Thermoelectric figure of merit for semiconductor submicron layers

We show that in semiconductors of submicron dimensions, for certain relations between the electronic and phonon boundary conditions a heat flux may occur only through the electronic subsystem. Consequently, the thermoelectric figure of merit z does not involve the total thermal conductivity, but only its electronic component. In the ideal case of isothermal contacts, this leads to a sharp increase in z compared with massive semiconductors.Ya. A. Galan Ternopol' Pedagogical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 68–72, September, 1993.     

Thermoelectric figure of merit of a material consisting of semiconductor or metal particles

It is found that the dimensionless thermoelectric figure of merit of a material consisting of a large number of ball-shaped semiconductor or metal particles can be much more than unity. The introduction of an insulator into the space between the particles is shown to sharply increase the power of the converter of heat energy into electric current energy.     

Thermoelectric figure of merit in a quantum wire coupled to a graphene sheet between ferromagnetic leads

We theoretically investigate the figure of merit ZT for a quantum wireside-coupled by a graphene sheet and sandwiched between two ferromagnetic electrodes withnoncollinear magnetic moments. By using the nonequilibrium Green’s function combining withthe tight-binding Hamiltonian, we demonstrate that the ZT for the system developsan oscillating behavior and weakly depends on the wire-graphene coupling strength as wellas magnetic configuration of the leads. On the contrary, it is strongly dependent ontemperature and the polarization strength of the leads. Importantly, the maximum value ofZT for thesystem without the polarization strength (p = 0) is about 1.1 at temperature k _B T =0.015Γ ₀, which is in agreement with theexperimental measurements for silicon nanowires.     

Sn-based type-VIII single-crystal clathrates with a large figure of merit

Single-crystal samples of type-VIII Ba₈Ga_{16 - x}Cu_xSn₃₀ (x=0, 0.03, 0.06, 0.15) clathrates were prepared using the Sn-flux method. At room temperature the carrier density, n, is 3.5-5×10¹⁹ cm^-3 for all the samples, the carrier mobility, μ_H, increases to more than twice that of Ba₈Ga₁₆Sn₃₀ for all the Cu doping samples, and consequently the electrical conductivity is enhanced distinctly from 1.90×10⁴ S/m to 4.40×10⁴ S/m, with the Cu composition increasing from x=0 to x=0.15. The Seebeck coefficient, α , decreases slightly with the increases in Cu composition. The κ values are about 0.72 W/mK at 300 K and are almost invariant with temperature up to 500 K for the samples with x=0 and x=0.03. The lattice thermal conductivity, κ_L, decreases from 0.59 W/mK for x=0 to 0.50 W/mK for x=0.03 at 300 K. The figure of merit for x=0.03 reaches 1.35 at 540 K.     

Sn-based type-VIII single-crystal clathrates with a large figure of merit

Single-crystal samples of type-VIII Ba 8 Ga 16 x Cu x Sn 30 (x=0,0.03,0.06,0.15) clathrates were prepared using the Sn-flux method.At room temperature the carrier density,n,is 3.5-5×10～(19)cm～3 for all the samples,the carrier mobility,μ H,increases to more than twice that of Ba 8 Ga 16 Sn 30 for all the Cu doping samples,and consequently the electrical conductivity is enhanced distinctly from 1.90×10～4 S/m to 4.40×10～4 S/m,with the Cu composition increasing from x=0 to x=0.15.The Seebeck coefficient,α,decreases slightly with the increases in Cu composition.The κ values are about 0.72 W/mK at 300 K and are almost invariant with temperature up to 500 K for the samples with x=0 and x=0.03.The lattice thermal conductivity,κ L,decreases from 0.59 W/mK for x=0 to 0.50 W/mK for x=0.03 at 300 K.The figure of merit for x=0.03 reaches 1.35 at 540 K.     

Large transverse thermoelectric figure of merit in a topological Dirac semimetal

The Seebeck effect encounters a few fundamental constraints hindering its thermoelectric(TE)conversion efficiency.Most notably,there are the charge compensation of electrons and holes that diminishes this effect,and the Wiedemann-Franz(WF)law that makes independent optimization of the corresponding electrical and thermal conductivities impossible.Here,we demonstrate that in the topological Dirac semimetal Cd3As2 the Nernst effect,i.e.,the transverse counterpart of the Seebeck effect,can generate a large TE figure of merit zNT.At room temperature,zNT≈0.5 in a small field of 2 T and it significantly surmounts its longitudinal counterpart for any field.A large Nernst effect is generically expected in topological semimetals,benefiting from both the bipolar transport of compensated electrons and holes and their high mobilities.In this case,heat and charge transport are orthogonal,i.e.,not intertwined by the WF law anymore.More importantly,further optimization of zNT by tuning the Fermi level to the Dirac node can be anticipated due to not only the enhanced bipolar transport,but also the anomalous Nernst effect arising from a pronounced Berry curvature.A combination of the topologically trivial and nontrivial advantages promises to open a new avenue towards high-efficient transverse thermoelectricity.     

厚针孔射线成像品质因数的研究

定义了一种评价针孔成像性能的品质因数,给出了数学表达式.此品质因数综合反映了针孔成像的总空间分辨和灵敏度因素.基于透射模型,给出了针孔成像中心点等效直径、 点扩散函数均方根半径数学表达式.给出了全视场和有效视场下针孔成像品质因数的计算方法,分析了直孔型、刀口型、船底型和长船底型等4种典型针孔的成像特性.     

Photoacoustic figure of merit for photothermal energy conversion efficiency

The photoacoustic signal detected near the non-illuminated back surface of a photothermal converter can be used for comparative studies of the conversion efficiency of the converter. The signal is caused not only by the thermal wave transmitted through the sample, but also by mechanical vibrations of the sample itself. While no absolute absorptance or emittance values can be obtained, the signal reflects the influences of both these quantities on the conversion efficiency. Thus this kind of method may lend itself to quality control of such converters, although direct comparisons between absorbing surfaces on different substrates are still problematic.     

Enhancement of figure of merit of thermoelectric materials: a new theoretical approach

Thermoelectric materials have attained importance because of the gargantuan energy crisis the world faces today. A thermoelectric material can be used efficiently and frequently, provided, its figure of merit ZT is increased. Also, easy availability, manufacturing, and low cost are the other factors to be considered for a novel thermoelectric material. A theoretical model is proposed in this paper for the enhancement of the figure of merit of thermoelectric materials.     

Lead Phthalocyanine: A one-dimensional conductor

The temperature dependence of the electrical resistivity of lead phthalocyanine (PbPc) films and the crystal structure of PbPc suggest that PbPc is a one-dimensional conductor.     

Stochastic properties of the resistivity in a one-dimensional disordered conductor

We investigate the stochastic properties of the resistanceR and its logarithm lnR for a one-dimensional disordered conductor of finite length and at zero temperature. In the model which we consider, the non-interacting electrons are scattered by a Gaussian random potential of vanishing correlation length. It is shown that for long samples, lnR is distributed according to a Gaussian law and the parameters of this distribution are calculated explicitly. For weak disorder potentials, we recover known relations between R>, ln<R>, and ln<R ^–1>, whereas for strong disorder new results are derived.     

Non-linear conduction in a quasi one-dimensional conductor at low temperatures

The current-voltage characteristics of a quasi one-dimensional organic system having asymmetric donor molecule like Qn-(TCNQ)₂ pellet with stoichiometry 1:2 grown from acetonitrile as a solvent have been studied atT=69 K. The characteristic curves show a pronounced deviation from ohmicity beyond a certain value of current. For higher values of currents a negative differential resistance region is observed.     

Design of dispersion-compensating Bragg fiber with an ultrahigh figure of merit

We report a novel idea for achieving highly efficient dispersion-compensating Bragg fiber by exploiting a modified quarter-wave stack condition. Our Bragg fiber yielded an average dispersion of approximately -1800 ps/(nm km) across the C band for the fundamental TE mode and an ultrahigh figure of merit of approximately 180,000 ps/(nm dB), which is at least 2 orders of magnitude higher than that of conventional dispersion-compensating fibers. The proposed methodology could be adopted for the design of a dispersion compensator across any desired wavelength range.     

On the enhancement of the figure of merit in bulk nanocomposites

In this paper we make a detailed comparison of the thermoelectric properties of quantum dot superlattices with those of equivalently doped bulk material and show that a major contribution to the enhancement of the figure of merit comes from the increase of the thermoelectric power over that of bulk, in addition to the lattice thermal conductivity reduction in quantum dot superlattices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoelectric properties of one-dimensional graphene antidot arrays

We investigate the thermoelectric properties of one-dimensional (1D) graphene antidot arrays by nonequilibrium Green?s function method. We show that by introducing antidots to the pristine graphene nanoribbon the thermal conductance can be reduced greatly while keeping the power factor still high, thus leading to an enhanced thermoelectric figure of merit (ZT). Our numerical results indicate that ZT values of 1D antidot graphene arrays can be up to unity, which means the 1D graphene antidot arrays may be promising for thermoelectric applications.     

Enhancement of conductivity due to local disorder in a one-dimensional conductor

We theoretically investigate electron transport in a one-dimensional conductor with a locally disordered potential by using the non-equilibrium Green's function theory. It is found that, by changing the energy of a site in a one-dimensional atomic chain, the electron conductivity can be larger when the modulated site energy is smaller than that of the other sites. This contradicts the conventional picture that an electron is scattered by the disorder of the potential, because such a scattering process usually causes resistivity. We show that the enhancement of conductivity that seems contradictory to the conventional picture of electron motion is explained by the change of energy of quasi bound states in the conductor.     

Chirped Bragg光纤光栅的优化设计与优化指数

根据线性ChirpedBragg光纤光栅的一般原理 ,探讨了不同啁啾系数 ,准高斯耦合函数对Bragg反射光栅的反射率及色散补偿特性的响应。结果表明 :增大啁啾系数以及选取适当的准高斯耦合函数 ,在保证反射率较大的情况下 ,能有效地改善色散补偿特性。通过引入微波领域的“优化指数”这一概念 ,量化了色散补偿器的补偿能力。     

Coupling-induced spectral splitting for plasmonic sensing with ultra-high figure of merit

We investigate a kind of spectral splitting effect in a plasmonic multilayer system, which consists of stacked Al_2 O_3 and SiO_2 layers, a thin metal film, and a dielectric prism substrate. The results illustrate that an obvious peak appears in the center of the surface plasmon resonance(SPR)-induced reflection spectral dip in the structure with the SiO_2/Al_2 O_3/SiO_2 layers. This spectral splitting response can be regarded as an electromagnetically induced transparency(EIT) like effect,which is attributed to the coupling and interference between the SPR on the metal film and guided-mode resonance(GMR)in the Al_2 O_3 layer. The theoretical calculations agree well with the numerical simulations. It is also found that the reflection spectrum will be further split by the introduction of another Al_2 O_3 layer into the multilayer structure. The reintroduced GMR in the Al_2 O_3 layer changes the coupling and interference process between the SPR and GMR field, giving rise to the generation of ultra-narrow reflection dip. Especially, the spectral splitting can facilitate the realization of plasmonic sensors with ultra-high figure of merit(583), which is about 5 times larger than that of traditional SPR sensors. These results will provide a new avenue to the light field manipulation and optical functionalities, especially biochemical and environmental sensing.