Adhesive failure of sandwich structures under combined fatigue and thermal loading

Adhesive failure of a sandwich beam made of steel and PTFE is studied under thermal and vibration environment. Superposed stresses model is presented in order to predict the effect of the combination environment. Then, the stress distributions are evaluated by an experimental method. Results show that the proposed model is effective for the study of thermal stress. The stress on the steel beam in the initial crack region is slightly low under thermal environment. Vibration excitation generates cyclic loading on the beam and stress distribution exhibits sensitivity to location and loading frequency. It is observed that the high-stress peak will be provided in combined environment. The fatigue crack growth is monitored, and it is shown that the adhesive failure is strongly temperature-dependent with invariable dynamic excitation. High temperature leads to both the high value of stress intensity and an increase in fatigue. Moreover, stick-slip behavior is observed at the interface of adhesive and base plate.     

Crosslinkable fluorinated hyperbranched polyimide for thermo‐optic switches with high thermal stability

A series of fluorinated hyperbranched polyimides (FHBPIs) were synthesized by condensation of a triamine monomer, 1,3,5‐tris(2‐trifluoromethyl‐4‐minopheoxy) benzene (TFAPOB) and various aromatic dianhydride monomers with different linear length for application on integrated optical devices. Near infrared absorption measurement shows that it has high transparency in optical communication wavelength region. The glass transition temperature and thermal decomposition temperature were 189°C and 596°C, respectively. According to the atomic force microscopy analysis, the surface roughness of the FHBPI films is 0.208 nm. A classic Mach–Zehnder interferometer thermo‐optic switch with single mode waveguide fabricated by FHBPIs represents excellent switching characteristic. The rise time and fall time of this device are 530 µs for both. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Nonlinear control of rapid thermal chemical vapor deposition under uncertainty

This article focuses on nonlinear control of a rapid thermal chemical vapor deposition (RTCV'D) process in the presence of significant model uncertainty and disturbances. Initially, a detailed mathematical model of the RTCVD process is presented consisting of a nonlinear parabolic partial differential equation (PDE) which describes the time evolution of the wafer temperature across the radius of the wafer, coupled with a set of nonlinear ordinary differential equations (ODEs), which describe the time evolution of the concentrations of the various species. Then, the synthesis of a nonlinearoutput feedback controller based on the RTCVD process model by following a control methodology for nonlinear parabolic PDE systems introduced in (Baker and Christofides, 1998) is discussed. The controller uses measurements of wafer temperature at four locations to manipulate the power of the top lamps in order to achieve uniform temperature, and thus, uniform deposition of the thin film on the wafer over the entire process cycle. The nonlinearoutput feedback controller is successfully implemented through computer simulations and is shown to attenuate significant model uncertainty end disturbances and to outperform a proportional integral (PI) control scheme.     

Designability and thermal stability of protein structures

Only about 1000 qualitatively different protein folds are believed to exist in nature. Here, we review theoretical studies which suggest that some folds are intrinsically more designable than others, i.e. are lowest energy states of an unusually large number of sequences. The sequences associated with these folds are also found to be unusually thermally stable. The connection between highly designable structures and highly stable sequences is generally known as the ‘designability principle’. The designability principle may help explain the small number of natural folds, and may also guide the design of new folds.     

Fabrication and thermal shock behavior of periclase-forsterite aggregates with micro-nanometer dual-pore-size structures

To improve the service life of periclase-forsterite refractories, it is important to develop aggregates with high thermal shock resistance. In this study, periclase-forsterite aggregates with good resistance to thermal shock and micro-nanopores were prepared using high-silicon magnesite, silica, and silica sol. Microcracks were generated in the multiphase aggregates, which inhibited the continuous propagation of cracks during thermal shock through mismatched thermal expansion coefficients. Based on Hasselman's thermal shock stability factor, the reduction in the average thermal expansion coefficient and improved mechanical characteristics were critical factors in improving the thermal shock resistance of the multiphase aggregates. As a binder, silica sol provided nano-SiO₂ and superplasticity, which facilitated the formation of micro-nanopores and strengthened the combination of the various phases in the aggregates.     

Evaluation of two new white silicone thermal control paints under atomic oxygen

The exterior optical surfaces of satellites are directly exposed to the harsh space environment. Thermal control paints are resistant to the conditions encountered at low earth orbit (LEO): vacuum, atomic oxygen, thermal cycling, and ultraviolet radiation. In this paper two white paints were prepared by ultrasonic dispersing method for application to space structures. The white paints include zinc-oxide-pigmented silicone elastomer SilGel 612 (U1) and a zinc-oxide-pigmented silicone elastomer RT604 (U2). These coatings are electrically non-conductive. We present the behavior of thermal control paints under atomic oxygen (AO). The effects of AO exposure were studied by the mass loss of paint specimens and complementary techniques including scanning electron microscopy (SEM) and atomic force microscopy (AFM). Also the optical degradation in the reflectance spectra, solar absorptance and thermal emittance for U1 and U2 samples before and after AO exposure were investigated. The paints have promising AO resistance properties that could be suitable for space applications.     

Comparison between tandel effect, and thermal capacity behaviour under alternative electric field

The behaviour of the thermal capacity of a LATGS crystal under an alternative field is compared with the Temperature Auto, stabilizing Non-Linear Dielectric Element (TANDEL) effect.

Assuming that the dissipation due to dielectric losses linearly depends on the temperature, the heat conduction equation in the sample is studied and the estabilization of its average temperature is expressed in terms of the dissipation and the thermal properties of the sample.     

Design and control of transesterification reactive distillation with thermal coupling

In the study, the design and control strategies of a reactive distillation process with partially thermal coupling for the production of methanol and n-butyl acetate by transesterification reaction of methyl acetate and n-butanol are investigated. Since methanol and methyl acetate formed an azeotrope, the products of a reactive distillation column include n-butyl acetate and the mixture of methanol and methyl acetate, which must be separated by an additional column. Partially thermal coupling can be used to eliminate the condenser of the second column. Not only energy reduction but also better operability and controllability can be obtained for the thermally coupled reactive distillation process. Proper selection and pairing of controlled and manipulated variables chosen for three control objectives were determined by using steady-state analysis. A simple control scheme with three temperature control loops is sufficient to maintain product purities and stoichiometric balance between the reactant feeds.     

Adversarially robust Bayesian optimization for efficient auto-tuning of generic control structures under uncertainty

The performance of optimization- and learning-based controllers critically depends on the selection of several tuning parameters that can affect the closed-loop control performance and constraint satisfaction in highly nonlinear and nonconvex ways. Due to the black-box nature of the relationship between tuning parameters and general closed-loop performance measures, there has been a significant interest in automatic calibration (i.e., auto-tuning) of complex control structures using derivative-free optimization methods, including Bayesian optimization (BO) that can handle expensive unknown cost functions. Nevertheless, an open challenge when applying BO to auto-tuning is how to effectively deal with uncertainties in the closed-loop system that cannot be attributed to a lumped, small-scale noise term. This article addresses this challenge by developing an adversarially robust BO (ARBO) method that is particularly suited to auto-tuning problems with significant time-invariant uncertainties in an expensive system model used for closed-loop simulations. ARBO relies on a Gaussian process model that jointly describes the effect of the tuning parameters and uncertainties on the closed-loop performance. From this joint Gaussian process model, ARBO uses an alternating confidence-bound procedure to simultaneously select the next candidate tuning and uncertainty realizations, implying only one expensive closed-loop simulation is needed at each iteration. The advantages of ARBO are demonstrated on two case studies, including an illustrative problem and auto-tuning of a nonlinear model predictive controller using a benchmark bioreactor problem.     

Complete control and strengthening over iron oxide porous structures freeze cast under oscillating magnetic fields

Porous scaffolds can be utilized in a variety of biomedical as well as mechanical applications. The process of freeze casting is a successful method to fabricate these porous structures but with ideal characteristics in only one direction (the ice-growth direction). The application of magnetic fields led to an increase in both the microstructural control and mechanical strength in an additional orthogonal direction. The application of these weak, uniform fields (≤20 mT), in particular oscillating fields from a Helmholtz coils setup, has led to increases in mechanical strength through microstructural alignment in multiple material types. However, structures fabricated from these uniform fields have primarily been compared to each other, with little research comparing them to structures fabricated under strong, non-uniform fields from permanent magnet setups. Therefore, iron-oxide scaffolds were fabricated under weak, uniform fields (≤20 mT) as well as strong, non-uniform fields (≥20 mT), and their mechanical and microstructural properties were compared to one another. The application of weak, uniform fields led to superior mechanical properties compared to those produced from the application of strong, non-uniform fields, no distortions in the physical structure of the freeze-cast scaffold, and the best microstructural alignment ever seen in freeze-cast structures.     

Flammability and thermal properties of rigid polyurethane foams with additionally introduced cyclic structures

Flammability, smoke evolution, thermal, and thermomechanical properties of low-density rigid polyurethane foams obtained from different aromatic polyols were investigated. The foams were prepared according to a standard formulation ensuring the same foam phosphorus content. Cellular polyurethanes with the best fire resistance were obtained from polyols containing disubstituted naphthalene and biphenyl rings. A linear equation was proposed to describe the influence of various structural units of the polyurethane (the content of cyclic structures C_c, nitrogen content C_N, and crosslinking equivalent M_c) upon its flammability, expressed in terms of its oxygen index (OI) Thermal stability of crosslinked polyurethanes was not found to influence significantly their thermomechanical properties, while crosslink density and the type and quantity of cyclic structures additionally introduced did have a pronounced effect upon these properties.     

Stereolithography additive manufacturing of multi-ceramic triangle structures with tunable thermal expansion

Ceramics and their structures with tunable thermal expansion are deemed necessary to develop. In this study, multi-ceramic structures with tunable thermal expansion, including negative thermal expansion (NTE), zero thermal expansion (ZTE), and positive thermal expansion (PTE), were developed for the first time. Firstly, multi-ceramic triangle structures with tunable thermal expansion were designed based on ZrO₂ and Al₂O₃ ceramics. Then, multi-ceramic triangle structures were fabricated by using the stereolithography additive manufacturing technology. Functionally gradient ceramic (FGC) layers were applied for releasing the thermal mismatch stress among the structure. Finally, the thermal expansion behavior of the multi-ceramic triangle structures were characterized and analyzed by using a homemade thermal expansion testing system. The proposed design, fabrication and characterization methods of multi-ceramic structures with tunable thermal expansion are believed to promote the engineering application.     

三类随机分形结构下干土壤有效热导率的介观研究

地埋管换热器的换热能力是设计地源热泵系统的关键,而环境土壤的有效热导率是影响地下传热量的重要参数。为探究土壤的介观结构参数对有效热导率的影响,提出三类随机分形结构,并结合格子玻尔兹曼方法,对土壤类多孔材料的传热特性进行了基础研究。通过对热探针实验结果和三类重构结构下模拟结果的对比分析,发现孔隙率仍然是影响干土壤有效热导率的主要因素,分形维度数和粒径比的影响则较小;干土壤介观结构的随机性对有效热导率有较大的影响,随机颗粒分布的微小变化会导致差异高达到24.5%。     

Novel ZrP2O7 ceramic foams with controllable structures and ultra-low thermal conductivity

This study demonstrated the synthesis of novel zirconium pyrophosphate (ZrP₂O₇) ceramic foams via a two-step method using a foam casting technique. The synthesised foams functioned as thermal insulators with a highly controllable performance. We investigated the effects of the addition of foaming and thickening agents as well as the solid content of the slurries on the slurry, mechanical properties, thermal conductivities, and microstructure of ZrP₂O₇ ceramic foams. The ZrP₂O₇ ceramic foams synthesised at 1473 K exhibited a porosity, compressive strength, and thermal conductivity of 75.2–89.1 %, 1.95–0.02 MPa, and 0.144–0.057 W/(m K) (298–573 K), respectively. The increase in the porosity to >60 % will facilitate applications based on the low thermal conductivities of the foams.     

Thermal control in graphene nanoribbons: thermal valve, thermal switch and thermal amplifier

We study the thermal transport in graphene nanoribbons by using nonequilibrium molecular dynamics simulations. It is reported that the three-terminal graphene nanoribbons can perform some functions of thermal devices such as thermal valve, thermal switch and thermal amplifier. Electronic devices have transformed almost all aspects of our lives. It has not escaped our attention that the graphene nanoribbons we have presented here may have similar surprising applications in devices that allow the flow of heat to be controlled in a short future.     

Gunited coating with high thermal conductivity for water-cooled structures in steel works

Conclusions The authors have devised a composition for gunited coatings with high thermal conductivity for protection of water-cooled structures in the linings of steel smelting systems at the points of contact with the slag. They show that addition of 20 or 40 wt. % of a heat-conductive filler, consisting of cast iron pellets measuring 2–5 mm, to magnesia guniting material increases the thermal conductivity of the gunited coatings by an average factor of 1.3 or 1.6, respectively.The dependence of the thermal conductivity of the coatings on the bulk concentration of pellets is satisfactorily represented by the equations of Euchen and Odelevskii, which were derived for a two-phase system consisting of a continuous matrix with randomly distributed inclusions of spherical particles. High-thermal-conductivity gunited coatings were tested in service in a water-cooled screen in a vertical channel of a 500-ton open-hearth furnace at the Zaporozhstal' works.Translated from Ogneupory, No. 2, pp. 31–36, February, 1979.     

Novel aluminum borate foams with controllable structures as exquisite high-temperature thermal insulators

In this contribution, a manageable foam-casting technique for the preparation of novel aluminum borate foams (ABFs) as thermal insulators with highly controllable performances is presented. ABFs were fabricated from α-Al₂O₃ and 2Al₂O₃·B₂O₃ with the addition of various amounts of foaming agents, thickening agents, and slurry solid contents. The dispersions and rheological properties of the slurries were then examined, followed by exploration of microstructural evolution and testing of mechanical/thermal properties. It should be noted that the generated micro-pores generated and interlocking rod-like 9Al₂O₃·2B₂O₃ crystals may lead to superior mechanical tolerances and lower thermal conductivities for the ABFs. In general, the as-prepared ABFs with porosities ranging from 73.8 to 96.3 vol%, compressive strengths of 8.20–0.15 MPa, and thermal conductivities of 0.228–0.046 W/(mK) (200–800 °C) could render them suitable for application as high-temperature thermal insulating materials.     

载人航天器环热控一体化仿真分析

采用系统建模及仿真方法搭建了一种典型载人航天器环热控一体化系统模型,分析了系统的性能。针对3人7天载人飞行工况开展了仿真分析,结果表明,经过合理设计,该系统可将舱内温湿度、压力、氧分压等参数控制在航天医学指标要求范围内。环热控系统仿真结果较好地预测了系统工作过程,显示了主要参数的变化情况,结果合理,验证了仿真方法、系统仿真模型的正确性。通过控制流体回路外回路旁通阀门开度,可准确控制外回路控温点温度,保证舱内温湿度在合理范围之内。此外,外回路控温点的设定会对环热控系统状态带来影响,通过合理设计外回路控温点,可保证舱内温湿度满足航天医学指标要求。     

Chemical structures and properties of low thermal expansion coefficient polyimides

The relationships, between chemical structures of various aromatic polyimides and their thermal expansion coefficients, were investigated and the properties of low thermal expansion polyimides were elucidated. Such low values were observed for polyimides obtained from pyromellitic dianhydride or 3,3′,4,4′-biphenyltetracarboxyIic dianhydride and aromatic diamines, which included only benzene or pyridine rings fused at para-positions without a flexible linkage. It was proposed that these low thermal expansion coefficients were related to the linearity of their polymer molecular skeletons. In particular, PIQ-L100 (Hitachi Chemical Co. Ltd) is one such low thermal expansion polyimide and, it has excellent mechanical properties, thermal stability, and low absorbed moisture content.     

Designing and temperature sensing characteristics of upconversion luminescence core-shell structures with negative and positive thermal expansion

Generally, lanthanum ions doped positive expansion and negative expansion materials exhibit thermal quenching and enhancement of upconversion luminescence (UCL), respectively. Combining the UCL characteristics of positive expansion and negative expansion lattices is of importance for developing efficient temperature sensing systems. Here, positive expansion TiO₂:Yb³⁺, Er³⁺ three dimensionally ordered macroporous film was prepared by the template-assisted approach, and the Yb₂W₃O₁₂: Er³⁺ solution was filled into the TiO₂: Yb³⁺, Er³⁺ three dimensionally ordered macroporous film. After secondary sintering, the shell of negative expansion Yb₂W₃O₁₂: Er³⁺was formed on the surface of TiO₂:Yb³⁺/Er³⁺ core. Under 980 nm excitation, the red and green UCL is predominate for the spectra of TiO₂:Yb³⁺/Er³⁺ core and Yb₂W₃O₁₂: Er³⁺ shell, respectively. With the measurement temperature increasing, the green UCL from negative expansion Yb₂W₃O₁₂: Er³⁺ shell increases, while the red UCL from positive expansion TiO₂:Yb³⁺, Er³⁺ core decreases. The performance of temperature sensing was characterized by the monitoring the UCL intensity ratio between 525 nm and 660 nm. The temperature sensitivity is about 1.12% K^?1, which is larger than that of thermally coupled FIR technology. We believed that the present work is instructive for developing new generation temperature sensor.