commons.m.wikimedia.org/wiki/File:Beach_at_Fort_Lauderdale.jpg#mw-jump-to-license https://commons.m.wikimedia.org/wiki/File:Beach_at_Fort_Laud... Iron oxide: https://commons.m.wikimedia.org/wiki/File:Hull%E2%80%93Rust%E2%80%93Mahoning_Open_Pit_Iron_Mine.jpg#mw-jump-to-license https://commons.m.wikimedia.org/wiki/File:Hull%E2%80%93Rust%...
- adrian_b 3y ago
  Even an extremely thin layer of SiO2 is impenetrable for oxygen, so the rest of the elemental silicon is not oxidized any more, exactly like aluminum, titanium or stainless steel. All these 4 common materials can persist in the environment essentially forever, despite the fact that their base material would react easily with oxygen, even burning in certain conditions.
  - ben_w 3y ago
    I don't know how I can be more clear about how much this sounds like the exact opposite of a problem, about how this completely fails to elucidate why silicon is being singled out and demonised — especially as you're now putting steel (which one of the things the other commenter was preferring) into the same bucket as silicon.
    - thsksbd 3y ago
      OP here. Believe it or not, Im aware what rust and sand are made of. Iron oxide is an essential nutrient your body craves and has billions of years of evolution to process. Sand has no biological use and is completely inaccessible to biological systems. While chemically inert, it is chemically active through its ability to provide a surface for catalysis and biologically active by being able to cut bio membranes. Furthermore, nanoscale "ceramics" (is SiO2 a ceramic? I think so) are far more catalytically active than their bulk counterpart due to the sharp vertices and edges. Nano scale rods of SiO2 is not sand. It only shares chemical formula, which is a sophomoric understanding of chemistry. Anyway steel and SiO2 are not even close to the same category in terms of chemical resistance. For starters the person you replied to said stainless steel, not steel which corrodes like crazy. But even if it were stainless steel, stainless steel isn't stainless and will corrode. Especially at the nanoscale. By comparison SiO2 will never degrade. AFAIK only HF attacks SiO2, and I know of no biological systems that use F. "Si is not SiO2!" you protest. So? SiO2's lattice habit and constant matches Si's almost exactly. This means that SiO2 will adhere to Si very well protecting it. Since O is everywhere, SiO2 will form. Since the material we're talking about here is a nanoscale and narrow aspect ratio, it's likely to fully oxide the Si turning this into nanoscale asbestos. This despite the reduced diffusion from the low T since: - there isn't much length to diffuse at the nanoscale - narrow, nanoscale structures have different bond, surface and volume energetics which favor passivation rxn by the O. In short, these researchers have to prove this isn't going to stay forever in the environment piercing bio membranes until evolution incorporates HF into our gene pool. Btw, I propose novel ways to make nanoscale thermite for a living. Im hardly an innocent party.
    - lazide 3y ago
      To add to what my sibling commentator said - PFAS compounds were originally considered awesome and safe exactly because of the same attributes - relative chemical inertness. Fluorine grabs so strongly onto other atoms, that it essentially makes the resulting compounds invulnerable and non reactive under normal circumstances. Just like sio2/silica/sand, actually. In many cases, this is actually fine because the molecules don’t happen to be shaped in a problematic way, or block some pathway due to them building up. In other cases, they are a huge problem because they are problematically shaped, or can bioaccumulate to problematic levels. Silicon dioxide happens to have such strong binding energy, it plays a similar type of role - and unlike PFAS compounds, which were very novel when first created and don’t exist in nature, we know it is carcinogenic in certain forms, since it is so prevalent in nature and we’ve had time to learn. Though we’re still learning a lot about these hazards. They are often not obvious or quickly apparent. Like, as the sibling poster noted, fine crystals (silicosis) and long thin nanoscale rods (mesothelioma from asbestos), albeit those are more complex silicates and not pure. So while cool that there is an antibiotic property coming from long thin pure silicon rods applied to surfaces, maybe don’t start applying it to everything until we’re sure it won’t cause cancer and accumulate in the environment? Since unlike iron, copper, etc. and more similar to PFAS compounds, it won’t just biodegrade or oxidize to a different form either.

ben_w 3y ago

> metalic Si will quickly grow a layer of passivating SiO2 whose lattice constant just happens to match Si's ensuring it doesn't flake off This sounds like the exact opposite of a problem. > Either way metallic Si is, essentially, a forever "chemical" Silicon dioxide: https://commons.m.wikimedia.org/wiki/File:Beach_at_Fort_Lauderdale.jpg#mw-jump-to-license https://commons.m.wikimedia.org/wiki/File:Beach_at_Fort_Laud... Iron oxide: https://commons.m.wikimedia.org/wiki/File:Hull%E2%80%93Rust%E2%80%93Mahoning_Open_Pit_Iron_Mine.jpg#mw-jump-to-license https://commons.m.wikimedia.org/wiki/File:Hull%E2%80%93Rust%...

adrian_b 3y ago
Even an extremely thin layer of SiO2 is impenetrable for oxygen, so the rest of the elemental silicon is not oxidized any more, exactly like aluminum, titanium or stainless steel. All these 4 common materials can persist in the environment essentially forever, despite the fact that their base material would react easily with oxygen, even burning in certain conditions.
- ben_w 3y ago
  I don't know how I can be more clear about how much this sounds like the exact opposite of a problem, about how this completely fails to elucidate why silicon is being singled out and demonised — especially as you're now putting steel (which one of the things the other commenter was preferring) into the same bucket as silicon.
  - thsksbd 3y ago
    OP here. Believe it or not, Im aware what rust and sand are made of. Iron oxide is an essential nutrient your body craves and has billions of years of evolution to process. Sand has no biological use and is completely inaccessible to biological systems. While chemically inert, it is chemically active through its ability to provide a surface for catalysis and biologically active by being able to cut bio membranes. Furthermore, nanoscale "ceramics" (is SiO2 a ceramic? I think so) are far more catalytically active than their bulk counterpart due to the sharp vertices and edges. Nano scale rods of SiO2 is not sand. It only shares chemical formula, which is a sophomoric understanding of chemistry. Anyway steel and SiO2 are not even close to the same category in terms of chemical resistance. For starters the person you replied to said stainless steel, not steel which corrodes like crazy. But even if it were stainless steel, stainless steel isn't stainless and will corrode. Especially at the nanoscale. By comparison SiO2 will never degrade. AFAIK only HF attacks SiO2, and I know of no biological systems that use F. "Si is not SiO2!" you protest. So? SiO2's lattice habit and constant matches Si's almost exactly. This means that SiO2 will adhere to Si very well protecting it. Since O is everywhere, SiO2 will form. Since the material we're talking about here is a nanoscale and narrow aspect ratio, it's likely to fully oxide the Si turning this into nanoscale asbestos. This despite the reduced diffusion from the low T since: - there isn't much length to diffuse at the nanoscale - narrow, nanoscale structures have different bond, surface and volume energetics which favor passivation rxn by the O. In short, these researchers have to prove this isn't going to stay forever in the environment piercing bio membranes until evolution incorporates HF into our gene pool. Btw, I propose novel ways to make nanoscale thermite for a living. Im hardly an innocent party.
  - lazide 3y ago
    To add to what my sibling commentator said - PFAS compounds were originally considered awesome and safe exactly because of the same attributes - relative chemical inertness. Fluorine grabs so strongly onto other atoms, that it essentially makes the resulting compounds invulnerable and non reactive under normal circumstances. Just like sio2/silica/sand, actually. In many cases, this is actually fine because the molecules don’t happen to be shaped in a problematic way, or block some pathway due to them building up. In other cases, they are a huge problem because they are problematically shaped, or can bioaccumulate to problematic levels. Silicon dioxide happens to have such strong binding energy, it plays a similar type of role - and unlike PFAS compounds, which were very novel when first created and don’t exist in nature, we know it is carcinogenic in certain forms, since it is so prevalent in nature and we’ve had time to learn. Though we’re still learning a lot about these hazards. They are often not obvious or quickly apparent. Like, as the sibling poster noted, fine crystals (silicosis) and long thin nanoscale rods (mesothelioma from asbestos), albeit those are more complex silicates and not pure. So while cool that there is an antibiotic property coming from long thin pure silicon rods applied to surfaces, maybe don’t start applying it to everything until we’re sure it won’t cause cancer and accumulate in the environment? Since unlike iron, copper, etc. and more similar to PFAS compounds, it won’t just biodegrade or oxidize to a different form either.