Change In Thermal Expansion Coefficient Research Articles

Properties of a reactive‐graphitic‐carbon‐nanofibers‐reinforced epoxy

AbstractOur previous studies showed that herringbone graphitic GNFs surface‐derivatized with reactive linker molecules bearing pendant primary amino functional groups capable of binding covalently to epoxy resins. Of special importance, herringbone GNFs derivatized with 3,4′‐oxydianiline (GNF‐ODA) were found to react with neat butyl glycidyl ether to form mono‐, di‐, tri‐, and tetra‐glycidyl oligomers covalently coupled to the ODA pendant amino group. The resulting reactive GNF‐ODA (butyl glycidyl)n nanofibers, r‐GNF‐ODA, are especially well suited for reactive, covalent incorporation into epoxy resins during thermal curing. Based on these studies, nanocomposites reinforced by the r‐GNF‐ODA nanofibers at nanofiber loadings of 0.15–1.3 wt% were prepared. Flexural property of cured r‐GNF‐ODA/epoxy nanocomposites were measured through three‐point‐bending tests. Thermal properties, including glass transition temperature (Tg) and coefficient of thermal expansion (CTE) for the nanocomposites, were investigated using thermal mechanical analysis. The nanocomposites containing 0.3 wt% of the nanofibers gives the highest mechanical properties. At this 0.3‐wt% fiber loading, the flexural strength, modulus and breaking strain of the particular nanocomposite are increased by about 26, 20, and 30%, respectively, compared to that of pure epoxy matrix. Moreover, the Tg value is the highest for this nanocomposite, 14°C higher than that of pure epoxy. The almost constant change in CTEs before and after Tg, and very close to the change of pure epoxy, is in agreement with our previous study results on a chemical bond existing between the r‐GNF‐ODA nanofibers and epoxy resin in the resulting nanocomposites. POLYM. COMPOS., 28:605–611, 2007. © 2007 Society of Plastics Engineers

Complex thermal expansion properties of Al-containing HZSM-5 zeolite: A X-ray diffraction, neutron diffraction and thermogravimetry study

Thermal expansion of re-hydrated, Al-containing HZSM-5 was studied by different diffraction techniques, as well as, through thermogravimetry. It is concluded that the nature of the remarkable shift of thermal expansion properties, that occurs in air, between 350 °C and 400 °C, where the coefficient of thermal expansion changes from positive to negative is due to dehydration of HZSM-5 and not due to dealumination. The intrinsic thermal expansion properties of dehydrated HZSM-5 are observed in the second cycle of neutron diffraction experiments performed in vacuum (∼10 −5 Torr). Near zero thermal expansion was observed in the temperature interval between 100 °C and 400 °C. These intrinsic thermal properties were rationalized in terms of the rigid unit mode model.

Thermally induced stress hysteresis and co-efficient of thermal expansion changes in nanoporous SiO2

The thermomechanical response of electron beam deposited nanoporous silicon dioxide is examined using substrate curvature measurements and nanoindentation. Analysis of the thin film bond angle strain distributions versus temperature indicates that low temperature (T < 100 °C) stress hysteresis and tensioning are primarily attributed to hydrogen bonded water desorption. However, at higher temperatures, the absence of water desorption suggests that the thermomechanical behaviour is related to thermally induced bond angle strain redistributions towards the local bonding environment of quartz and thermally grown silicon dioxide. This is supported by the co-efficient of thermal expansion data that trend lower with higher annealing temperatures. The re-absorption of water into the thin film accounts for the reproducibility of the open-loop stress hysteresis and tensioning observations.

Neodymium–cerium oxide as new thermal barrier coating material

Neodymium–cerium oxide (Nd2Ce2O7) was proposed as a new thermal barrier coating material in this work. Monolithic Nd2Ce2O7 powder was prepared by the solid-state reaction at 1400 °C. The phase composition, thermal stability and thermophysical properties of Nd2Ce2O7 were investigated. Nd2Ce2O7 with fluorite structure was thermally stable in the temperature range of interest for TBC applications. The results indicated that the thermal expansion coefficient (TEC) of Nd2Ce2O7 was higher than that of YSZ (6–8 wt.% Y2O3+ZrO2) and even more interesting was the TEC change as a function of temperature paralleling that of the superalloy bond coat. Moreover, the thermal conductivity of Nd2Ce2O7 is 30% lower than that of YSZ, which was discussed based on the theory of heat conduction. Thermal barrier coating of Nd2Ce2O7 was produced by atmospheric plasma spraying (APS) using the spray-dried powder. The thermal cycling was performed with a gas burner test facility to examine the thermal stability of the as-prepared coating.

Thermal Stability Investigation of a Newly Developed Sealing Glass as IT-SOFC Sealant

A sealing glass, based on the SiO2-B2O3-BaO-La2O3-ZrO2-Y2O3 system, was developed for intermediate temperature solid oxide fuel cells (ITSOFCS). The coefficient of thermal expansion (CTE) of the glass is 9.8 x 10(-6)/K (RT similar to 631 degrees C), very close to that of 8YSZ electrolyte. The glass shows a very good thermal stability, even after heat-treatment at 700 degrees C for 300h, little change in CTE detected. The investigations also reveal that the glass is chemically compatible with 8YSZ electrolyte and no obvious interfacial reaction can be observed after the glass contacting with 8YSZ at 700 degrees C for 300h.

Comparison Between Micrometer‐ and Nano‐Scale Glass Composites for Sealing Solid Oxide Fuel Cells

The present study compares the feasibility of controlling the properties of a glass composite seal by adding nano‐ or micron‐scale yttria‐stabilized zirconia (YSZ) powders to a borate glass used for sealing electrolyte‐supported solid oxide fuel cells (SOFCs). The crystallization of the glass composites was found to be independent of the volume fraction of added YSZ, for both sizes of the additive. The variation of the flow properties of both composite seals was measured using a wettability test, and an increase of the contact angle was observed when the volume fraction of additives was increased. The major factor found to decrease spreading of the glass composite was the additive particle size, where shape retention was observed for the nanometer (nm)‐YSZ composites while spreading of the micrometer (μm)‐YSZ composites was observed under the same testing conditions. Examination of the microstructure showed that initially the Ba‐containing glass reacted with YSZ to form a BaZrO3 compound. Long‐time exposure at 800°C caused a large reduction in the coefficient of thermal expansion (CTE), which can be explained by increased formation of BaZrO3 and further change in glass composition. This change in CTE occurs rapidly for the nm‐YSZ composites, which is not observed for the μm‐YSZ composites. However, the adverse reactions occurring between the additives and the glass matrix were found to reduce the CTE of the glass composites to a value lower than the recommended limit for a system used for sealing SOFCs.

Thermal properties of Pb[(Zn 1/3Nb 2/3) 0.93Ti 0.07]O 3 single crystals grown by the flux Bridgman method

Thermal properties of Pb[(Zn 1/3Nb 2/3) 0.93Ti 0.07]O 3 (PZNT) crystals have been investigated as a function of temperature. The DTA curve shows PZNT crystals begin to melt at 1154.9 °C. The TGA curve shows the weight loss in the PZNT crystals is about 1.24% at 1290 °C. At room temperature, the specific heat of PZNT crystals is about 0.358 J g −1 K −1. The thermal diffusivity and the thermal conductivity for PZNT crystals are about 0.326 mm 2 s −1 and 1.022 W m −1 K −1 at 298 K, respectively. The thermal expansion coefficient of PZNT crystals decreases quickly from 298 to 326 K, and an anomaly appears at 376 K due to the ferro–ferroelectric phase transition. The thermal expansion coefficient changes slowly from 544 to 1070 K.

Experiments and theory of thermally-induced stress relaxation in amorphous dielectric films for MEMS and IC applications

Plasma-enhanced chemical vapor deposited (PECVD) silane-based silicon oxide (SiO x ) films were chosen as an example to study the thermally-induced stress relaxation phenomena of amorphous dielectric films. Wafer curvature was measured optically both during and after various thermal conditions, including temperature cycling, constant peak temperature annealing, and varying peak temperature annealing experiments. From these measurements, we are able to obtain a complete stress evolutional history in the thin film, and further derive a series of mechanical/material properties such as viscoelasticity, density and viscosity, coefficient of thermal expansion (CTE), etc. We found that the stress in the amorphous silicon oxide films is sensitive to both viscosity and density changes. Stress relaxation was then theoretically investigated by a viscoelastic model, which was associated with a defects-based microstructural causal mechanism. Our theoretical model elucidates defects movement and consequent microstructure rearrangement as a source of damping, accompanied by viscous flow. This theory was applied to explain a series of experimental results, including stress hysteresis generation and reduction, stress relaxation, and CTE changes, etc.

Effect of the carbon isotope composition on the properties of diamond

Parameters of the interatomic interaction potential for 12C and 13C carbon isotopes in the crystal lattice of diamond have been determined. Based on these data, the isotope dependence of the properties of diamond such as the Debye temperature, molar heat capacity, thermal expansion coefficient, vacancy formation energy, self-diffusion activation energy, surface energy, and longitudinal sound velocity is described. This approach is used for estimating a change in the bulk compression modulus of lithium crystals upon the passage from 7Li to 6Li. As the temperature increases, the isotope dependence of the heat capacity at constant volume vanishes; at a certain temperature, the isotope dependence of the thermal expansion coefficient changes from growth to decay. The expected isotope dependence of the parameters of phase transitions is predicted. It is shown that carbon condensates formed upon deposition from the gas phase must be enriched with the heavy isotope.

Density change and viscous flow during structural relaxation of plasma-enhanced chemical-vapor-deposited silicon oxide films

The structural relaxation of plasma-enhanced chemical-vapor-deposited (PECVD) silane-based silicon oxide films during thermal cycling and annealing has been studied using wafer curvature measurements. These measurements, which determine stress in the amorphous silicon oxide films, are sensitive to both plastic deformation and density changes. A quantitative case study of such changes has been done based upon the experimental results. A microstructure-based mechanism elucidates seams as a source of density change and voids as a source of plastic deformation, accompanied by a viscous flow. This theory was then used to explain a series of experimental results that are related to thermal cycling as well as annealing of PECVD silicon oxide films including stress hysteresis generation and reduction and coefficient of thermal-expansion changes. In particular, the thickness effect was examined; PECVD silicon oxide films with a thickness varying from 1to40μm were studied, as certain demanding applications in microelectromechanical systems require such thick films serving as heat∕electrical insulation layers.

Model for BGA and CSP Reliability in Automotive Underhood Applications

Fine-pitch ball grid array (BGA) and underfills have been used in benign office environments and wireless applications for a number of years, however their reliability in automotive underhood environment is not well understood. In this work, the reliability of fine-pitch plastic ball grid array (PBGA) packages has been evaluated in the automotive underhood environment. Experimental studies indicate that the coefficient of thermal expansion (CTE) as measured by thermomechanical analyzer (TMA) typically starts to change at 10-15/spl deg/C lower temperature than the T/sub g/ specified by differential scanning calorimetry (DSC) potentially extending the change in CTE well into the accelerated test envelope in the neighborhood of 125/spl deg/C. High T/sub g/ substrates with glass-transition temperatures much higher than the 125/spl deg/C high temperature limit, are therefore not subject to the effect of high coefficient of thermal expansion close to the high temperature of the accelerated test. Darveaux's damage relationships were derived on ceramic ball grid array (CBGA) assemblies, with predominantly solder mask defined (SMD) pads and 62Sn36Pb2Ag solder. In addition to significant differences in the crack propagation paths for the two pad constructions, SMD pads fail significantly faster than the non solder mask defined (NSMD) pads in thermal fatigue. The thermal mismatch on CBGAs is much larger than PBGA assemblies. Crack propagation in CBGAs is often observed predominantly on the package side as opposed to both package and board side for PBGAs. In the present study, crack propagation data has been acquired on assemblies with 15, 17, and 23mm size plastic BGAs with NSMD pads and 63Sn37Pb on high-T/sub g/ printed circuit boards. The data has been benchmarked against Darveaux's data on CBGA assemblies. Experimental matrix also encompasses the effect of bis-maleimide triazine (BT) substrate thickness on reliability. Damage constants have been developed and compared against existing Darveaux Constants. Prediction error has been quantified for both sets of constants.

Thermal expansion and indentation-induced free volume in Zr-based metallic glasses measured by real-time diffraction using synchrotron radiation

Abstract Changes in thermal expansion coefficient and excess free volume content during in situ heating–cooling cycles of metallic glasses in the as-quenched state and after indentation were measured in the temperature range between 350 and 670 K by real-time diffraction. It was found that deformation produced by an array of ball indentations generate more than 37% increase of free volume in Zr-based glassy ribbons.

X-ray diffraction studies of thermal properties of the core and surface shell of isolated and sintered SiC nanocrystals

The effect of the presence of surface strains on the apparent lattice parameters (alp) obtained experimentally for nanocrystalline SiC is discussed. The alp values were determined for two kinds of powders with an average crystallite size of 11nm and related sintered samples. The measurements were done in a wide range of the diffractions vector up to Q = 12Å−1, allowing for evaluation of the internal pressure in the grains. Based on in situ high-temperature measurements, the thermal expansion coefficient and overall temperature factor BT were evaluated. It is shown that while the thermal expansion coefficient changes very little upon sintering, there is a large difference in the amplitude of the atomic oscillations between powders and sintered SiC reflected in a difference between respective Debye temperatures. It is concluded that the overall thermal properties of nanocrystals are determined by two components: thermal properties of the crystallite surface and its interior. The atoms at the surface vibrate much stronger than those in the bulk, and their behavior is strongly affected by the crystallite’s environment.

Thermal behavior and glass transition of Zr-based bulk metallic glasses

Abstract The thermal behaviour of Zr-based bulk metallic glasses has been investigated in situ through the glass transition by means of differential scanning calorimetry, high temperature X-ray synchrotron diffraction, electrical resistivity, and dilatometry. The temperature dependence of the X-ray structure factor can be well described by the Debye theory. The Debye temperature of the glassy and the supercooled liquid state of Zr 52 Ti 5 Cu 18 Ni 15 Al 10 is θ =412 and θ =162 K, respectively. The temperature coefficient of the electrical resistivity and the thermal expansion coefficient change also at the calorimetric glass transition temperature. The results point to a significant changes in the dynamics of molecular motion at T g .

Thickness-dependent Structural Relaxation of Plasma-Enhanced Chemical Vapor Deposited Silicon Oxide Films during Thermal Processing

ABSTRACTThis paper presents a microstructure-based mechanism which elucidates seams as a source of density change and voids as a source of plastic deformation, accompanied by a viscous flow. This theory was then applied to explain a series of experimental results that are related to thermal cycling of amorphous dielectric films, such as plasma-enhanced physical vapor deposited (PECVD) silicon oxide (SiOx) films, including stress hysteresis generation and reduction and coefficient of thermal-expansion changes. In particular, the thickness effect was examined; PECVD SiOx films with a thickness varying from 1 to 40 m were studied, as certain demanding applications in Microelectromechanical Systems (MEMS) require such thick films serving as heat/electrical insulation layers.

Change in Thermal Expansion Coefficient of Bulk Metallic Glasses Tg Measured by Real-Time Diffraction Using High-Energy Synchrotron Light

Change in Thermal Expansion Coefficient of Bulk Metallic Glasses Tg Measured by Real-Time Diffraction Using High-Energy Synchrotron Light

高強度コンクリートの若材齢における熱膨張係数に関する実験

Thermal expansion coefficient is very important value to analyze the thermal deformation behavior of structural element made by high-strength concrete in early age. As autogenous shrinkage increases in high-strength concrete, the evaluation of autogenous shrinkage in the condition that receives high temperature is important. Therefore this study considered the calculation method of the thermal expansion coefficient by taking this deformation into consideration. In this study, the experiments on thermal deformation behavior using full scale column and small specimens were carried out. As the result, we obtained the result that thermal expansion coefficient changes with time. And we showed that the thermal coefficient could be evaluated by using the equation considered of change in the relative humidity inside concrete.

Thermal Expansion of Low-pressure Chemical Vapor Deposition Polysilicon Films

Polysilicon films were deposited using low-pressure chemical vapor deposition (LPCVD) onto oxidized silicon substrates, after which substrate curvature as a function of temperature was measured. The curvatures changed with temperature, implying that the thermal expansion of LPCVD polysilicon differs from that of the single crystal silicon substrate. Further, polysilicon films with tensile residual stresses displayed an increased thermal expansion, while polysilicon films with compressive residual stresses displayed a decreased thermal expansion. Following high temperature annealing, the residual stresses of the polysilicon films were reduced to near zero, and the thermal expansion of the polysilicon films matched that of the single crystal substrate. The apparent change in thermal expansion coefficient due to residual stress was much larger than predicted theoretically.

Preparation and thermal expansion behavior of glass coatings for electronic applications

PbO–B 2O 3–SiO 2 based alkali-free glass coatings were prepared and deposited on alumina ceramics, in order to: (1) decrease the sintering temperature of the coatings, and (2) keep a close match of thermal expansion between the coatings and the ceramics. Influence of different dopants on sintering temperature and thermal expansion behavior was studied, and an optimized composition was obtained. The effects of annealing processing on the coefficient of thermal expansion (CTE) and softening temperature ( T s) of the glass were investigated by using X-ray diffraction (XRD) spectra and thermal–mechanical analysis (TMA). The results showed that high annealing temperature would be helpful to obtain the closely packed structural with lower CTE, but long annealing time would lead to increasing of CTE. This could be due to the structural rearrangement of glass during the thermal treatment. Furthermore, the variation in coordination of the glass network would also be responsible for the change of CTE. The results showed that an optimized annealing scheme must be determined for producing glass coatings that can match to the ceramic.

Thermomechanical head performance

Mismatch of thermal expansion of various materials used in the transducer and slider of giant magnetoresistive (GMR) recording heads causes, at higher operating head temperature, mechanical stresses and, in particular, protrusion toward the media [thermal pole tip recession (T-PTR)]. Low T-PTR is necessary for low head-media spacing without mechanical contact. Impact of the magnitude of the Poisson's ratio of a photoresist coil insulator on thermal protrusion is shown to be large, due to large variation of the volume compressibility. Three-dimensional FE (3-D FE) thermomechanical modeling shows that the distribution of thermal stress in shields caused by mismatch of coefficient of thermal expansion changes completely close to the air bearing surface due to protruded head surface. During head operation, the primary heat source arises from the writer coil. The maximum temperature and the particular temperature distribution depends on the ability of the head components to dissipate effectively the generated heat. As the transducer continues to scale down in size with increasing areal density and data rate, the power dissipated per unit volume grows due to the larger coil resistance in the core region.