I've posted about this once in the past, I think in an Instructables comment, though I can't find it again with Google. But I've never posted about it here, and it's been on my list for two years now.
I shall begin by describing a conventional fluorescent lamp [1], and its failure modes.
It consists of a glass tube filled with mixture of mercury vapor and argon gas (or other noble gas) at low pressure, with the inside surface coated with phosphors, and a filament at each end. The filaments are coated with an emission mix to promote thermionic emission. After heating them, an electric arc is struck between the filaments, through the mercury vapor. This causes the mercury vapor to emit ultraviolet light, primarily at 253.7 and 185 nm. This ultraviolet light is absorbed by the phosphors, which then re-emit that energy as visible light (usually at a mix of wavelengths designed to give the appearance of white light).
This conventional fluorescent lamp can fail in a number of ways. The filaments can sputter their emission mix coating away. They can also burn out like those of incandescent bulbs. The phosphor and glass can adsorb the mercury vapor (which both reduces the available mercury, the main problem, and blocks some UV from getting to the phosphor). The phosphor can wear out through any of several phosphor degradation processes [2]. Also, the ballast can fail, but I am considering that out of scope for this idea.
The first two problems, those of the filaments, have already been solved by the electrodeless fluorescent lamp [3]. This uses a sealed induction coil to couple energy into the mercury vapor. Therefore, it has no electric parts exposed to the arc, and no resistive heating is used either. Wikipedia also claims uncitedly that it has another advantage, namely that you can use higher-efficiency "light-generating substances" (vapor? phosphors? IDK) because they don't have to be chemically compatible with the electrodes.
My invention is an improved version of the electrodeless fluorescent lamp. It solves the remaining problems: adsorption of the mercury onto the glass and phosphor, and phosphor degradation.
First, the phosphor is encased inside the glass. This keeps the mercury from adsorbing onto it. Also, with proper chemical design, it should be able to prevent phosphor degradation. The oxidation process is blocked by the phosphor being encased in glass. (The outer glass must be a non-oxygen-permeable one, obviously.) The crystal lattice degradation and activator migration processes could be blocked by a close encapsulation by the glass—multiple alternating molecular-thickness phosphor layers and thicker glass layers might work. This will depend on the crystal lattice of the glass, and different phosphors have different degradation processes, so the phosphor mix and glass chemistry must all be designed holistically.
Obviously, the layer of glass between the phosphor and the mercury vapor must be UV- transparent. There is a well-known UV-transparent glass, used in blacklights, called Wood's glass. [4] However, Wood's glass gets less transparent to UV the more UV you expose it to, meaning it would make the lamp mortal. Therefore, some candidate materials are fused quartz (silica) [5], sapphire [6], alumina [7], and aluminium oxynitride [8]. With these, there should be enough flexibility to engineer a way to incorporate phosphors into their crystal lattices. As well, these materials are quite hard and strong, resulting in durability of the envelope. (And no, I have no idea how to make a bulb or tube out of sapphire.)
The one remaining failure mode (as far as I know yet) is adsorption of the mercury vapor onto the glass. This is solved by a simple bakeout process, performed whenever the ballast detects low mercury vapor level (difficult to start) or at every startup. This consists of heating the glass, using a radiant heater at the center of the tube/bulb (sharing space with the induction coil if it's in the center). The radiant heater is ceramic rather than filament-based, to avoid burning out.
In the case of fused quartz, you can get both UV-grade and IR-grade, and each one is not very transparent to the other band. Therefore, using UV-grade fused quartz for the inner glass would result in the inner glass getting heated efficiently. The glass covering the radiant heater and induction coil should probably be IR-grade. It will absorb more IR anyway due to being closer to the heater, and will also be heated by conduction. The mechanical properties of the two grades are pretty much identical, so they should be easily weldable together.
(A resistive heater embedded throughout the envelope (similar to the rear window defroster of a car) could be used instead, but oxygen could get in through where it entered the glass, oxidize the metal, and, even if it didn't crack the glass, result in the heater failing.)
Please point out any failure modes I've missed. :)
34/198-- notexactly, Mar 30 2017 [1] Conventional fluorescent lamp https://en.wikipedi...ki/Fluorescent_lamp [notexactly, Mar 30 2017] [2] Phosphor degradation processes https://en.wikipedi...hosphor_degradation [notexactly, Mar 30 2017] [3] Electrodeless fluorescent lamp https://en.wikipedi.../Electrodeless_lamp [notexactly, Mar 30 2017] [4] Wood's glass https://en.wikipedi...wiki/Wood%27s_glass [notexactly, Mar 30 2017] [5] Fused quartz https://en.wikipedi...g/wiki/Fused_quartz [notexactly, Mar 30 2017] [6] Synthetic sapphire https://en.wikipedi...#Synthetic_sapphire [notexactly, Mar 30 2017] [7] Alumina https://en.wikipedi...iki/Aluminium_oxide [notexactly, Mar 30 2017] [8] Aluminium oxynitride https://en.wikipedi...luminium_oxynitride [notexactly, Mar 30 2017] I congratulate on an attempt to create an object of timeless perfection.
Does nature's tiny random rain have a failure say?-- wjt, Mar 31 2017 fused quarts is a nice material UV curing lamps at work are made of it. I will point out one issue the heat required to "bakeout" Mercury tends to lead to the failure mode I have with are lamps at work, you end up quite literally burning dust on to the surface of the quarts and it goes frosted getting darker and darker till it no long serves it's function as a light source but now just works as a heat source. On a second note we have mercury loss to the quarts and are lamps run temps of 900-1000c surface temps.-- dev45, Mar 31 2017 Don't get me wrong I like the Idea. +-- dev45, Mar 31 2017 Could you clean the outside of the lamp with, say, oven cleaner?-- notexactly, Mar 31 2017 Mechanical stress is a risk. what if someone plays ruler-vibrate with it on the edge of a table and it breaks?
one way around that is to make it like a ruperts drop, with the brittle part tucked inside, like a klien bottle.-- beanangel, Apr 01 2017 Byron!-- calum, Apr 01 2017 I'm not selling it as an invincible lamp, just an immortal one. Whatever invincibility it has is a side benefit.-- notexactly, Apr 01 2017 random, halfbakery