Photonic Structure Increases Light Bulb Efficiency

An Anonymous Coward writes "A new experimental microscopic tungsten lattice can increase the efficiency of an incandescent electric bulb from 5 percent to greater than 60 percent. This is done by converting waste heat into visible light. "

Comments

Think of the children...

[weave]

It works by "converting waste heat into visible light?" This is going to ruin all of those Easy Bake Ovens.

And what about when my daughter finds a birds nest that has fallen out of a tree and we need to fabricate a incubator out of a box and a 25 watt light bulb to keep it warm?

This is horrible news. Think of the children. Call your congressman and ban this insanity.

Re:Question

[maraist]

Second, I love this. They don't even have a THEORY on why this works. It just does.

Well, I'm an undergraduate Electrical Engineer, so I only have superficial understandings of how semi-conductors interact with light, but it doesn't seem too great a stretch of the imagination.

First, semi-conductors work based on the principle of the band-gap (which they even mentioned) (correct me if I'm wrong with any of this, I'm doing it straight from rusty memory).

A little background:
The outer 8 electrons held by an atom are the most important (the valence) - They are responsible for the bonding of other atoms. The configuration of all the electron orbitals in free space is nicely geometric; the first two electrons form a spherical shell (s-shell), the second 6 form dumb-bells in each of three axis's (p-shell). These types of configurations affect the geometries of the connection of the atoms. Configurations get more complex as the number of electrons grow (which is somewhat independent of the atomic number (number of protons), but such ionized atoms are unstable; especially when the number of electrons differs dramatically from the #protons). The important thing to understand here is that each additional electron takes more energy. Instead of worrying about the geometries, you can plot each electron orbital at a different (successively higher) energy level. Different atoms (characterized by atomic-number and even, to a small degree, the number of neutrons present), have differing characteristic energy-levels. The discrete nature of atoms includes the probabilistic nature wherein electrons have an extremely high probability of occupying the exact energy levels (which can be thought of as the distance away from the center of the nucleus). There is a chance that an electron will pass through any point around the shell of an atom, but it's highly unlikely that it will deviate from its characteristic point.

But, since different atoms have different characteristic levels, warping an atom will warp its points. Warping can occur by simply placing two atoms near each other (such as in an ionic or covalent bond). As it happens, when you squeeze atoms closer and closer together, the discrete lines that represent the energy levels start to merge together. Eventually the 8 outer valence bands merge into one continuous band... As you squeeze them even closer together, this band breaks into two continuous pieces. As you get even closer together, these pieces get further and further apart (I would presume that eventually one of these bands starts to merge with preceding energy levels, but that's not relevant here). This gap of continuous energy levels is called the band-gap.

As it turns out, in perfectly bonded atoms (those where every electron in the valence layer are bonded, and each atom has exactly 8 outer electrons; such as carbon, Silicon, etc) we have a total of 4 electrons that fill the inner continuous shell and 4 electrons that are void in the outer continuous shell. BUT, that outer shell is looped across neighboring atoms. When a diamond-lattice is organized (which is as close as you can possible get multiple atoms to sit next to each other), you have the greatest band-gap you can get for that particular element. Different elements (or even molecules) that can form the diamond-lattice will have differing characteristic band-gaps. What we have here are 4 electrons that are tightly tied to a core atom, and 4 potentially absorbed electrons that can freely be shared across every single atom in the entire crystalline lattice. In semi-conductor crystals, the problem is that every electron is accounted for so there are no free electrons to put into the outer band (which could roam free as current through an almost zero-resistance substrate; due mostly to quantum effects). Impurities are therefore inserted into the crystalline lattice which act as ionic donators of electrons or ionic acceptors of electrons (namely atoms not in the 4-column of the periodic table). Thermal excitation (heat) causes an electron to be ripped from donor atoms and those which are then quickly swept up in the outer-most continuous band.

Normally, electrons must have a precise energy-value in order to live in an atomic orbital. When an atom absorbs an electron, it gives off a photon of the remainder of the energy. To change orbital-levels, it has to accept a photon of exactly the correct amount of energy. It can accept a larger energy photon, but it will again give off the remainder of energy. Eventually that excited electron will fall back to its lower energy level, giving off another photon which will have the exact energy as the distance between the two energy levels.

In the continuous region of these silicon atoms, excitation between energy levels isn't apparent, since an electron can have any value within the region. The only difference is that separating the band gap... An electron from the inside can jump into the outer band if it's given at least enough energy to make the jump... This gap is usually enormous for semi-conductors. I believe its 1.2 electron Volts for Silicon, and 10 electron Volts for Carbon. The 1.2V is within the range of thermal excitation. That means that heat (in the form of vibrating atoms in the crystal) is enough to shake an electron free; e.g. jump the gap (like water successfully spitting to the lid of a boiling pot). In carbon, however, room-temperature heat is no where near enough to make the jump. This property (along with others) is why we don't use carbon-based semi-conductors. Germanium and silicon are much more practical in our particular earth climate.

There is another aspect to the band-gap that is relevant to our discussion. Each electron has not only an associated energy, but a quantum-form of momentum. You must not only have conservation of energy, but conservation of momentum. I'm a little fuzzy on this topic, but this momentum is represented by the letter k, and we can plot energy verses k for different things. For semi-conductors, we get parabolas, and inverted parabolas, but with discrete points. This says that while we have a continuous set of energy levels within a region, we have only a certain set of allowable energy+momentum values for the electrons. And like the discrete energy levels of atomic orbitals, you can only have one electron occupying a given state. In a rather unfulfilling way, I'll stop talking about things that I don't fully understand and simply say that this multitude of characteristic parabolas says that in order to have an electron jump, you have to not only have a precise amount of energy absorbed or emitted, but you have to be able to transition your momentum somehow. Energy transition occurs through photons, and momentum transition occurs through phonons - which is energy present in lattice vibrations (e.g. packets of heat).

Gallium arsenide is an example where the lowest point of the upper parabola and the highest point of the lower inverted parabola are aligned with respect to momentum. This means that the smallest amount of energy needed to make an electron jump the gap requires zero change in momentum. Because of this, gallium arsenide crystals easily will easily absorb or emit light with no dependence on the heat of a lattice. For this and other reasons, GaAs is great for laser diodes. Silicon, on the other hand requires a momentum change for its lowest energy transfer. Thus lots of heat is generated and absorbed; (Not to mention that silicon doesn't conduct heat as well as some other semi-conductors).

Given this superficial description, what I get out of this is that heat of a certain resonant point (in the form of vibrating atoms in the crystal) could provide the proper momentum shift needed for efficient electron excitation. You'd still need to provide photonic energy for the transition, but you'd have a perfect combination of heat + light absorption. Eventually (due to statistical decay), the electrons would fall back to their lower-energy-level states. But they'd give off light of specific frequencies.

Putting all this together, my initial impression from the article was that that the tungsten injection into a silicon substrate change the characteristic e-k curves enough to absorb the phonon-heat generated by IR light. The result is a 60% efficient absorption of the heat + light (e.g. nearly perfect efficiency). That energy is retransmitted as diode light (e.g. an exact energy level transition, producing a constant level of energy photons, which requires an equally constant frequency of light).

What I don't know at the moment is if this is actually emitting mono-chromatic light, or if a multitude of frequencies (e.g. white-light) permeates. The only way I could see white-light emitting is if the standard tungsten light-bulb is making it, and the Tungsten semi-conductor is amplifying a particular frequency.

Light emitting technology

[wowbagger]

The science of turning electric power into light has really changed in the past decade. I've seen a graph in one of my engineering trade journals showing the efficency of LEDs in lumens per watt. Just a decade ago, the best LEDs were two orders of magnitude less efficent than flourescent bulbs. Now, the new generation of blue and white LEDs are more efficent than flourescent, and are approching the levels of low pressure sodium lights.

If we extrapolate from the given 5%->60% levels given in the article, that would raise incandescent lights to nearly the levels of flourescent, without the warm-up time flourescent has.

Now, the problem with LED vs. flourescent is cost - LEDs are much more expensive in terms of lumens per doller than flourescent. Would microstructured tungsten be any cheaper?

Re:Question

[maraist]

Damnit.. Forgot to append the best link in the world that describes this in detail.

Britney Spearse Guide to semiconductors