Papers by Keyword: Coefficient of Thermal Expansion

Abstract: This paper presents the results of an experimental determination of the coefficient of thermal expansion of hardened alkali-activated slag pastes. In the experiment, three different activators were used – liquid sodium silicate, sodium carbonate and sodium hydroxide. The slag volume fraction for all paste types was set to 0.52. The specimens’ internal moisture content and shrinkage strains were plateaued at ambient laboratory conditions (temperature = 22 ± 2 °C, relative humidity = 55 ± 5 %) at the testing time. All of the specimens were wrapped into the polyethylene foil before the start of thermal dilation measurements and stored in the Q-Cell incubator in which they were subjected to temperature cycling from 5 °C to 30 °C. Length changes caused by the temperature changes were measured with dilatometers supplemented by continuous strain gauges measurement. The results show different values of the coefficient of thermal expansion depending on the activator type. Moreover, the strains recorded by the dilatometer through the steel pins embedded into the ends of specimens and by the strain gauges placed in the core of the test specimens led to different resulting values of the coefficient of thermal expansion.

Abstract: AlN has high thermal conductivity (TC) and low coefficient of thermal expansion (CTE), which is an ideal reinforcement phase for strengthening the thermophysical properties of magnesium alloys to be used as packaging substrate material. However, the poor wettability between AlN and magnesium melt makes the preparation of AlN/Mg composites with high AlN content very difficult. In this work, the AlN/Mg-5Zn-4.9Cu composites were prepared by semi-solid stir casting method to address this problem, and the effect of AlN particle content on the thermophysical properties of the composites was investigated. The results show that AlN can reduce the CTE of magnesium alloy significantly and weaken the TC of magnesium alloys slightly. When 20 wt.% of AlN was added, the TC of the composites was 125.1 W/(m·K) and the CTE was only 19.83×10^-6 K^-1, which was 26.7% lower than that of the matrix alloy.

Abstract: For the purpose of this investigation, carbon and Kevlar fiber-reinforced plastics were subjected to dynamic testing. Carbon-Kevlar fibers are included into the LY 556 resin and AR 951 hardener. Fiber makes for 40% of the study's total weight. The carbon and Kevlar composites were created by hand lay-up. The orientation considered in this research is 0/0 and 0/90 for both carbon and Kevlar composites. Advanced characterizations such as TMA and DMA has been integrated in this research. By using DMA (Dynamic Mechanical Analyzer) and the thermomechanical Analyzer (TMA), the dynamic properties were acquired. ASTM E756 was used to conduct the tests. Composite heating at room temperature to 120⁰ C was the temperature variable under investigation. The studies were conducted on TMA and DMA with a force of 0.002N, respectively. DMA uses a three-point bending loading design for its samples. Temperature affects the dimensions, storage modulus, loss modulus, and tan delta.

Abstract: The 10 vol% SiC_p/Mg composites were prepared by external addition and stirring-casting method, and the hybrid reinforced (10 vol% SiC_p+10 vol% Mg₂Si)/Mg composites were prepared by combining in-situ method. The effects of melt ultrasonic treatment (UT) and forming processes on the thermophysical properties of the two composites were studied. The results show that UT can effectively disperse SiC particles in molten magnesium and reduce the casting porosity, while squeeze casting can significantly reduce the porosity of the composites, which can also significantly improve the thermal conductivity. The thermal conductivity (λ) of 10 vol.% SiC_p/Mg composites squeeze casted after UT is 135.3 W/(mK) and the average coefficient of thermal expansion (CTE) is 19.95×10^-6 K^-1 at 293-373 K. Compared with gravity casting, the λ is increased by 17% and the CTE is reduced by 0.8%. The λ of (SiC_p+Mg₂Si)/Mg composite squeeze casted after UT is 132.4 W/(mK), and the CTE is 18.95×10^-6 K^-1, which is 27% lower than the CTE of pure magnesium.

Abstract: This experimental study deals with the problematics of thermal expansion α [10^-6/K] of the composite systems based on recycled carbon fibres reinforced epoxy resin. The epoxy resin CHS – EPOXY 520 (EPOXY 15), cured with the hardener P11 (Districhem, s.r.o.), was chosen as a sample matrix. Recycled carbon fibres with a diameter of 7 μm and a length of 100 μm (Easy Composites Ltd.) were the filler. In the experiment, samples with the fulfilment of 10, 20, 40, 60, 80, 90 and 100 phr were prepared. The samples were being poured into silicone molds, cured at an overpressure of 0.7 MPa and a temperature of 23 ± 2 °C for 24 hours. A thermomechanical analyzer was used to determine the thermal expansion of composite systems. The addition of recycled carbon fibers to epoxy resin can reduce the coefficient of linear thermal expansion at 20°C in half, from the original α = 45 to 55 × 10^-6/K for non-filled epoxy resin to α = 25 to 27 × 10^-6/K for filled epoxy resin. Optimal filling is at the level of 40 phr, with higher filling there is no further reduction of the linear thermal expansion coefficient.

Abstract: This paper describes a method for detecting defects inside high-refractive index gems. This method consists in immersion of a precious stone inside low-melting chalcogenide glass. After cooling, molten glass turns into a solid phase and is an optical cube. This cube can be photographed in layers and using OctoNus equimpment a 3D model of gemstone defects can be built. The proposed method allows you to effectively and accurately determine the coordinates of the defects in diamond and to offer the most profitable option for polishing a precious stone.

Abstract: The study of glass components influence on the physical and chemical properties of As–S–Se–I glasses has the potential to help design glasses with the required properties. A change of certain glass properties upon the introduction of selenium into AsSI glass was demonstrated. Non-stoichiometric compounds along the As₂S₃-AsI₃-Se diagram section were investigated by differential thermal analysis, ultrasonic measurements, dilatometric measurements. It has been shown that the introduction of selenium decreases the glass solubility in water. Change in the glass properties with the change in the short-range structure studied by Raman spectroscopy was shown. These glasses can be used for creation of immersion lenses and optical adhesives which change radiation characteristics of mid-IR LEDs.

Abstract: Metallography analysis of invar alloys crystallized with different cooling rates has been carried out. The study has demonstrated that velocity of crystallization has an impact on the dispersity of graphite. The higher velocity of cooling, the more dispersive graphite is. The volume percentage of graphite in alloy, crystallized with high cooling rate, is lower than compared with low cooling rate. Crystallization with low cooling rate leads to the reduction of the amount of carbon into γ-phase. The coefficient of thermal expansion is basically depends on the amount of carbon into γ-phase.

Abstract: Solid state sintered pellets of barium hexaferrite were performed. The Curie temperature of BaFe₁₂O₁₉ pellets were measured with differential scanning colorimetry (DSC). The coefficient of thermal expansion (CTE) of pellets was defined above and below the Curie temperature. The Curie temperature calculated basedon the dilatometry is are in good agreement with the DSC measurements.

Abstract: Samples of VT23 β-metastable titanium alloy quenched from temperatures in a range between critical and β-transus were studied in situ during heating on X-ray diffractometer. The anisotropy of orthorhombic martensite lattice was investigated. Abnormal behavior of «b» lattice spacing, which demonstrated a negative value of thermal expansion, has been revealed.