Thermal Backfill Research Articles

Oxidation of TiAl surface with hyperthermal oxygen molecular beams

We report results of our detailed studies on the initial oxidation process of TiAl with a 2eV hyperthermal oxygen molecular beam (HOMB) and thermal O2 in the backfilling. The oxidation processes are monitored by X-ray photoemission spectroscopy (XPS) measurements in conjunction with synchrotron radiation (SR). Oxidation of both Al and Ti occurs during the oxidation. The incident-energy and surface-temperature dependences of oxidation reveal that the precursor-mediated dissociative adsorption is the dominant initial step of oxidation in the thermal O2 backfilling. Thus, the efficiency of oxidation is higher for the thermal O2 backfilling than for the HOMB dose. The result is quite different from that on TiNi where the HOMB dose has the advantages in oxide layer growth at high O coverage. We succeeded in fabricating blue colored TiO2 and Al2O3 containing layers, combining HOMB and surface annealing.

Non-equilibrium Green’s function calculation for GaN-based terahertz-quantum cascade laser structures

We theoretically investigated GaN-based resonant phonon terahertz-quantum cascade laser (QCL) structures for possible high-temperature operation by using the non-equilibrium Green’s function method. It was found that the GaN-based THz-QCL structures do not necessarily have a gain sufficient for lasing, even though the thermal backfilling and the thermally activated phonon scattering are effectively suppressed. The main reason for this is the broadening of the subband levels caused by a very strong interaction between electrons and longitudinal optical (LO) phonons in GaN.

Wall-plug efficiency of mid-infrared quantum cascade lasers

We present a comprehensive analysis of the wall-plug efficiency of mid-infrared (λ ∼ 4–10 μm) quantum cascade (QC) lasers, by taking into account the transition efficiency of the active region, the light out-coupling efficiency from the emission facet, the voltage efficiency across the superlattice, and the current efficiency including the leakage and thermal backfilling current densities at threshold. All relevant parameters including their interdependencies have been taken into account and analyzed in order to optimize the wall-plug efficiency of practical QC lasers. Expressions for optimizing the extraction energy Δinj and the mirror loss αm are deduced, which are then used to maximize the wall-plug efficiency of QC lasers. Finally, predictions for the wall-plug efficiency of realistic QC lasers are made both at room temperature and at low temperature. Good agreement is found with corresponding experimental data from the literature.

Temperature dependence of electron transport on a bound-to-continuum terahertz quantum cascade laser

We investigate the temperature effects on electron transport in a bound-to-continuum terahertz quantum cascade laser using Monte Carlo simulation which includes electron–electron and electron–phonon scattering. The effects of carrier transport paths and mechanisms, especially for those related to laser levels, on the device's temperature performance are evaluated. The simulation shows that the parasitic leakage of carriers from the upper laser level and the thermal backfilling to the lower laser level is the main limiting factor for high-temperature operation. The calculations are in good agreement with experimental results.

프리스트레스트 콘크리트 거더 일체식 교량의 교대 거동 해석과 예측

이 연구는 교량의 생애 동안의 온도 변화와 콘크리트의 시간 의존 영향을 고려하여 PSC 거더 일체식 교량의 해석 방법과 교대의 변위를 예측하는 모델 개발에 관한 것이다. 비선형 수치 해석 모델은 지반-구조물의 상호작용을 고려하며, 재료의 비선형 또한 고려되었다. 개발된 수치 해석 모델을 이용하여 총 243가지의 경우에 대하여 변수 연구를 하였다. 고려된 변수는 (1) 열팽창 계수, (2) 교량 길이, (3) 뒤채움재의 높이, (4) 뒤채움재의 강성, 그리고 (5) 말뚝-지반 강성이다. 변수 연구의 결과는 열팽창 계수, 교량 길이, 말뚝-지반의 강성이 지배적인 영향을 나타내는 것으로 드러났다. 또한, 교량의 길이는 교대의 윗부분의 변위에 지배적인 영향을 미치며 자유팽창·수축과 유사하였다. 하부의 변위에는 다른 변수들의 영향으로 추정이 쉽지 않았다. 개발된 교대의 변위 추정 모델은 기본 설계시에 사용될 수 있을 것이다.

Integral abutment bridge response under thermal loading

This paper presents a parametric study and approximate integral abutment bridge (IAB) response prediction models. IABs are complex structures due to the nonlinearity and uncertainties in bridge materials and soil boundaries. Current design specifications and guides do not provide clearly defined analysis methods; therefore, there is a need for easily implemented preliminary analysis and accurate two-dimensional (2D) analysis methods. Based on a calibrated, nonlinear, 2D numerical modeling methodology including backfill–abutment interaction, soil–pile interaction and critical construction joints, a parametric study of 243 analysis cases was performed, consistent with the AASHTO 75-year bridge life. Analyses were performed under thermal load and gradient, and backfill pressure with time-dependent effects of concrete creep and shrinkage and prestressing steel relaxation included. The parametric study considered five parameters: (1) thermal expansion coefficient, (2) bridge length, (3) backfill height, (4) backfill stiffness, and (5) pile soil stiffness. Each of the five parameters was evaluated at three distinct magnitudes to cover the normal range of bridge construction. The parametric study revealed that the thermal expansion coefficient, bridge length and pile soil stiffness significantly influence the IAB response as measured by: (1) girder axial force, (2) girder bending moment, (3) pile lateral force, (4) pile bending moment, and (5) pile head/abutment displacement. The influences of backfill height and backfill stiffness are not relatively significant. The study results provide practical, preliminary estimates of the bridge response and ranges for preliminary IAB design and analysis.

Steric Effects in Dissociative Adsorption of Low-Energy CH3Cl on Si(100): Orientation and Steering Effects

We report the rotational-state dependence of the sticking probability and a study of the steric effect for the 65 meV CH3Cl incidence on Si(100). We found that the steric effects are quite small and independent of the rotational state of |J,K,M〉 = |111〉 or |212〉 at such a low incident energy as the thermal backfilling condition. The steering effects smear the orientation effects upon the system being trapped into a precursor state.

N-Type Ge–SiGe Quantum Cascade Structure Utilizing Quantum Wells for Electrons in the $L$ and $\Gamma$ Valleys

In this letter, we propose an n-type vertical transition bound-to-continuum Ge-SiGe quantum cascade structure utilizing electronic quantum wells in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> and Gamma valleys of the Ge layers. The optical transition levels are located in the quantum wells in the L valley. Under a bias of 80 kV/cm, the carriers in the lower level are extracted by miniband transport and L -Y tunneling into the subband in the Gamma well of the next period. And then the electrons are injected into the upper level by ultrafast intervalley scattering, which not only effectively increases the tunneling rate and suppresses the thermal backfilling of electrons, but also enhances the injection efficiency of the upper level. The performance of the laser is discussed.

High temperature operation of λ≈3.3μm quantum cascade lasers

We report quantum cascade lasers emitting at a wavelength near 3.3μm up to a temperature of 400K in pulsed mode. A fine tuning of the InAs∕AlSb active region design allowed to maintain efficient electron injection and to reduce thermal backfilling at high temperatures. The threshold current density is 3kA∕cm2 at 300K with characteristic temperature T0 of 175K. The lasers emitted up to 1W peak power in a single lateral mode at room temperature.

High-Performance Quantum Cascade Lasers: Optimized Design Through Waveguide and Thermal Modeling

We present a comprehensive model to study the thermal effects in quantum cascade (QC) lasers for continuous-wave (CW) operation at and above room temperature. This model self-consistently solves the temperature-dependent threshold current density equation and heat equation to determine the CW threshold current density, maximum heat sink temperature, and core temperature at threshold for a given laser design. The model includes effects from temperature dependence on thermal backfilling, thermal conductivity, phonon lifetimes, gain bandwidth, thermionic emission, and resistive heating in waveguide layers. Studies on these effects yield results not simultaneously considered by previous models. By including these results in laser designs, lasers with lower core temperatures, with higher operating temperatures, and requiring lower electrical power than current high-performance lasers are predicted. Additionally, experimental results are presented, exploring various methods of improving CW laser performance for a lambda ~ 8 mum QC laser and are compared to the model.

Terahertz quantum cascade lasers with double-resonant-phonon depopulation

We present two different terahertz quantum cascade laser (QCL) designs based on GaAs∕Al0.3Ga0.7As heterostructures that feature a depopulation mechanism of two longitudinal-optical phonon scattering events. This scheme is intended to improve high temperature operation by reducing thermal backfilling of the lower radiative state. The better of these two devices displays a threshold current density of 170A∕cm2 at 5K and lases up to 138K in pulsed mode and 105K in continuous-wave mode. However, contrary to expectation, we observed no improvement in temperature performance compared to single-resonant-phonon designs, which suggests that the thermal backfilling is not yet a limiting factor for high temperature terahertz QCL operation.

Terahertz quantum-cascade lasers based on an interlaced photon-phonon cascade

A THz (λ∼80 μm) quantum-cascade laser utilizing alternating photon- and phonon-emitting stages has been developed to achieve efficient extraction of electrons from the lower laser level. Thermal backfilling of electrons is drastically reduced leading to an operation up to 95 K and a weak temperature dependence of the power versus current slope efficiency. The threshold current density is 280 A cm−2 at 6 K and increases to 580 A cm−2 at 90 K. Peak output powers of 10 mW at 30 K and 4 mW at 80 K are obtained.

Intersubband staircase laser

A quantum-cascade laser is presented in which the injection minibands are omitted. The intersubband laser operates as a four-level staircase and has potential for higher gain devices due to an increased confinement factor compared to conventional quantum-cascade lasers. A high threshold current density of 5.9 kA/cm2 with peak output powers of 230 mW at 77 K is observed. A large wavelength shift from 9.3 to 10 μm and broadband emission are observed at higher bias fields. This modification to existing quantum-cascade laser designs has potential for higher gain devices, but problems with thermal backfilling and doping limit the operating temperature currently to 200 K.

Thermal characteristics of a class F fly ash

It is necessary to evaluate thermal properties of geotechnical materials, viz. soils and rocks, for power cables and oil pipe lines and disposal of nuclear wastes. A situation may arise where the heat dissipation through the soils and rocks may not be satisfactory, leading to adoption of a backfill. Fly ash can be used in conjuction with aggregates to design a proper backfill material. The thermal characteristics of this material, in terms of its resistivity, play an important role in the design of a thermally stable backfill, i.e., Fluidized Thermal Backfill (FTB). As such it is desired to establish thermal response of the fly ash based on its physical and moisture- holding characteristics. In this paper an effort has been made to evaluate the thermal resistivity of a Class F fly ash using a laboratory thermal needle/probe. The effect of density of compaction and the moisture content on the thermal response of the fly ash has been also studied.

Transient ratings of buried power cables. I. Historical perspective and mathematical model

The authors report the results of a recently completed major project on power cable transient calculations. It used a fundamental analytical method using a lumped parameter model for computation of transient ratings of buried power cables. An account is given for past work in the field of thermal transients in buried cables. A full description of the mathematical model is given. A summary of the state-of-the-art is provided. Developments relevant to the analysis of groups of unequally loaded and/or dissimilar cables and cables located in ductbanks or thermal backfills are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A HYBRID FINITE ELEMENT — BOUNDARY ELEMENT METHOD (FEM‐BEM) FOR THERMAL ANALYSIS IN ENERGY INSTALLATIONS

This paper presents a hybrid finite element — boundary element method for the steady state thermal analysis of energy installations. The coupling of the two techniques is presented: finite elements are used in a bounded region containing thermal sources while the complementary domain is treated with boundary elements. With such a combination the number of unknowns is reduced and an accurate prediction of temperature is obtained. As an example, the temperature rise is computed for the case of three power cables laid in a thermal backfill: the finite element method (FEM) is used for the cables and the backfill while the homogeneous soil is taken into account with the boundary element method (BEM).

Heat Transfer of Buried Pipe for Heat Pump Application

It is generally felt that the application of line source theory for ground coil design usually resulted in excessive overdesign. It was anticipated that in order for the ground coil heat pump systems to be economically competitive with other residential heating and cooling systems, ground coil overdesign had to be kept to a minimum. A new ground coil model was derived, which based on energy balance rather than the traditional line source theory. It was aimed to more accurately predict the operation of ground coils. It is the intention of this study to compare this ground coil model with models based on line source theory, a simple line source model and a modified line source model, by using them to simulate the same field test data for both summer and winter ground coil operations. The results indicated that for winter coil operation, the new model predicted the coil liquid exit temperature less than 2°C maximum deviation from the measured values, with an average deviation less than 1°C. The modified line source model had an average deviation of more than 1.5°C. For summer operation, all models underpredicted the measured soil temperatures because the effect of thermal backfill material was not included in the models. The new model still predicted the test results better than the other two models. However, when the effect of sand thermal backfill was included in the new model, which was not easy for the other two models, the calculated soil temperatures were almost identical to the test results.

A unit cell model for predicting the thermal conductivity of a granular medium containing an adhesive binder

A unit cell model is proposed which can be used to predict the effective thermal conductivity of a granular medium composed of sand particles, a thermal binder, and air. The unit cell model is described by three dominant thermal conductances which account for the heat transport across the interfaces among the components comprising the medium. The stereological concept of contiguity is used to quantitatively link the macroscopic effective thermal conductivity to the microscopic interfacial contact area among the components. Utilizing a geometric model of a representative contact between two particles, an analytical expression for the contiguity is developed. This expression is used in the unit cell model to calculate the effective thermal conductivity of the medium as a function of the thermal conductivity of the components and their volumetric concentration. The predictive accuracy of the unit cell model for sand-like media is verified with numerous quantitative microscopic measurements and thermal conductivity data obtained with the transient, cylindrical probe technique. The unit cell model is applied to the design of thermal backfill materials. Both analysis and experiment demonstrate that thermally coupling contiguous sand particles with a binder can significantly increase the effective thermal conductivity of dry sand. Four basic microstructure regimes of the medium are identified, and these regimes are quantitatively related to the volume fraction of the binder. The greatest increase in the effective thermal conductivity with respect to the volume fraction of the binder is shown to occur in a regime where the sand particles become thermally coupled with the binder. The unit cell model identifies quantitative guidelines for the thermal conductivity and volume fraction of the binder that can be used to produce the greatest increase in the effective thermal conductivity of dry sand. Thus, the results of this study have practical application to the design and development of thermal backfill materials for increasing the heat transfer between the earth and underground heat sources such as buried electric power cables and ground source heat pump coils.

Optimization of Power Cable and Thermal Backfill Configurations

The design problem of selecting the optimal parameter values associated with cable trench and thermal backfill is formulated as a nonlinear programming problem and solved using a multi-dimensional gradient optimization method. The technique considers all design parameters simultaneously and provides the optimal solution which either minimizes the cost of backfill and trench production subject to an ampacity constraint or maximizes the cable ampacity subject to a cost constraint. In either formulation, upper and lower bounds on design parameters as well as other thermal and physical constraints are included. The important role of backfill parameters in reducing the effects of environmental fluctuations on cable ampacity is exploited in the optimization analysis via defining functional sensitivity profiles which may be used to ensure secure and reliable designs. The paper includes a description of the computer program which was developed for the optimization of general cable configurations in thermal backfill.

Optimization of Power Cable and Thermal Backfill Configurations

Optimization of Power Cable and Thermal Backfill Configurations