Thermal Expansion Of Composites Research Articles

Investigation of high-temperature effects on the strengthening and degradation of mechanical property in sandstone

The underground coal gasification (UCG) mining technology constitutes an innovative form of clean energy technology pertaining to coal resources. The sandstone specimens from the roof and floor of coal seam were collected and heat treated at 25 °C–800 °C in laboratory. According to the test results of physical parameters, 400 °C is the temperature turning point for the physical parameters such as mass, longitudinal P-wave velocity, porosity, and water absorption of sandstone specimens to change from “slow” to “drastic”. The uniaxial compression test showed that the uniaxial compressive strength (UCS) of sandstone specimens decreases by 3.22% when the temperature rises from 25 °C to 200 °C, and by 5.70% when the temperature rises from 600 °C to 800 °C, both of which show temperature degradation effect. However, in the range of 200 °C–600 °C, the average UCS of sandstone specimens increases from 122.80 MPa to 162.54 MPa, showing obvious strength strengthening effect. In order to fully elucidate the mechanism of mechanical properties strengthening and degradation of tunnel surrounding rock under high temperature, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were combined in the experiment. We concluded that these two intensity effects can be explained by physical and chemical reactions, including mineral composition thermal expansion, thermally induced microcrack and healing effects, phase transformation of the rock-forming mineral quartz, dehydration and thermal decomposition of mica, as well as dehydration and dehydroxylation of illite. The research results can provide theoretical basis for UCG roof and floor disaster prevention evaluation and tunnel support system design.

Investigation on Optical Absorption and Reflection of Carbon Nanotubes Mixed Copper Composites for Laser Sintering Process Improvement

Selective laser sintering (SLS) of copper components manufactured via powder metallurgy is widely studied due to minimal material wastage. However, copper has poor optical absorption when exposed to infrared (IR) lasers, such as in laser-based additive manufacturing or laser surface processing. To address this issue, an innovative approach to enhance the optical absorption of copper powders during infrared laser sintering is presented in this study. Carbon nanotubes (CNTs) have several unique properties, including their high surface area, plasmonic response, excellent conductivity, and optical absorption properties. CNTs were mixed with copper powders at different weight percentages using an acoustic method. The resulting Cu-CNT compositions were fabricated into pellets. The Box-Behnken Design of Experiments methodology was used to optimize the IR laser processing conditions for sintering. Spectroscopic analysis was conducted to evaluate the reflection and thermal absorption of the IR wavelengths by the Cu-CNT composites. Density and hardness measurements were taken for the laser-sintered Cu-CNT pellets. The coating of copper powders with CNTs demonstrated enhanced optical absorption and correspondingly reduced reflection. Due to the enhanced optical absorption, increased control and sensitivity of the laser sintering process was achieved, which enabled improvement in the mechanical properties of strength, hardness, and density, while also enabling control over the composite thermal expansion coefficient. A maximum average hardness of 66.5 HV was observed. Indentation test results of the samples revealed maximum tangential and radial stresses of 0.148 MPa and 0.058 Mpa, respectively.

Design of near-zero thermal expansion composites with superior mechanical properties in a wide temperature region

Materials that exhibit very small dimensional changes with temperature, i.e., near-zero thermal expansion (NZTE) properties, find attractive prospects in fields such as precision instrumentation and aerospace. However, how to manually design and synthesize NZTE materials with high performance instead of laboriously exploring composition has been a long-standing challenge. In this study, based on the excellent bonding and forming abilities of metallic glass (MG), a composite with continuous regulation of thermal expansion was obtained by combining different volume ratios of MG with beta-LiAlSiO4 (beta-LAS) possessing negative thermal expansion (NTE). Interestingly, excellent NZTE performance of 0.693 and 1.587 ppm/K was achieved over the 150–700 K temperature range when the volume ratio of MG was 40% and 50%, respectively. In addition, the compressive strength of the NZTE composite reached 320 and 436 MPa for the above two ratios, respectively. The thermal shock resistance results show that the composite can withstand thermal cycling tests of not less than 80 times. The proposed strategy not only provides new NZTE materials with high performance but also facilitates a shift in the preparation of high-performance NZTE composites from laborious search to customizable design.

Analysis of Coefficient of Thermal Expansion in Carbon Black Filled PDMS Composite

Polymer composites are gaining attention due to their superior thermal properties. Especially carbon black /carbon nanotubes/ graphene filled polymer composites are used in energy harvesting, thermal actuators and MEMS. The coefficient of thermal expansion (CTE) is one of the most important properties in the polymer composite. In the present study, thermal expansion of polydimethylsiloxane (PDMS) matrix is filled with carbon black particle of varied volume fraction is modeled. Two-dimensional finite element (FE) model is computed in order to explain the thermal expansion behavior of the polymer composite and same is carried out for ambient to 70 K temperature. A 2D regular arrangement of circular particle packing model is set up and simulated. The FE model predicts that filler geometry has a little effect on the thermal expansion than the percentage of filler in the composite. Thermal expansion of composite is compared with the theoretical model. It shows that the CTE of composite reduces as the filler percentage increase, also gives good agreement in the both models. Hence, it is found that the addition of carbon black to the polymer composite could make it perform significantly better in thermal expansion.

Hydrothermal Synthesis and Study of the Properties of the System ZrW<sub>2-x</sub>Mo<sub>x</sub>O<sub>7</sub>(OH)<sub>2</sub>·2H<sub>2</sub>O (0≤x≤2)

The ZrW2-xMoxO8 (0≤x≤2) system has a unique property - a negative coefficient of thermal expansion (CTE), that is it shrinks when heated. Such materials solve the problem of undesirable thermal expansion of composites in many areas. In contradistinction to ZrW2O8 and ZrMo2O8, this system has anomalously low CTE values [1,2] and adverse phase transition is shifted to a lower temperature, unlikely area for operation [3,4]. At the same time, the fundamental question remains about the mechanism of formation of ZrW2-xMoxO8(0≤x≤2). According to [5,6], pure cubic ZrW2O8 and ZrMo2O8 are obtained by thermal decomposition of the precursor ZrM2O7(OH)2· 2H2O, where (M = Mo, W). However, the structure of the precursor is extremely sensitive to various changes in the synthesis process. To date, there are a number of works devoted to the hydrothermal synthesis of precursors ZrW2-xMoxO7(OH)2·2H2O (0≤x≤2) solid solutions, however, not all formulations were obtained during the hydrothermal reaction. The study of their thermal properties, as well as structural and phase transformations are not fully studied. Within the paper it was first time proposed to obtain full number of nanosized materials by the hydrothermal method and also to study the nature of the thermal expansion of each member of the solid solution. The obtained data will solve many technical problems associated with dimensional nonvariance in the electronic, oil and gas, optical industry, medicine and rocket engineering.

High Pressure Behavior of Chromium and Yttrium Molybdate (Cr2Mo3O12, Y2Mo3O12).

The high pressure behavior of negative thermal expansion materials continues to be of interest, as their potential use in controlled thermal expansion composites can be affected by irreversible pressure-induced phase transitions. To date, it is not possible to predict the high pressure behavior of these compounds, necessitating measurements on each composition. In this work, high pressure synchrotron powder X-ray diffraction studies of Cr2Mo3O12 and Y2Mo3O12 were conducted in a diamond anvil cell. Chromium molybdate, which adopts the monoclinic P21/a structure under ambient conditions, was found to not undergo any crystalline-crystalline transitions up to 8.9 GPa. The orthorhombic ambient pressure polymorph of yttrium molybdate was found to undergo a phase transition to the monoclinic P21/a scandium tungstate structure below 0.13 GPa. This structure is frequently observed for related materials at low temperatures, but has never been reported for Y2Mo3O12. No additional changes in this material were observed up to 4.9 GPa. The fact that the monoclinic polymorphs of these materials do not undergo phase transitions within the studied pressure range makes them unique among A2M3O12 materials, as most isostructural compositions undergo at least one phase transition to crystalline high pressure phases.

Coefficients of Thermal Expansion for Composites with Agglomerated Carbon Nanotubes

In this paper, a micromechanics model is presented, which can predict the thermal expansion coefficients (CTE) of composites with agglomerated carbon nanotubes (CNTs). In the present model, two aggregation parameters are introduced. The Mori-Tanaka method and rule of mixture model are adopted to estimate the effect of CNT agglomeration on the thermal expansion coefficients of composites. The aim of the present paper is to investigate the CTEs of CNTs reinforced composites, with an emphasis on the influence of two agglomeration parameters on the CTEs. It is noted that the carbon nanotube agglomeration has a strong influence on the thermal expansion coefficients of composites.

Effects of Al particles and thin layer on thermal expansion and conductivity of Al–Y2Mo3O12 cermets**Project supported by the National Natural Science Foundation of China (Grant Nos. 10974183 and 11104252), the Doctoral Fund of the Ministry of Education of China (Grant No. 20114101110003), the Fund for Science & Technology Innovation Team of Zhengzhou, China (Grant No. 112PCXTD337), the Industrial Science and

Low thermal expansion composites are difficult to obtain by using Al with larger positive thermal expansion coefficient (TEC) and the materials with smaller negative TECs. In this investigation, Y2Mo3O12 with larger negative TEC is used to combine with Al to obtain a low thermal expansion composite with high conductivity. The TEC of Al is reduced by 19% for a ratio Al:Y2Mo3O12 of 0.3118. When the mass ratio of Al:Y2Mo3O12 increases to 2.0000, the conductivity of the composite increases so much that a transformation from capacitance to pure resistance appears. The results suggest that Y2Mo3O12 plays a dominant role in the composite for low content of Al (presenting isolate particles), while the content of Al increases enough to contact each other, the composite presents mainly the property of Al. For the effect of high content Al, it is considered that Al is squeezed out of the cermets during the uniaxial pressure process to form a thin layer on the surface.

Calculation of the effective thermal properties of the composites based on the finite element solutions of the boundary value problems

One of the key problems of mechanics of composite materials is an estimation of effective properties of composite materials. This article describes the algorithms for numerical evaluation of the effective thermal conductivity and thermal expansion of composites. An algorithm of effective thermal conductivity evaluation is based on sequential solution of boundary problems of thermal conductivity with different boundary conditions (in the form of the temperature on the boundary) on representative volume element (RVE) of composite with subsequent averaging of the resulting vector field of heat flux. An algorithm of effective thermal expansion evaluation is based on the solution of the boundary problem of elasticity (considering the thermal expansion) on a RVE of composite material with subsequent averaging of a resulting strain tensor field. Numerical calculations were performed with the help of Fidesys Composite software module of CAE Fidesys using the finite element method. The article presents the results of numerical calculations of the effective coefficients of thermal conductivity and thermoelasticity for two types of composites (single-layer fiber and particulate materials) in comparison with the analytical estimates. The comparison leads to the conclusion about the correctness of algorithms and program developed.

Thermal expansion, electrical conductivity and hardness of Mn3Zn0.5Sn0.5N/Al composites

Abstract Mn3Zn0.5Sn0.5N/Al composites were successfully prepared by sintering at 623 K and 723 K. It is found that the thermal expansion of Al is effectively decreased with the addition of negative thermal expansion material Mn3Zn0.5Sn0.5N, which plays the role of the thermal expansion compensator in the composite system. The thermal expansion of composites can be tailored by adjusting the volume fraction of Mn3Zn0.5Sn0.5N. In particular, 40 vol.%-Mn3Zn0.5Sn0.5N/Al composite sintered at 723 K has a low thermal expansion coefficient of 2.38×10−6 K−1. The thermal expansion of Mn3Zn0.5Sn0.5N/Al composites matches well with those calculated from the rule of mixture (ROM) estimation. The ROM model can be used to predict the thermal expansion of Mn3Zn0.5Sn0.5N/Al composite and design Mn3Zn0.5Sn0.5N/Al composite with a particular thermal expansion. Mn3Zn0.5Sn0.5N/Al composites exhibit excellent electrical conductivities at the level of 104 S/cm. The hardness of the as-prepared composites increases by adding Mn3Zn0.5Sn0.5N in Al.

Research in the Field of Preparing Molded and Unmolded Refractories Based on High-Alumina HCBS. Part 6. Mullitization and Thermal Expansion of Materials Based on Compound Composition HCBS1

Sintering and mullitization are studied for specimens prepared on the basis of compound composition HCBS in the system bauxite – very fine quartz glass (VFQG), and also with addition of refractory clay. The nature of sintering and mullitization is governed to a considerable extent by SiO2 content in the original composition (VFQG + clay). Dilatometric studies of specimens previously fired at 1600°C established that depending on composition thermal expansion values in the range 20 – 1000°C are within the limits of 0.5 – 0.6%, which is close or comparable with similar data for pure mullite.

Zero thermal expansion, electrical conductivity and hardness of Mn3Zn0.5Sn0.5N/Cu composites

Mn3Zn0.5Sn0.5N/Cu composites were successfully prepared by sintering at 723 K and 823 K. The results show that the thermal expansion of Cu decreases effectively by mixing Mn3Zn0.5Sn0.5N within its negative thermal expansion temperature range. The thermal expansion of composites can be tailored by adjusting the volume fraction of Mn3Zn0.5Sn0.5N. When the volume fraction of Mn3Zn0.5Sn0.5N is 40%, the composite shows a near zero thermal expansion, with the coefficient of thermal expansion of 0.268 × 10−6 K–1 Mn3Zn0.5Sn0.5N/Cu composites exhibit excellent electrical conductivity at the level of 104 S/cm, which is less than that of the matrix Cu. The hardness of composite is improved by adding Mn3Zn0.5Sn0.5N with high stiffness. Some classic models are used to predict the properties of composites and corrected according to the experiment data.

Iridescent cellulose nanocrystal/polyethylene oxide composite films with low coefficient of thermal expansion

Simultaneous control over optical and thermal properties is particularly challenging and highly desired in fields like organic electronics. Here we incorporated cellulose nanocrystals (CNCs) into polyethylene oxide (PEO) in an attempt to preserve the iridescent CNC optical reflection given by their chiral nematic organisation, while reducing the composite thermal expansion. The hydrophilic nature and long-range self-organisation of CNCs facilitated structural control in the PEO matrix. The coefficient of thermal expansion (CTE) was determined by using contrast-enhanced microscopy digital image correlation (CEMDIC) based on textural features revealed within CNC/PEO composites under polarised light. The attained composite films were iridescent and exhibited a significant reduction in CTE even with low CNC addition (∼50% CTE reduction at ∼10 wt.% CNC load). With further control over nanoparticle processing, such composites promise potentials for selective optical bandgap materials while tuning the CTE.

Thermal expansion of Y2W3O12/Al metal matrix composites

Most materials will expand upon heating, while rare ones contract. Such negative thermal expansion materials have great potential in controlling thermal expansion composites. Squeeze casting was used to fabricate fully dense 60 vol.-%Y2W3O12/Al composite in this study. The results showed that negative thermal expansion Y2W3O12 was not reacted with Al due to advantage of squeeze casting fabrication. Upon coefficient of thermal expansion (CTE) test, the composites showed the CTE is significantly larger than predicted from theory. Different sizes of particles were chosen to fabricate high volume ceramic reinforced metal matrix. The thermal expansion of composite shows complex transition with the change of temperature. On the basis of X-ray diffraction, some peak vanishing in composite is shown to result from the thermal mismatch stress between matrix and reinforcement.

Effect of 3D-Braided Structure on Thermal Expansion of PIP–Cf/SiC Composites

3D3d, 3D4d, 3D5d-braided Cf/SiC composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor. The coefficient of thermal expansion (CTE) of Cf/SiC composites was measured in longitudinal and transversal directions in the temperature range from –150°C to 25°C. The longitudinal CTE varies in the range (0.09–0.68) × 10–6/°C, and the transversal CTE varies in the range (0.21–1.95) × 10–6/°C. Various CTEs of 3D-braided Cf/SiC composites were mainly determined by different braided structures of carbon fibers. The longitudinal CTE is lower than transversal CTE for the negative axial expansion of carbon fibers at cryogenic temperature. Microcracks were examined to understand the effect of structure on the thermal expansion of composites.

Development of low thermal expansion Sc2(WO4)3 containing composites

Negative thermal expansion (NTE) oxides are interesting materials for use in controlled thermal expansion composites. Since Sc2(WO4)3 shows stronger NTE (−11×10−6°C−1) behaviour between −263 and 927°C compared with other NTE materials, this work develops low thermal expansion composites using ZrO2 or Cu as positive thermal expansion materials and Sc2(WO4)3 as NTE material respectively. Sc2(WO4)3 containing composites were synthesised using solid state method by co-sintering of Sc2O3, WO3 and ZrO2 mixture or Sc2(WO4)3 and Cu mixture with various ratios. The thermal expansion coefficient of the Sc2(WO4)3 containing composites can be controlled, being positive, negative or even zero. The Sc2(WO4)3/ZrO2 and Sc2(WO4)3/Cu composites show almost zero thermal expansion (0·3×10−6 and 0·14×10−6°C−1) when the volume fraction of Sc2(WO4)3 is 25 and 70% respectively. The zero expansion of these composites allows them to be utilised as ceramic substrate of optical fibre gratings and some components in microelectronics to eliminate thermal stresses due to the thermal gradient or transient temperature change.

Ab-initio calculation of the coefficients of thermal expansion for MeB2 (Me–Ti, Zr) and LaB6 borides and LaB6–MeB2 eutectic composites

The coefficients of thermal expansion of transition-metal borides, lanthanum hexaboride, and eutectic composites thereof are calculated using the a-priori pseudopotential method. To determine the energy of the electron–ion system as a function of the lattice parameters at nonzero temperatures, it is proposed that the energy scales of the system be compared at absolute zero and nonzero temperatures. The coefficient of thermal expansion of the composites shows an abrupt increase in the temperature range from 750 to 1000 K.

Two Decades of Negative Thermal Expansion Research: Where Do We Stand?

Negative thermal expansion (NTE) materials have become a rapidly growing area of research over the past two decades. The initial discovery of materials displaying NTE over a large temperature range, combined with elucidation of the mechanism behind this unusual property, was followed by predictions that these materials will find use in various applications through controlled thermal expansion composites. While some patents have been filed and devices built, a number of obstacles have prevented the widespread implementation of NTE materials to date. This paper reviews NTE materials that contract due to transverse atomic vibrations, their potential for use in controlled thermal expansion composites, and known problems that could interfere with such applications.

Low Thermal Expansion Composites Prepared from Polyimide and ZrW<su>2O8 Particles with Negative Thermal Expansion

Inorganic/polymer composite films with adjustable small thermal expansion were prepared by incorporating sub-m sized particles of zirconium tungstate (ZrW2O8) into aromatic polyimide (PI). ZrW2O8 particles exhibit negative coefficients of thermal expansion (CTE) between 4.8 to 8.7 ppm/K. The cross-sectional SEM images of ZrW2O8/PI composite films indicate that filler particles are homogeneously dispersed in PI matrix without aggregation. The CTEs of these films are linearly decreased with increasing the volume fraction of ZrW2O8. The CTE of a composite film containing 60 vol% of filler is as low as that of metallic copper (17 ppm/K). Assuming that the thermal expansion behavior is three-dimensionally isotropic, PI composite films containing more than 50 vol% of ZrW2O8 could achieve coefficients of volumetric thermal expansion smaller than 50 ppm/K. Keyword: Polyimide / thin film / zirconium tungstate / thermal expansion / Organic-inorganic composite

The Electronic Transport Properties of the Composites with Nanosized Carbon Fillers

Electric resistance of epoxy composite materials where thermally exfoliated graphite (TEG) with different dispersion degree and multi-walled carbon nanotubes (MWCNT) used as fillers has been investigated in the temperature range 77–300 K. The filler content was 5, 10 wt.%. It was shown that the behavior of the resistivity percolation curves of CMs depends on the structure of carbon fillers and becomes more linear with smaller size of particles (nanoTEG). The temperature dependences of electric resistance of CMs depend on the temperature changes of the electrical resistivity of carbon filler and contact resistance between filler particles and electroconductive network formed by fillers in CMs which change as a result of thermal expansion of composite.