1. Field of the Invention
The present invention relates to a method and a structure for heat transport, and more particulary to a method and a structure for spontaneous emission enhanced heat transport for cooling, sensing and power generation. The present invention also relates to methods and structures for cooling, sensing and power generation.
2. Discussion of the Background
For a long time the spontaneous emission of light was considered a natural and immutable property of radiating atoms. However, Purcell showed that an atom in cavity would radiate faster than an atom in free space (E. M. Purcell, Physical Review, 69, 681 (1946)). Purcell indicated that the spontaneous emission at wavelength λ will be increased by a factor f given by, f˜(λ3/a3), where a is the dimension of the cavity. For example, Purcell suggested that incorporating metal particles of 10−3 cm diameter in a matrix can cause the spontaneous emission rate at radio frequencies, 107 Hz (λ˜3×103 cm), to increase by a whopping f˜λ3/a3=(3×103)3/(10−3)3˜2.7×1019 times. Thus the thermal equilibration time constant at radio frequencies will come down from 5×1021 sec to only 3 minutes.
From Planck's radiation law, the spontaneous emission at frequency ν is derived from a probability Aν, given byAν˜[8πhν3/c3]  (1)The above coefficient Aν gets modified by the Purcell factor, f, indicated above,
Now, consider spontaneous emission at or near room temperature. Wein's Law gives the peak emission wavelength (λT) at a temperature T:
            λ      T        -          2.89      ×              10                  -          3                    ⁢                          ⁢      Km            T    ⁢                  ⁢          (              in        ⁢                                  ⁢                  K          .                    )      
For the purpose of this discussion, consider the radiative emission at this peak wavelength. The following equation is obtained:λ300k˜9.67 μm or ν300k˜3.1×1013 Hz.Compared to spontaneous emission at radio frequencies (ν˜107 Hz discussed above), the probability of the spontaneous emission (at 300K) at far-infrared wavelengths (ν˜3.1×1013 Hz) is already high from eqn. (1). It is for this reason, all bodies are observed to radiate significant amount of radiation at 300K. This is the basis for imaging using IR wavelengths.
Even though the above spontaneous emission at IR wavelengths is significantly useful for IR imaging purposes, the energy loss (dissipative transfer) from spontaneous emission of a body even at a slightly higher temperature than 300K is rather small. The energy flux (Φ) radiating from a blackbody at temperature (T) including all frequencies is given by the Stefan Boltzmann Law:
                                          where            ⁢                                                  ⁢            ɛ                    =          emissivity                ,        and                                                            σ        =                  Stefan          ⁢                                          ⁢          Boltzmann          ⁢                                          ⁢          constant                                    =                  5.672          ×                      10                          -              5                                ⁢                                          ⁢          erg          ⁢                      /                    ⁢          sec          ⁢                                          ⁢          1          ⁢                      /                    ⁢                      cm            2                    ⁢                      deg            4                                                                              =                  5.672          ×                      10                          -              12                                ⁢                                          ⁢          W          ⁢                      /                    ⁢                      cm            2                    ⁢                                          ⁢          1          ⁢                      /                    ⁢                      deg            4                              For a blackbody with ε=1 at T=300K, the following is obtained:Φ≅4.39×10−2 W/cm2  (2)
From a heat-spreader point of view, as used in many electronics and other sensitive cooling applications around 300K, this Φ is small. Hence most of the heat that is removed from the electronics (like a modern day 1.2 GHz Pentium® processor) is achieved through a convective process (blowing air across fins) or through other conductive/heat-transfer process (flowing liquid) as in many top-of-the-line servers and main-frame computer electronics. These heat transfer processes are at best modest just enough that the Pentium® chip does not overheat or that the reliability of the servers are not in doubt. However such solutions are insufficient for many future cooling applications in computer electronics operating in the 2 GHz range and above. This will be especially true if high-performance thin-film thermoelectrics are used to actively pump the heat from the chip, when the power density levels that need to be dissipated (taking into account some heat-spreading effects from the source to the spreader-sink side) can easily be in the range of several to tens of watts/cm2. See L. H. Dubois, Proc. of 18th International Conference on Thermoelectrics, 1, (1999), IEEE Press. Catalog No. 99 TH8407 and references cited in this article.
For spot-cooling of high-power electronics and high-power VCSELS, simple convective cooling processes are insufficient. Also, the above methods of cooling with blowing air or flowing liquid are cumbersome and introduce unwanted complexities to systems.