D4RR3N
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Temperature to breakup BN crystal lattice?
Boron Nitride (hexagonal) has a crystal lattice structure similar to graphite consisting of layers of molecules arranged in hex structure.
I have been trying to find some technical info on BN but have been unable so was thinking someone hear might know?
What temperature would a sample of BN have to be heated to in order to release the BN molecules from the hex structure?
I know the temperature will be below the melting point and just above the cure temperature. I have however been unable to find the cure temperature of
BN anywhere.
Dose anyone know, can it be calculated?
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12AX7
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Cure temperature???
If you mean atomic diffusion, that starts around half the absolute melting point, ballpark.
I don't know anything about the vapor phase of BN, which by the sound of it, you seem to know it exists in diatomic form? If you don't and are just
assuming, realize it's a bad assumption, as BN isn't made of molecules, it's a crystalline solid. (It may well be true, as many crystalline and ionic
substances have molecular vapor phases, but I don't know about BN.)
By definition, the temperature at which a crystal lattice "breaks up" is when it melts.
Tim
[Edited on 4-22-2008 by 12AX7]
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D4RR3N
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If I use an analogy perhaps it will make clearer what I’m getting at. Lets say we have water and we start to cool it, as we cool it a crystal
structure starts to form, this happens at a specific temperature. The reverse is also true, we have some ice and we start to heat it and the crystal
structure starts to break up. My question is, is the temperature at which this takes place slightly below the melting point. Is it necessary to heat
to melting point?
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JohnWW
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By analogy with graphite, and also diamond, it would take a very high temperature indeed, for both hexagonal BN and borazon (cubic diamond-like BN,
which is about as hard as diamond) to melt or sublime. Although the melting-points of all these, and B4C and SiC, are given as being of the order of
2,500 to 3,500ºC, they supposedly "boil" at 3,500 to 4,200ºC; so there is either a very short liquid range, or else the carbon sublimes as C2
molecules (and BN as BN molecules, which being strongly polar are likely to be not as volatile). Like graphite, hexagonal BN is used as a
high-temperature lubricant.
According to the following references, hexagonal BN sublimes at 2,700ºC (no liquid range). Unlike graphite, which is electrically conducting, black,
and absorbs all wavelengths of light, it is a white solid, indicating that the polar B-N bonds greatly reduce the mobility of electrons over its
crystal lattice and hence conductivity and light absorption, due to a larger conduction-band gap. Cubic BN is preferred to diamond as an abrasive and
for high-temperature uses, because diamond forms carbides with steels, and unlike BN begins to revert to graphite at 871ºC.
http://en.wikipedia.org/wiki/Boron_nitride
http://en.wikipedia.org/wiki/Borazon
http://www.britannica.com/eb/article-9000210/boron-nitride
http://www.thomasnet.com/products/boron-nitride-7030406-1.ht...
http://www.bn.saint-gobain.com/
http://www.esk.com/en/materials/boron-nitride-bn.html
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12AX7
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I say again, crystal breaking up == melting.
It sounds like you want ionic diffusion, which is as I've said.
Tim
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franklyn
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The unspoken constraint is at ambient pressure of the atmosphere, 1 Bar.
Pressure is also a variable and if I recall correctly at around 1.4 Megabar
even diamond becomes soft and runny and at room temperature.
BN being very similar would behave much the same.
.
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D4RR3N
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Perhaps if I make my goal clearer it will help on this matter of Boron Nitrate.
Boron Nitrate is a widely available material in Engineering, you can purchase it in powder, rod or sheet form. I would like to make an electret using
BN Sheet.
An electret as you know is the electrical equivalent of a bar magnet. Usually they are made of plastics as they are easy to work with but I want to
see if I can make one out of BN.
In order to polarize the BN I need to heat it up to a temperature at which the crystal lattice is weak, then apply a high voltage across the material.
The BN molecules are dipolar so they spin around and align themselves to the external electric field. The material is allowed to cool whilst the
voltage is maintained. The crystal lattice now rearranges itself, the BN molecules cant go back to the original formation as they are held in position
by the electric field. Upon cooling the material now has a permanent electric field much like a bar magnet has a permanent magnetic field.
Question is what temperature will I need to reach for that to happen? Lets also take into account the electric field, if the crystal lattice is
sufficiently weakened then a blast from a high voltage source may be enough to brake up the hex crystal structure without having to heat to melting
point. Obviously the higher the voltage the grater the polarising force on the BN dipole molecules.
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Nicodem
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Boron nitride is not composed of molecules so I can not see how you could orientate the dipoles of molecules that aren't there. A single crystal of
boron nitride, just like a crystal of diamond, behaves like a single enormous molecule. Therefore its dipole moment originates only from the
polarization of covalent bonds and atom polarisability, but not from molecular orientation change (like in gasses and liquids) or crystal lattice
deformation (like in ionic crystalline materials). But even with crystals made of dipolar molecules I can't see how you could freeze a charge by
orienting their dipoles (which, in crystals, are already oriented by default). Dipoles bear no formal charge.
Also an electret, can not really be permanent since the charge is not fixed in the material and isolated but is free to interact with other charges.
Hence the uses of nonconductive materials where the charge can not travel from one end of a rod to the other where opposite charges reside. An upgrade
using dielectric properties of the supporting material would be a long rod made of barium titanate with both tips consisting of a conductive material
(essentially a capacitor with a tiny capacitance but high voltage resistance). Then you would have to charge the tips, which would not be as easy as
mechanically self charge a wool cloth in dry weather.
So in short, the closest thing to what you appear to be up to is called high voltage capacitors (HVC). Provided you can isolate the contacts, they
behave like (more or less) permanent electrets. Unfortunately the maximum achievable potential of commercially available HVC's is only in the range of
few tens of kV (but at least the potential is long lasting and capacitors in the pF range can be enough if all you want is maintain an electric
field). More can be achieved by connecting a couple of HVC's in a row, but as far as finding an easy way to charge them...
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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D4RR3N
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I was clearing out the garage last week and came across a sheet of BN I had leftover from something I was doing a few years back. Thought it would be
interesting to see if I could electrically polarise it. I thought it may work as an electret.
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franklyn
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| Quote: | Originally posted by D4RR3N
Perhaps if I make my goal clearer it will help on this matter of Boron Nitrate. |
It would help if you could decide wether you're refering to Boron Nitride or Boron nitrate.
You cannot make a Peizo electric material as for example Lead Titanate from a material
of only two elements, since there is no dipole and it is covalent. In extreme ( 100's of
millions of volt ) potentials, polarization effects would be created but only maintained in
the presence of the applied field and would disappear when the field is removed.
You may alter the basic properties by doping it with a third inclusion element but in this
case you would just end up with a semiconductor.
http://en.wikipedia.org/wiki/Boron_nitride
[img]http://www.sciencemadness.org/talk/viewthread.php?action=attachment&tid=6538&pid=74921[/img]
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D4RR3N
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Sorry franklyn, I was considering Boron Nitride (hex) BN, but now that you mention it I was wondering about Boron Nitrate. I Google searched but did
not find much. Is Boron Nitrate more correctly called Boron Oxynitride, sometimes given as BNO and also BON?
[Edited on 24-4-2008 by D4RR3N]
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D4RR3N
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| Quote: | Originally posted by franklyn
You cannot make a Peizo electric material as for example Lead Titanate from a material
of only two elements, since there is no dipole and it is covalent. |
I was not trying to make a peizo electric material, I wanted to make an electret. I know that its possible to make an electret from polarised lead
monoxide PbO, this has just two elements.
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franklyn
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Your initial notion invokes more than the simple atomic structure alone, which is what I refered to only. Electret
properties may also be created in a material by the nanoscale structure , unrelated to the crystal structure.
I should proof read what I write more closely - I can't believe I wrote this " You cannot make a Piezoelectric
material - from a material of only two elements." , wrong , I confused Electret which has intrinsic polarization with
Piezoelectric which need not be polarized ( until strained ). Most notably quartz SiO2 , ZnO , exhibit Piezoelectric
properties. Lead oxide PbO alone is not an Electret itself, ( although it can exhibit this property - See below ∆ )
it is always used in combination with other materials, lithium for example in thin films. PbO is dielectric as is any
metal oxide and can serve as the charge separating barrier of a capacitor as well as Aluminum Oxide or Boron
Nitride can. Thus an Electrolytic capacitor is an electret, but only when charged.
Making a KNO3 capacitor - I don't know about Boron nitrate
http://www.doitpoms.ac.uk/tlplib/ferroelectrics/fabrication....
Also I said " You may alter the basic properties by doping it with a third inclusion element but in this case you would
just end up with a semiconductor." this is simplistic , I again was thinking of the atomic scale crystal. Engineered
materials with a nanoscale structure can be made of seemingly anything , imagination being the only limitation.
In this case the novel approach is to form voids ( like swiss cheese) which can retain a charge.
∆ See -> http://en.wikipedia.org/wiki/Ferroelectret
And also - 14 Ferroelectrets – Cellular piezoelectric polymers , page 138 , here _
https://dspace.lib.cranfield.ac.uk/bitstream/1826/2115/1/New...
Nanoferroelectrics
http://www.polecer.rwth-aachen.de/News.2005/JF_Scott_IMF11.p...
Some definitions
See Electret types
http://en.wikipedia.org/wiki/Electret
http://books.google.com/books?id=_M8B6ISHxFUC&pg=PA184&a...
Ferroelectric
property of certain nonconducting crystals, or dielectrics, that exhibit spontaneous electric polarization
(separation of the centre of positive and negative electric charge, making one side of the crystal positive
and the opposite side negative) that can be reversed in direction by the application of an electric field,
this hysteresis is only within a definite temperature range , the transition temperature are it's Curie points.
Ferroelectric Electret that is Piezoelectric -
The property of some materials to store a permanent electric field, by analogy with the storage of a magnetic field
by ferromagnetic materials. The BaTiO3 in a Perovskite structure is used to create Ferroelectrics in the lab under
the imposition of a strong electric field which permanently creates electric dipoles. The Perovskite structure is
amenable to this treatment because the Ti atom is slightly too small to come into bonding contact with the surrounding
oxygen atoms, and can "slip" so that it touched only three of them, creating a net dipole. Striking a ferroelectric
will cause the generation of an extremely high voltage (but low current) two-peaked current (one for the compression
and one for the rebound).
http://www.doitpoms.ac.uk/tlplib/ferroelectrics/phase_change...
Piezoelectric and Electret effects ( scroll to middle of page )
http://www.resonancepub.com/piezoele.htm
________________________________________________
I point all this out because Boron Nitride has distinct crystal phases a cubic form ( wurtzite ) called borazon
and the plane form dipicted in the post above, this is as different as graphite a conductor is from diamond an
insulator.
http://en.wikipedia.org/wiki/Boron_nitride
Crystal cubic structure
http://en.wikipedia.org/wiki/Wurtzite
http://cst-www.nrl.navy.mil/lattice/struk.picts/b4.s.png
Dielectric constants
http://clippercontrols.com/info/dielectric_constants.html
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D4RR3N
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Thanks for that franklyn, Electret and Piezoelectric materials are not necessarily the same in that many electret materials are made from waxes,
plastics and cured resins which make good electrets but dont have a marked piezoelectric effect.
On the other hand I have heard that there are many piezoelectric materials which can be made into electrets, So whilst not all electrit materials
exhibit piezoelectricity many piezoelectric materials if processed correctly can be polarised thus forming an electret.
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