Light-polymerized compomers: Coefficient of thermal expansion and microhardness

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

This work investigated the linear thermal expansion properties of a multi-material specimen fabricated with Invar M93 and A36 steel. A sequence of tests was performed to investigate the viability of additively manufactured Invar M93 for lowering the coefficient of thermal expansion (CTE) in multi-material part tooling. Invar beads were additively manufactured on a steel base plate using a fiber laser system, and samples were taken from the steel, Invar, and the interface between the two materials. The CTE of the samples was measured between 40 °C and 150 °C using a thermomechanical analyzer, and the elemental composition was studied with energy dispersive X-ray spectroscopy. The CTE of samples taken from the steel and the interface remained comparable to that of A36 steel; however, deviations between the thermal expansion values were prevalent due to element diffusion in and around the heat-affected zone. The CTEs measured from the Invar bead were lower than those from the other sections with the largest and smallest thermal expansion values being 10.40 μm/m-K and 2.09 μm/m-K. In each of the sections, the largest CTE was measured from samples taken from the end of the weld beads. An additional test was performed to measure the aggregate expansion of multi-material tools. Invar beads were welded on an A36 steel plate. The invar was machined, and the sample was heated in an oven from 40 °C and 160 °C. Strain gauges were placed on the surface of the part and were used to analyze how the combined thermal expansions of the invar and steel would affect the thermal expansion on the surface of a tool. There were small deviations between the expansion values measured by gauges placed in different orientations, and the elongation of the sample was greatest along the dimension containing a larger percentage of steel. On average, the expansion of the machined Invar surface was 42% less than the expansion of the steel surface. The results of this work demonstrate that additively manufactured Invar can be utilized to decrease the CTE for multi-material part tooling.

Relevance. Coatings based on bioactive glass are widely used in maxillofacial and orthopedic implantology due to their ability to osseointegration, degradation control and bone compatibility. At the same time, when implants are operated in loaded areas, glass coatings experience mechanical loads, thermal fluctuations and the effects of an aggressive biological environment, which places high demands on their crack resistance, adhesion and thermo-mechanical stability. Objective. To carry out a comparative evaluation of the performance characteristics of the new bioactive coating for the dental implant FAR-5X and its analogue, aimed at identifying potential advantages in mechanical stability and compatibility with the titanium base. Materials and methods. This study presents a comparative analysis of the mechanical properties of two glass–ceramic composite coatings, FAR-5X and FAR-5.3/E, designed for application on titanium implants. The analysis focused on key mechanical parameters, including the coefficient of thermal linear expansion, Vickers microhardness, fracture toughness, Young’s modulus, and adhesion strength. All measurements were performed on seven samples of each coating type using standard micro-mechanical testing methods. Statistical processing was conducted using one-way ANOVA and Tukey's post-hoc test. Results. The developed glass composite coating FAR-5X demonstrates a set of high-performance characteristics due to its nanostructured morphology obtained through optimized thermal treatment. This approach promotes a homogeneous distribution of the crystalline phase within the amorphous matrix, enhancing the material’s mechanical integrity. The results revealed a statistically significant higher coefficient of thermal linear expansion value in the FAR-5X coating compared to FAR-5.3/E (p < 0.05), indicating improved thermomechanical compatibility with the titanium substrate. A statistically significant difference between the coatings was found only in the coefficient of linear thermal expansion, indicating better thermal compatibility of FAR-5X with the titanium base. No significant differences were observed in microhardness, fracture resistance, elastic modulus, or adhesion strength, confirming comparable mechanical integrity between the coatings. Conclusion. The findings suggest that the newly developed FAR-5X coating offers advantageous thermal expansion behavior while maintaining the necessary mechanical robustness, supporting its potential application as a bioactive coating in orthopedic and dental implant systems.

The thermomechanical analyzer (TMA) is eminently suitable for the measurement of the thermal expansion coefficient of rubbers, provided the equipment is calibrated with materials of similar expansion coefficient. FKM or other thermally stable and amorphous polymers (of independently determined thermal expansion characteristics) can be used as calibration standards. The relative simplicity and precision of the TMA method for the determination of the coefficient of isotropic linear expansion, and hence, of volumetric expansion of rubbers, makes this important thermodynamic parameter easily accessible experimentally. Because of the frequently observed anisotropy of molded elastomeric compounds, measurements in all three dimensions are required to define linear, isotropic expansion and shrinkage which, to an excellent approximation, are represented by arithmetic averages of the three unidirectional values. Good agreement has been found between the average (isotropic) shrinkage determined experimentally and that calculated from the average (isotropic) coefficient of linear thermal expansion, α. The effect of fillers, such as carbon black, on α is additive. This makes it possible to predict α and shrinkage of all filled compounds from a single value of α of the unfilled rubber. The linear thermal expansion coefficients of the tested rubbers (FKM, NBR, EPDM) increase linearly with temperature, the increase being generally stronger for FKM than for the other elastomers. The appreciably higher mold shrinkage of FKM relative to NBR is directly due to the higher α of FKM for comparable levels of fillers. This must be kept in mind when expedience dictates the use of the same molds for FKM and NBR compounds.

The effect of cracks on the thermal expansion of rocks

Copper–matrix composites reinforced with Al2O3, SiO2, SiC and MgO nanoparticles were fabricated using powder metallurgy. The effects of the different nanoparticles on the thermal expansion behaviour of the composites were examined. The thermal expansion properties of the composites were evaluated using the coefficient of thermal expansion from 50 to 500°C. The relationship between interface stress and coefficient of thermal expansion of the copper–matrix composites with various reinforced phases was determined using theoretical models and experimental data. The results show that introducing dispersed nanoparticles into the copper matrix can significantly reduce the coefficient of thermal expansion. By contrast, increasing the temperature increases the thermal expansion coefficient of the copper–matrix composites and the interfacial pressure between the reinforcements and the copper matrix. The calculated pressure values at the nanoparticle–copper matrix interface suggest that plastic deformation occurred in the temperature range from 200 to 250°C for the SiO2/Cu and SiC/Cu composites and at 350°C for the MgO/Cu and Al2O3/Cu composites. These values are consistent with the variations observed in the measured coefficient of thermal expansion.

High density polyethylene (HDPE) is a thermoplastic polymer which is classified as one of the highly consumed types of plastics. One major advantage of thermoplastic materials is their ability of recycling and reprocessing which will bring considerable economic and environmental benefits. The present paper, therefore, endeavors to explore the practical possibility of using recycled HDPE hollow section as a replacement of virgin HDPE made by the extrusion process. The main focus of the study was to evaluate the mechanical performance of the recycled HDPE and compare the results with virgin or non-recycled HDPE. The modulus of elasticity, tensile yield and ultimate strength, compressive yield and ultimate strength, flexural yield and ultimate strength and the coefficient of thermal expansion were the main parameters to be checked against the respective mechanical properties. Thus, pursuant to the results, it was found out that the modulus of elasticity and the tensile yield strength are lower in recycled HDPE compared to the non-recycled HDPE. However, there is no significant difference between the recycled and non-recycled HDPE for the tensile ultimate strength, compressive yield strength and compressive ultimate strength. The flexural yield strength and flexural ultimate strength properties of the recycled HDPE proved to be superior to those of the non-recycled HDPE. The coefficient of linear thermal expansion of the recycled HDPE sample was 130 μm/(m.°C) and that for the non-recycled HDPE was 142 μm/(m.°C).

Thermal expansion and thermal diffusivity properties of Co-Si solid solutions and intermetallic compounds

Thermal expansion is an important design parameter of materials, which should be taken into account when developing, producing and using products. To control this significant characteristic of new materials, it is necessary to develop equipment, methods and measurement procedures that take into account the novelty of the specific behavior of these materials. The production of specially shaped samples is the main limitation in thermal expansion measurements. This primarily applies to new materials (heterogeneous, composite, poorly processed) developed for special tasks. These factors necessitated the development of a method for measuring the temperature coefficient of linear expansion of products and materials with arbitrary sample shapes, which was not available in previous implementations of interferometric methods. An interference speckle dilatometer SD was developed for measuring samples with irregular shape in the temperature range from 200 to 400 K. When creating the speckle dilatometer SD, the speckle interferometry method was implemented to measure the elongation of samples with an irregular surface with nanometer sensitivity. Since this device has no analogues, a study was conducted on the uncertainty components arising when measuring the thermal coefficient of linear expansion of materials on this device. The analysis of the uncertainty components of the measurement of the thermal coefficient of linear expansion of materials on the speckle dilatometer SD was carried out. An experimental determination of the measurement uncertainty of the thermal coefficient of linear expansion of TCLE measures of arbitrary shaped samples was obtained using the SD speckle dilatometer. After research and initial certification, the developed measurement instrument was approved and included in the State Secondary Standard of the unit of thermal coefficient of linear expansion of solids in the range of values from 0.05 · 10–6 to 100.0 · 10–6 K–1 in the range of temperature values from 90 to 1,900 K.

The geometry and constituent materials of metastructures can be used to engineer the thermal expansion coefficient. In this thesis, we design, fabricate, and test thin thermally stable metastructures consisting of bi-metallic unit cells and show how the coefficient of thermal expansion (CTE) of these metastructures can be finely and coarsely tuned by varying the CTE of the constituent materials and the unit cell geometry. Planar and three-dimensional finite element method modeling is used to drive the design and inform experiments, and predict the response of these metastructures. We demonstrate computationally the significance of out-of-plane effects in the metastructure response. We develop an experimental setup using digital image correlation and an infrared camera to experimentally measure full displacement and temperature fields during testing and accurately measure the metastructures’ CTE. We experimentally demonstrate high aspect ratio metastructures of Ti/Al and Kovar/Al which exhibit near-zero and negative CTE, respectively. We demonstrate robust fabrication procedures for thermally stable samples with high aspect ratios in thin foil and thin film scales. We investigate the lattice structure and mechanical properties of thin films comprising a near-zero CTE metastructure. The mechanics developed in this work can be used to engineer metastructures of arbitrary CTE and can be extended to three dimensions.

Preparation of low-CTE composite using new alkoxysilyl-functionalized bisphenol A novolac epoxy and its CTE enhancement mechanism

In this study, the effects of Nb and O contents on the negative thermal expansion behaviors including the coefficient of negative thermal expansion and matensite (α) stability of cold rolled (CR) Ti-34Nb alloys was investigated by optical microscopy, X-ray diffraction and Thermomechanical analysis. Results show that texture strength of CR-Ti-xNb-xO alloys is related to β stability, the addition of Nb and O (β stabilizers) enhances the texture strength. XRD results indicate that Nb and O inhibit the formation of α. The coefficient of negative thermal expansion of CR-Ti-xNb-xO alloys decrease with the increase of Nb and O contents. Moreover, cyclic thermal expansion results reveal that Nb and O promote the decomposition of α.

The nonequilibrium Green's function method is applied to investigate the coefficient of thermal expansion (CTE) in single-walled carbon nanotubes (SWCNT) and graphene. It is found that atoms deviate about 1% from equilibrium positions at T=0 K, resulting from the interplay between quantum zero-point motion and nonlinear interaction. The CTE in SWCNT of different sizes is studied and analyzed in terms of the competition between various vibration modes. As a result of this competition, the axial CTE is positive in the whole temperature range, while the radial CTE is negative at low temperatures. In graphene, the CTE is very sensitive to the substrate. Without substrate, CTE has large negative region at low temperature and very small value at high temperature limit, and the value of CTE at T=300 K is $-6\times 10^{-6}$ K$^{-1}$ which is very close to recent experimental result, $-7\times 10^{-6}$ K$^{-1}$ (Nat. Nanotechnol. \textbf{10}, 1038 (2009)). A very weak substrate interaction (about 0.06% of the in-plane interaction) can largely reduce the negative CTE region and greatly enhance the value of CTE. If the substrate interaction is strong enough, the CTE will be positive in whole temperature range and the saturate value at high temperature reaches $2.0\times 10^{-5}$ K$^{-1}$.

Flip chip ball grid array (FCBGA) is one of the most broadly used package types in different types of electronic packages. In a FCBGA package, the die/ silicon is flipped and attached to substrate with help of copper pillars. The material property coefficient of thermal expansion (CTE) mismatch between die/ silicon and substrate materials like core, dielectric material results in package thermal warpage and may lead to thermo-mechanical related failures in electronic packages. CTE can be defined as the rate at which a material expands with change in the temperature. Material expansion increases with a higher CTE. The CTE of die/ silicon is lower than the overall CTE of organic substrate. During reflow, as an electronic package cools down it warps in a convex or crying shape which indicates positive package warpage, and as the electronic package heats up it warps in a concave or smiling shape which indicates negative package warpage. Induced thermal warpage during the electronic package reflow assembly process is an important factor affecting the package assembly and thermo-mechanical reliability of FCBGA packages; therefore, precise package thermal warpage prediction is one of the most significant parameters for the electronic package design processes. In this study, the effect of different electronic package parameters (core material CTE, core thickness, package type) of the FCBGA package on its warpage are assessed by using finite element analysis (FEA) mechanical simulation methodology. A bare die FCBGA package having substrate core material with lower coefficient of thermal expansion (CTE) shows lower package warpage as the mismatch between die/ silicon and substrate core material CTE becomes dominant factor to drive the package warpage. To understand the package warpage trend, warpage is evaluated at room temperature & high/ reflow temperature. The package warpage trend is reversed for larger FCBGA packages with stiffener ring. Substrate core material with higher CTE shows lower package warpage as CTE mismatch between substrate core material and stiffener ring material (steel/copper) CTE becomes the dominant factor to drive the package warpage. The counter intuitive package warpage data from mechanical simulation is compared with shadow-moire test data and it shows good correlation and validates the simulations findings. Based on this work, it is recommended that a thorough investigation for large size packages be conducted before choosing packaging materials. This work shall provide new insights for package designers to optimize thermal warpage and stress during package initial design phase.

Reducing the thermal expansion anisotropy in Mo 5Si 3 by Nb and V additions: theory and experiment