- #1
Enthalpy
- 667
- 4
Hello you all!
What about a slightly exotic idea? Here I propose to measure the "effective" mass of charge carriers by centrifugal force.
Electrons in vacuum have a mass, and when moving in a solid an other mass, often called "effective" (as if the vacuum mass were ineffective). Centrifugal force creates unequal voltages across dissimilar materials that give a different mass to electrons, and with a proper setup, this voltage seems measurable - which I feel funny.
Referring to the attached sketched (click to magnify if logged in):
Along a radial leg, the centrifugal force creates a voltage of mA * 0.5*(V2-v2) /q in the material A, or mB etc in the material B, with V the outer speed and v the inner one. By making the odd legs of material A and even legs of material B, and putting many leg pairs in series, we get a significant voltage.
At least with metals, the electron work function won't vary with the minute amount of electrons added or subtracted, and nor will the contact potential; other materials need an ohmic contact. And yes, I believe electric power could be harvested, which would be provided mechanically by the shaft, but is technologically uninteresting.
One excellent choice for the disk is a silicon wafer; I take D=2 inches here. An other choice would be a platter of a hard disk drive with its spindle already. Silicon can rotate at 600 m/s (and much more); the inner speed shall be 400 m/s. Take materials that give masses of 1.5*m0 and -1.2*m0 for instance, then each pair of legs offers 1.5 µV; a pitch of 100µm permits 1000 pairs of legs (not all drawn here) resulting in 1.5 mV.
Metal thermocouples can develop 20 µV/K for instance, so the outer and inner temperatures must be much closer than 0.1K: nothing special within metals or silicon, but it must impose to rotate in vacuum.
1.5 mV DC is easy to measure, but not on a disk rotating at 3800 Hz (226,000 rpm). Capacitive coupling with the stator simplifies it and can serve as a welcome chopper. Take a gap of 0.2mm and an electrode width of 0.5mm between r = 10 mm and R = 15 mm: you can put 2*40 of them, resulting in 4.4 pF /2 and 150 kHz, so the signal is 1.5 mV pk at 150 kHz through -j*480 kohm, so easy.
Take a Fet or Mos amplifier, polarize its inputs with 100 Mohm, you get 2.3 kohm noise equivalent from the polarization - or use a pair of diodes for that. The amplifier's noise is similar and the legs can sum to 40 kohm if made of 2 µm thick metal. Noise over a 10 Hz band is 0.1 µV only; with semiconductor legs, shunt the resistance at 150 kHz by a rotating capacitor of few pF on silicon. A 3.5" platter at 7200 rpm would still provide some 15 µV signal.
Hydrodynamic bearings of proper dimensions dampen vibrations, see Dubbel for instance. Silicone and fluorosilicone oils have a negligible vapour pressure but beware they're very under-Newtonian at high shear.
In metals, electron mass may be less exciting, but it is important in semiconductors. Known materials like silicon may serve as an electron standard, possibly with a metal (silver?) as an intermediate mass standard. What about superconductors, where heavy holes are allegedly essential? Or graphene and nanotubes? Or even electrolytes, to determine the degree of solvation?
Marc Schaefer, aka Enthalpy
What about a slightly exotic idea? Here I propose to measure the "effective" mass of charge carriers by centrifugal force.
Electrons in vacuum have a mass, and when moving in a solid an other mass, often called "effective" (as if the vacuum mass were ineffective). Centrifugal force creates unequal voltages across dissimilar materials that give a different mass to electrons, and with a proper setup, this voltage seems measurable - which I feel funny.
Referring to the attached sketched (click to magnify if logged in):
Along a radial leg, the centrifugal force creates a voltage of mA * 0.5*(V2-v2) /q in the material A, or mB etc in the material B, with V the outer speed and v the inner one. By making the odd legs of material A and even legs of material B, and putting many leg pairs in series, we get a significant voltage.
At least with metals, the electron work function won't vary with the minute amount of electrons added or subtracted, and nor will the contact potential; other materials need an ohmic contact. And yes, I believe electric power could be harvested, which would be provided mechanically by the shaft, but is technologically uninteresting.
One excellent choice for the disk is a silicon wafer; I take D=2 inches here. An other choice would be a platter of a hard disk drive with its spindle already. Silicon can rotate at 600 m/s (and much more); the inner speed shall be 400 m/s. Take materials that give masses of 1.5*m0 and -1.2*m0 for instance, then each pair of legs offers 1.5 µV; a pitch of 100µm permits 1000 pairs of legs (not all drawn here) resulting in 1.5 mV.
Metal thermocouples can develop 20 µV/K for instance, so the outer and inner temperatures must be much closer than 0.1K: nothing special within metals or silicon, but it must impose to rotate in vacuum.
1.5 mV DC is easy to measure, but not on a disk rotating at 3800 Hz (226,000 rpm). Capacitive coupling with the stator simplifies it and can serve as a welcome chopper. Take a gap of 0.2mm and an electrode width of 0.5mm between r = 10 mm and R = 15 mm: you can put 2*40 of them, resulting in 4.4 pF /2 and 150 kHz, so the signal is 1.5 mV pk at 150 kHz through -j*480 kohm, so easy.
Take a Fet or Mos amplifier, polarize its inputs with 100 Mohm, you get 2.3 kohm noise equivalent from the polarization - or use a pair of diodes for that. The amplifier's noise is similar and the legs can sum to 40 kohm if made of 2 µm thick metal. Noise over a 10 Hz band is 0.1 µV only; with semiconductor legs, shunt the resistance at 150 kHz by a rotating capacitor of few pF on silicon. A 3.5" platter at 7200 rpm would still provide some 15 µV signal.
Hydrodynamic bearings of proper dimensions dampen vibrations, see Dubbel for instance. Silicone and fluorosilicone oils have a negligible vapour pressure but beware they're very under-Newtonian at high shear.
In metals, electron mass may be less exciting, but it is important in semiconductors. Known materials like silicon may serve as an electron standard, possibly with a metal (silver?) as an intermediate mass standard. What about superconductors, where heavy holes are allegedly essential? Or graphene and nanotubes? Or even electrolytes, to determine the degree of solvation?
Marc Schaefer, aka Enthalpy