Boris Y. Moyzhes

Refrigeration by combined tunneling and thermionic emission in vacuum: Use of nanometer scale design

We consider new possibilities for cooling by vacuum tunneling. We examine a nanogeometry and find that large cooling currents can be obtained by a combination of energy selective tunneling of electrons and thermionic emission. The energy selective tunneling is a result of the special form of a potential barrier which has wider gap for low energy electrons, which results in electrons above the Fermi level being the principal tunneling component. Numerical calculations show that available material with work functions about 1.0 eV are useful for cooling. For gaps of 5–15 nm, which are well within the present state of the art, only a small external voltage (1–3 V) is required to create large currents and a useful Peltier coefficient of about 0.3, and cooling power of 100 W/cm2.

THERMOELECTRIC FIGURE OF MERIT OF METAL-SEMICONDUCTOR BARRIER STRUCTURE BASED ON ENERGY RELAXATION LENGTH

The thermoelectric figure of merit is calculated for a compound material comprising thin semiconductor and wider metallic layers. The layers are perpendicular to the direction of current. The semiconductor barriers exclude electrons with energies e<μ from the current. This exclusion increases thermopower. One may obtain a material with a very high ZT if the distance between the barriers is on the order of the energy relaxation length. This material should have the resistivity characteristic of a metal and the thermopower characteristic of a semiconductor. An additional significant rise in ZT can be achieved by increasing the contact area at the metal–semiconductor interface.

Measurements of cooling by room-temperature thermionic emission across a nanometer gap

We have completed an investigation of cooling at room temperature by thermionic emission. The use of a small nm-sized gap lowered the vacuum barrier between the electrodes, enabling emission from surfaces with work functions of ∼1 eV at room temperature. We utilized a microfabricated cantilever with a cesiated metal coating on the tip, and an integrated thermometer to initiate and control an emission current of 1–10 nA, and to detect the resulting temperature changes. Using a lock-in technique, temperature changes of 0.1–1.0 mK were observed, corresponding to cooling power of 1–10 nW. The amplitude of this signal and its dependence on emission current and bias voltage are in good agreement with our model. Possible applications for cooling and energy conversion are discussed.

Vacuum thermionic refrigeration with a semiconductor heterojunction structure

We consider possibilities for refrigeration by emission of electrons into vacuum using a semiconductor layered heterostructure and applying electric field in the order of 106 V/cm. Under the influence of a strong electric field, the height of the vacuum potential barrier is significantly reduced due to the Schottky effect and penetration of electric field into the semiconductor layer allowing high emission current. Joule heating inside the semiconductor layer can be minimized by creating a heterostructure with decreasing electron affinity from the metal–semiconductor boundary to the semiconductor–vacuum boundary. We find it is possible to obtain large Peltier currents while minimizing joule heating in the semiconductor. We find it is realistic to expect cooling of 10–100 W/cm2 at room temperature and down to 100 K by adjusting the thickness, and electron affinities of the semiconductor within practical ranges.

Qualitative understanding of the highest Tc cuprates

Abstract We use two old ideas for which layered structures are particularly relevant. The first is that the pairing can occur in different parts of the unit cell and interact symbiotically, behaving in some ways as a natural realization of the sandwich models discussed in the early days of this conference. Pairing and large current flow in the CuO2 layers are enhanced by additional pairing in other layers. The second idea is that additional pairing interactions can be due to negative U-atoms (Tl+3-Tl+1), molecules ( H g 2 + 2 − 2 H g + 2 ) , or due to interactions in the quasi one-dimensional Cu O chain layers of the YBCO-123 and 248 cuprates. Further, coulomb interactions in the CuO2 planes between pairs moving in a and b directions lead to d-wave superconductivity.

On O16–O18 Isotope Effects in Manganese Perovskites

Large isotope effects which have been found by others [1,2] in some colossal magneto resistance manganite perovskites, AMnO3, are evidence of unusually strong interactions between the lattice and magnetism. We offer a model which is based upon the approximate degeneracy of two Mn+3 states: one with high spin S = 2, and the other with low spin S = 1. These states have different radii and different electron form factors. They thus have different force constants governing the interaction with neighboring oxygen ions which provide the sought for link between magnetism and oxygen mass. The experiments can be understood with the reasonable assumption that the LS has greater force constants than HS. The dependence of changes in Tc with isotopic substitution as a function of the A-ion radius, the metal insulator transitions and the Mossbauer effect changes, are discussed in terms of this model.