Thermal Expansion Characteristics Research Articles

Design methodology for porous composites with tunable thermal expansion produced by multi-material topology optimization and additive manufacturing

Abstract To realize negative thermal expansion (NTE), porous composites made of two materials with different coefficients of thermal expansion are being actively researched. NTE can be realized by taking advantage of the thermal deformation mechanisms of a composite material's internal geometry. However, in addition to negative thermal expansion, materials with anisotropic and large positive thermal expansion are also desirable for various applications. Also, additive manufacturing provides new ways to fabricate composites by layering multiple materials at arbitrary points in three-dimensional space. In this study, we developed a design methodology for porous composites, which showed defined thermal expansion characteristics, including negative and positive thermal expansion as well as isotropic and anisotropic thermal expansion. Our approach was tested based on the fabrication of a multi-material photopolymer by additive manufacturing. The internal geometries required to produce such characteristics were designed by topology optimization, which is the most effective structural optimization method for realizing macroscopic inward deformation and for maintaining stiffness. The designed structures were converted to three-dimensional models and fabricated by multi-material photopolymer additive manufacturing. Using laser scanning dilatometry, we measured the thermal expansion of these specimens, revealing well-ordered thermal expansion, from anisotropic positive thermal expansion to anisotropic negative thermal expansion, over a wide range of about −3 × 10−4 K−1 to 1 × 10−3 K−1.

Temperature-dependent thermal properties of Ru/C multilayers.

Multilayers made of Ru/C are the most promising candidates when working in the energy region 8-20 keV. The stability of its thermal properties, including thermal expansion and thermal conduction, needs to be considered for monochromator or focusing components. Ru/C multilayers with periodic thicknesses of 3, 4 and 5 nm were investigated in situ by grazing-incidence X-ray reflectometry and diffuse scattering in order to study their thermal expansion characteristics as a function of annealing temperature up to 400°C. The thermal conductivity of multilayers with the same structure was also measured by the transient hot-wire method and compared with bulk values.

Effect of YSZ Coatings as Diffusion Barrier between Glass Sealing and Steel

In solid oxide fuel cell (SOFC) stacks the sealing is a critical element which has to ensure gas tightness between different gas streams as well as electrical insulation in several parts of the system depending on its design. It has therefore to establish stable interfaces with different materials such as the ferritic stainless steel (FSS) used for interconnects frame or gas manifolds and the ceramic materials of the cell electrolytes. Therefore, sealants have to be chemically and mechanically stable in contact with such materials at the SOFC working conditions in terms of temperatures, thermal and stress gradients, and exposition to different gas compositions. Glasses are widely used as sealing material because of their thermal expansion characteristics, mechanical properties, the possibility to obtain gas tight structures and ease of application. However, due to the operating temperatures, reactions and diffusion processes can occur at the interfaces resulting in the formation of undesirable phases that could compromise the glass properties increasing the risk of failures. The application of a barrier layer between the two components is considered as suitable countermeasure avoiding the migration of elements at the interfaces. In this work the barrier properties and compatibility at the interfaces of Yttria-stabilized zirconia (YSZ) has been investigated. YSZ layers have been applied by means of plasma spraying on as-rolled FSS substrates. Several glass compositions, applied on substrates with and without YSZ coating, have been tested at 780°C in static air up to 1000 h to investigate the barrier layer effects, the compatibility of the materials and the stability of the interfaces. Microstructural changes and migration of elements have been characterized post-experiment in cross-section by means of scanning electron microscopy- energy dispersive X-ray analysis.

Theoretical Study on Arrangement Depth of U-Tube Heat Exchanger under Building Foundation

In recent years, ground-coupled heat pump system has been widely used in engineering practice. For saving the area, arrangement of buried pipes under building foundation began to attract public attention. However, under the influence about the alternation of soil temperature field and the base compression, the overall strength and stability of the foundation is weakened, the foundation is settled again with the use of the building, which may affect the use of the building structure. In this paper, the settlement has been analyzed with the effect by porosity and temperature field. The best placement depth of U-tube under the foundation is also explored. The results are as follow. The settlements of cold and heat accumulation are consistent with the characteristics of soil thermal expansion and contraction. Until the settlements of buildings whose foundation have cold accumulation don’t satisfy the standard requirements, the depth of the U-tube heat exchanger need to be buried under the depth of settlement calculation regardless without the effects of temperature.

Initial thermal expansion characteristics of insulating refractory concretes

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Vitrification of hazardous Fe-Ni wastes into glass-ceramic with fine crystalline structure and elevated exploitation characteristics

Hazardous residues from ferronickel smelting plant are mixed with glass cullet and are vitrified for 2h at 1400°C. The obtained glass is characterized with a very high crystallization trend. As a result, it is inappropriate for sinter-crystallization, but a fine crystalline glass-ceramic can be obtained by bulk crystallization after short heat-treatment at low temperatures.Due to the presence of 1.5wt.% chromium oxide in the parent glass, together with high amounts of iron oxides and magnesium oxides, the crystallization process is peculiar. It starts during the melt cooling with the precipitation of preliminary Fe-Mg-Cr spinel crystals, which then act as centers for further epitaxial growth of pyroxene phase. It is also highlighted that, due to liquid–liquid immiscibility, the main amorphous phase is characterized by a nonhomogeneous binodal structure, which becomes finer after the nucleation treatment. As a result, pyroxenes with sizes below 1μm are formed as main crystal phase after the crystallization step.Notwithstanding of the high amount of hazardous wastes in the batch the obtained glass is characterized with high chemical durability. At the same time, the obtained glass-ceramics is characterized by a suitable coefficient of thermal expansion and attractive mechanical characteristics.

Thermo-mechanical analysis of heat exchanger design for thermal energy storage systems

Significant tensile stresses inside solid thermal energy storage media are induced due to incompatible thermal expansion characteristics. These stresses can cause damage to the often brittle storage material which is associated with a performance loss of thermal properties or the partial loss of long-term mechanical stability. In the present paper, a previously introduced analytical approach is extended to estimate the effects of the dominant physical and geometrical quantities on critical tensile stresses around tubular heat exchangers. Results are presented in terms of three composite dimensionless parameters representing the geometrical and material parameters of the system. Analytical sensitivities furthermore provide a direct quantification of how these sensitivities depend on selected system parameters, thus giving clues regarding the most promising optimisation handles. A representative case study was performed and can serve as a guide-line for making design decisions from a mechanical perspective as a complement to the typically performed thermodynamic design.

Mitigation of anomalous expansion of carbon xerogels and controllability of mean-pore-size by changes in mold geometry

Distribution of pore sizes in xerogels is a very relevant property of materials when it comes to applied research and commercial implementations requiring precise pore control at the meso–macro scales (10–1000nm radii). Resorcinol–formaldehyde (RF) xerogels with mean pore radii in the elusive intermediate range of few to several hundreds of nanometers are rarely reported. One of the reasons is that controlling pore size with chemical concentrations becomes very sensitive in that range. We report the manipulation of a less sensitive, and controllable variable, mold geometry, as a way to synthesize resorcinol–formaldehyde xerogels with mean pore radii ranging from 300 to 700 nm. Squared molds with a depth-to-side ratio of 1:3.33, for example, led to a final RF substrate with pore radii of 321±48 nm. Similarly, a squared mold with a depth-to-side ratio of 1:3.55 resulted in samples with radii of 704±50. This variation in pore size due to mold geometry was found to be related to the RF skin creation process. Additionally, since thermal expansion characteristics between heating and cooling phases have a noticeable discrepancy in RF materials, thermal expansion properties and mitigation of anomalous coefficients of thermal expansion were also studied in order to avoid fracture of substrates under applications that require dimensional stability under thermal loads.

Laser Heating of the Core-Shell Nanowires

AbstractThe induced thermal stress in a heating process is an important parameter to be known and controlled in the magnetization process of core-shell nanowires. This paper analyses the stress produced by a laser heating source placed at one end of a core-shell type structure. The thermal field was computed with the non-Fourier heat transport equation using a finite element method (FEM) implemented in Comsol Multiphysics. The internal stresses are essentially due to thermal gradients and different expansion characteristics of core and shell materials. The stress values were computed using the thermo elastic formalism and are depending on the laser beam parameters (spot size, power etc.) and system characteristics (dimensions, thermal characteristics). Stresses in the GPa range were estimated and consequently we find that the magnetic state of the system can be influenced significantly. A shell material as the glass which is a good thermal insulator induces in the magnetic core, the smaller stresses and consequently the smaller magnetoelastic energy. These results lead to a better understanding of the switching process in the magnetic materials.

Study of optical and thermo-optical properties of a hybrid photopolymer material based on thiol-siloxane and tetraacrylate oligomer

Optical (refractive index and absorption coefficient) and thermo-optical (linear thermal expansion and thermo-optic coefficients) characteristics of a new hybrid organic-inorganic photopolymer material “Hybrimer-TATS” based on thiol-siloxane and tetraacrylate oligomers are studied. Variation of the ratio of initial components makes it possible to change the thermo-optic coefficient from −0.7·10−4 to 0,66 · 10−4 K−1, which offers prospects for synthesizing athermal optical components and optical elements with a high sensitivity to temperature variation for thermal sensors.

Integration of additive-free Ni–Cu–Zn ferrite layers into LTCC multilayer modules

The sintering behavior of sub-micron Ni0.30Cu0.20Zn0.52Fe1.98O3.99 ferrite with and without Bi2O3 addition was studied. Ferrites with 0.5wt% Bi2O3 exhibit enhanced shrinkage at T<900°C with significant grain growth. Additive-free ferrite powders also sinter to high density at 900°C, however, grain growth is very limited. Both ferrites exhibit a permeability of μ=400–450. Multilayers consisting of ferrite and low-k dielectric LTCC layers were prepared by co-firing at 900–915°C. The shrinkage and thermal expansion characteristics of ferrite and LTCC tapes are similar. However, the permeability of integrated ferrite layers, made from ferrite tapes with Bi2O3 additive, significantly drops after co-firing with LTCC layers compared to separately fired monolithic ferrite multilayers. Contrarily, the permeability of integrated, Bi2O3-free ferrite layers, co-fired with dielectric tapes, is identical to that of monolithic ferrite multilayers. This finding is an important step toward ferrite integration into complex LTCC multilayer architectures.

Thermo-Mechanical Analysis of Chosen Epoxy Resins Used in Aviation Technology

In this paper the results of storage modulus (E’), loss modulus (E’’) and damping parameter tan (δ)=E''/E' of epoxy resins Epidian 57 and L285 with curing agents Z1 and LH285, respectively are presented. In addition to this the stress-strain and thermal expansion characteristics of Epidian 53, 57 and L285 were obtained experimentally in order to compare Dynamic Mechanical Analysis (DMA) results. Temperature range of DMA investigations using Netzsch (Germany) DMA 242C analyzer was from-120 °C to +110 °C at the heating rate of 1 K/min with frequency of {0.1, 1, 10} Hz, respectively. Netzsch DIL 402C dilatometer was used to study the thermal expansion of the tested samples within temperature range from 30 °C to 80 °C at 1 K/min of heating and cooling rates, respectively and Huang TA computer servo control material testing machine HT-2402 was applied to determine the stress-strain characteristics. Measurements of sample elongation ΔL and physical α* were performed twice in heating and cooling cycles. The glass transition temperature Tg determined from maximum of tan (δ) curve at f = 1Hz was equal to 76.7 °C for E57 and 87.2 °C for L285. It has been observed durable deformed shape of L285 sample with deflection in the middle about 5 mm just after finishing the DMA first run of heating which significantly affected DMA results during the second run of heating

Thermal and hygric expansion characteristics of mortars and bricks used in the dome structures of Turkish Baths from 14th and 15th centuries

Thermal and hygric expansion characteristics of mortars and bricks from the dome structures of three Turkish Baths from 14th and 15th centuries were investigated. The linear thermal expansion coefficients of the mortars and bricks were determined in the temperature range from −10°C to 50°C and hygric expansion characteristics of the same samples were studied in the relative humidity range from 30% to 80%. The results showed that the thermal and hygric expansion characteristics of mortars used in the dome structures of the studied baths are very similar to those of bricks used within the same structure. The thermal expansion coefficients of the mortars are 2.5% to at most 10% greater than the bricks of the same dome. The linear displacement of the mortars and bricks due to relative humidity changes are less pronounced in comparison with the displacement due to thermal changes, but similarity in the expansion trends in between the mortar and bricks of the same structure is also observed for the hygric expansion test results. Since movement of the materials resulting from cyclic changes in temperature or relative humidity may cause stresses at the interfaces where different materials are in contact, eventually resulting in cracks and detachments, it is important to consider the thermal and hygric expansion behavior of the original materials during the selection of repair materials to be used in the conservation works of these dome structures.

펄스전해가공을 이용한 인바 박판의 가공 형상 및 Overcutting 현상에 관한 연구

Invar is a compound metal of Fe-Ni system contained 36.5% Ni. The characteristic of invar is that the coefficient of thermal expansion is 1.0×10 -6 cm/°C. It is approximately 10 times smaller than series of steel. Because of this low thermal expansion characteristic of Invar, it is used to shadow mask of display device such as UHDTV or OLED TV. In this study, pulse current from pulse generator instead of DC current is used to overcome the disadvantages of the conventional electrochemical machining. Pulsed current with different duty factor in PECM affect the precise geometry. Pulse electrochemical machining is conducted to machine the micro hole to the invar sheet with different duty factor. The machined shape and overcut of invar sheet with different duty factor is observed by optical microscope and scanning electron microscope (SEM).

Magnetic behavior of Joule-heated magnetic core–shell nanowires with positive magnetostrictive core material

Temperature field is an important parameter to be known and controlled in the magnetization process of the core–shell nanowires. The paper analyzes the temperature dependence of hysteretic process in a core–shell nanowire subjected to a dc Joule heating process. An electrical current that passes through the wire induces a temperature and a thermal stress field in the system. Spatial and temporal evolution of the temperature in system was analyzed using a model based on time-dependent heat conduction equation. The stresses determined by thermal gradients and different expansion characteristics of core and shell materials were computed. The temperature and stress depend on the size parameters of the system, dc Joule current and the initial temperature of the system. The magnetic behavior of the nanowire was analyzed using the Micromag application. The magnetic state of the core is influenced by the temperature field induced by a dc current applied to the system. For core materials with positive magnetostriction coefficient the coercive field increases at the increase of dc current intensity passed through the system.

Thermal expansion characteristics of Fe–9Cr–0.12C–0.56Mn–0.24V–1.38W–0.06Ta (wt.%) reduced activation ferritic–martensitic steel

The lattice and bulk thermal expansion behavior of an Indian version of reduced activation ferritic–martensitic (INRAFM) steel has been quantified using high temperature X-ray diffraction and dilatometry. The lattice parameter of tempered α-ferrite phase exhibited a smooth quadratic increase with temperature, while that of γ-austenite remained fairly linear up to 1273K. The results suggest that α-ferrite+Carbides→γ-austenite transformation occurs upon continuous heating in the temperature range, 1146⩽T⩽1173K. Further, this transformation is found to be accompanied by a reduction in average atomic volume. The mean linear thermal expansion coefficients of tempered α-ferrite and γ-austenite phases are estimated to be about 1.48×10−5 and 2.4×10−5K−1 respectively. The magnetic contribution to relative thermal dilatation (Δl/l298)mag is found to be small and negative, as compared to phonon contribution.

Progress on matrix SiC processing and properties for fully ceramic microencapsulated fuel form

The consolidation mechanism and resulting properties of the silicon carbide (SiC) matrix of fully ceramic microencapsulated (FCM) fuel form are discussed. The matrix is produced via the nano-infiltration transient eutectic-forming (NITE) process. Coefficient of thermal expansion, thermal conductivity, and strength characteristics of this SiC matrix have been characterized in the unirradiated state. An ad hoc methodology for estimation of thermal conductivity of the neutron-irradiated NITE–SiC matrix is also provided to aid fuel performance modeling efforts specific to this concept. Finally, specific processing methods developed for production of an optimal and reliable fuel form using this process are summarized. These various sections collectively report the progress made to date on production of optimal FCM fuel form to enable its application in light water and advanced reactors.

비선형 열팽창 특성을 고려한 이종 접합 복합재의 열변형 해석

큰 온도 변화를 받는 이종 접합 복합재는 재료의 서로 다른 열팽창 특성으로 인해 열에 의한 형상 왜곡이 발생되기 쉽다. 성형 과정에서 이종 접합 복합재 구조물의 제작 공정 중의 형상 왜곡 현상을 고려하기 위해서 구성하는 각각의 복합재료들에 대한 열팽창 특성 분석이 우선적으로 요구된다. 본 논문에서는 Carbon/Epoxy와 Silica/Phenolic의 이종접합 복합재료 시편의 열변형 특성을 측정하기 위해 디지털 영상 상관 기법(DIC)을 활용하였다. 이종 접합 복합재 시편의 열변형에 대해 수치 해석을 수행하였고 이를 실험 결과와 비교하였다. 수치해석을 통한 예측 결과는 실험을 통하여 입증되었다. The co-bonded dissimilar composite under a wide range of temperature change shows thermal distortion due to the differences in thermal expansion characteristics of the composite materials. Analysis of the thermal expansion characteristics of each composite is required for the design of co-bonded dissimilar composite structure with considering the shape distortion during the manufacturing process. In this work, digital image correlation (DIC) technique is introduced for measuring the thermal distortion characteristics of co-bonded dissimilar composite specimen, carbon/epoxy and silica/phenolic. The thermal distortion of co-bonded dissimilar composite specimen is numerically estimated and compared with the experiments. The estimated results is successfully validated using the measured results.

Synthesis of LaNi&lt;sub&gt;0,6&lt;/sub&gt;Fe&lt;sub&gt;0,4&lt;/sub&gt;O&lt;sub&gt;3-δ&lt;/sub&gt; (LNF64) by Pechini Method for Use as Cathode in SOFC´S

The solid oxide fuel cell are an alternative of production clean and efficient energy, because converts chemical energy in electrical energy. A fuel cell is formed basically by an electrolyte, a cathode and an anode. The main electrolyte used for SOFC manufacturing is the ZrO2-Y2O3. The materials for electrode manufacturing must possess thermal expansion characteristics close to electrolyte and have high electrical conductivity in operating temperature. Recently, the perovskite LaNi0,6Fe0,4O3-δ, has attracted interest for application as cathode in SOFC ́s for has a high electronic conductivity and a thermal expansion coefficient whish for the zirconia electrolyte. This work aimed to LaNi0,6Fe0,4O3-δobtained by Pechini method. The powders were characterized by differential thermal analysis, thermogravimetric analysis, x-ray diffraction, energy dispersion X-ray fluorescence spectroscopy, helium pycnometry and scanning electron microscopy. The results showed powders obtained with perovskite formation when calcined 600°C during 2 hours.

The effect of strontium oxide replacing calcium oxide on the crystallization and thermal expansion properties of Li2O–CaO–SiO2 glasses

The crystallization process and thermal expansion characteristics of glass compositions based on 17.80Li2O–(32.20−x) CaO (xSrO)–50SiO2 system (where x=3, 6, 9 and 12mol%) were investigated. The crystallization characteristics of the glasses, the type of crystalline phases formed as well as the resulting microstructure were traced by means of differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The addition of SrO at the expense of CaO has led to a slight decrease in both the transition (Tg) and softening (Ts) temperatures of the glasses. The main crystalline phases, formed after controlling heat-treatment are lithium metasilicate-Li2SiO3, wollastonite-CaSiO3 together with pseudowollastonite-like phase. The incorporation of Sr into CaSiO3 structure has partially promoted has partially promoted the phase transition from wollastonite to pseudowollastonite-like phase. A slight increase of thermal expansion coefficients (α-values) of the glasses and their corresponding glass–ceramics has been detected. The α-values of the obtained glasses are 114 to 130×10−7°K−1 in the 25–300°C temperature range while those of the crystalline materials are 90 to 121×10−7°K−1 in the 25–700°C temperature range. The present results provide valuable information about the role of Sr2+ in determining the type of crystalline phases formed in the glasses and the thermal expansion data of the material were obtained.