About this report
Is this considered a metal?
and does that mean there are clear or tranlucent metals?
No, it says it’s a ceramic.
Transparent substances can have some metal. A lot of common glasses are doped with metals.
Having lots of free electrons doesn’t necessarily make a substance opaque.
The electrodes in LCD panels are transparent and yet have lots of free electrons to conduct the current.
LEDs also are transparent (how does the light get out otherwise) and they are also conductive (or at least semi-conductive)
I’ll bite. What’s a ceramic? Are ceramics, by definition, not “metals?” This stuff looks like “rusty” aluminum to me. Is rusty iron a ceramic, too?
“Any of various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing a nonmetallic mineral, such as clay, at a high temperature.”
I had always assumed that the light emission from an LED chip(chip, not package - the transparent bit of the package is usually just a blob of epoxy plastic) was a surface phenomenon - is this not true? Are Gallium-based semiconductors transparent?
It’s a surface phenomenon, but the surface in question is the P-N junction, not the exterior surface of the semiconductor device.
“Transparent” can mean different things. Silicon and gallium are both fairly opaque to visible light. But gallium is relatively transparent to IR, and its transparency continues up to the visible red region. So GaAs can be used for red LEDs. Silicon is relatively transparent to UV, and I’ve seen a perfectly ordinary silicon transistor cut open and emitting visible violet light.
Just for the record, the alkali metals are transparent in the ultraviolet. The wavelengths at which they “turn on” can extend surprisingly far into the visible. Lithium “turns on” at 205 nm and Sodium at 210, but Rubidium is transparent by 360 nm, and Cesium at a whoppingly high 440 nm. See Born and Wolf’s Principles of Optics, section 13.3.
As for Medusa’s :petrification rays", I think you have misconstrued the phenomenon. For a proper treatment, you need a book by someone versed in both Optics and Mythology. I doubt if such an odd individual exists.
Back to the first message. Alumina is not a metal, it’s a ceramic, Al2O3. It’s a polycrystalline ceramic made by sintering smaller particles together. 1200°C is close to the melting point of alumina, so in effect, the report is discussing a less polycrystalline alumina.
The logical extension is that a purely crystalline alumina would be quite transparent, which is borne out by the fact that single crystal alumina is often known as sapphire.
To clear up some misinformation about metals. Many metals and conductors can be “transparent” in the visible if sufficiently thin. Gold, being an excellent conductor is quite transparent in the visible when thinned to around 100 angstroms. Likewise, semiconducting silicon is also transparent visible when thinned below 5000 angstroms. Note that silicon and germanium are already transparent in the 3-5 micrometer and 3-14 micrometer wavelengths respectively.
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Once again, what is the scientific definition of a ceramic?
dictionary definition:
Any of various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing a nonmetallic mineral, such as clay, at a high temperature.
How is Aluminium Oxide (Al[sub]2[/sub]O[sub]3[/sub]) a nonmetallic mineral?
Nonmetallic doesn’t mean “not containing any metal”. It means “not having the properties of a metal”. Aluminum oxide does not conduct heat or electricity particularly well, it’s not ductile or malleable, it’s not lustrous, and it does not contribute electrons in chemical bonds with other substances, so it’s nonmetallic.
For comparison, would you consider chalk to be metallic? It’s calcium carbonate, and calcium is a metal.
Oh, how I love this threads that mess around with lots of terms and concepts.
Let me share some of my chemical knowledge:
Metals are substances that are hold together by metal-bonding, which is: The atoms sit in a crystal lattice and shed 1-2 of their electrons that float around freely in the whole crystal.
Minerals are things you dig out of the ground, occurring naturally in the inanimate nature. There is no specification to the chemical or physical composition. They can be elements with or without metal-bonding (sulphur, gold), oxides (iron ore, sand, bauxite (which is actually a hydroxide)) and most are “salts” in the chemical meaning (salt, chalk, malachite…). Stones are complex mixtures of different salts and oxides. (Chemically spoken you could call oxides the salts of the acid called water.)
Ceramic: Now, I’d say that’s a technical term, there is no all time valid scientific definition for it. Historically it stands for manmade products out of burned clay. This are, chemically spoken, complex mixtures of different salts and oxides (sound familiar?).
As technology ventured into High-Tec salts and oxides (artificial sapphire) they just got also called ceramics.
Probably there are some that call even silicon-carbide and boron-nitride ceramics, which are definitely no oxides and not what a chemist would call salts either.
Transparent in lay terms obviously refers to the visible light, electromagnetic waves with wavelength =400-800nm. Clear glass is opaque in the UV, =300nm, see thread about tanning behind windows.
The black elemental Silicon is very transparent in the near infrared. Water is opaque around =125 000nm, i.e. the microwave.
But here it gets interesting: Water is also opaque in the deep UV and near infrared.
There are three different absorption processes behind it: Rotation (microwave), stretching (infrared) and, to say it simple, electron jumping in the UV. We live in a quantum universe!
But now back to the metals: As the electrons float freely, they have a continuum of quantum states (translate that to energy states) where they can exist – they absorb at any wavelength.
And now pack together the Aluminium with the Oxide. The later grabs and locks the electrons. The absorption continuum is reduces to several fixed energy = wavelength states.
Every ceramic, every stone!, would be transparent if it was not multi-cristytalin and would not lose all the light by scattering.
On second thought we would live in a pretty transparent world, if it was not for the light-scattering.
Obviously the visible wavelengths are very susceptible to scattering, much more than the other wavelengths. Also it is easy to see that is practical, I will not try to venture on an explanation.
Will you, Chonos, that’s a physics question?
Before someone starts to correct me: Absorption occurs when there are two quantum states and the wavelength precisely matches the energy difference. Then the radiation “is used” to switch from the one to the other. Usually quickly after that the molecule falls back to lower energy level. Quantum laws require that exactly the same energy gets re-emitted.
So, where is it?
The molecules use it to joggle around, a process called heat-loss. An ill-coined term as the heat actually increases.
The perfect re-emission is tricky and works only in a Light Amplification be Stimulated Emission of Radiation machine.
Thanks, Chronos I think I get it now. It also helps me put the claim that there are no “transparent” metals in perspective. First, some metals are “transparent,” just not so much in the narrow visible range. Secondly, to say that “metals” aren’t “transparent” is a tautology. “Metals are things that are shiny/lustrous” Things that are lustrous are not transparent. Therefore, metals are not transparent." Perfectly true, perhaps, but not very earth-shaking since it is built into the definition.
thank you. Partyofone and MummyCave, thanks.
It’s the “Alumina” I mistook for Aluminium…
BTW: Hydrogen is considered to be a metal in some aspects; it has a single outer electron and tends to donate electrons rather than recieve them, and may actually have a classicly metallic phase at very high pressures and temperatures. So ice, being the oxide of a “metal”, can technically be considered a ceramic! It certainly sounds like pottery when you drop a piece of it.
I wouldn’t say that it is the definition, just that it’s a part of it. If had a sample of some substance that met the other properties (conductivity, ductility, malleability, and electropositivity), but was transparent, I would probably call it a metal. The point is, though, that it’s difficult to get a material which meets those conditions. Specifically, it’s almost impossible for a good conductor to be transparent. The materials used to make an LCD are pretty poor conductors, and even at that, have at most 50% transparency. So it does have meaning (beyond just the definition) to say that metals are opaque.
To expand on this, one very important material for these display technologies is ITO (indium tin oxide). It is transparent and is a pretty good electrical conductor. Of course, it probably becomes opaque at some place in the infrared or lower frequencies but it is transparent in the visible range.
The reason the ITO is transparent, and that’s relatively, not absolutely, is that it’s too thin to properly interact with the photons going through it.
The higher conductance ITO used in LCD’s has >80% transmission out to 2 um, while lower conductance ITO can have transparency out past 5 um. The thickness seems to be not much more than 100 angstroms.
A 10 angstrom thick layer of gold has >80% transmission from 300 nm to past 10 um.