Papers by Keyword: Thermal Expansion

Abstract: In this paper, we synthesized the CdSe quantum dots (QDs) about 4.5 nm, and using the in-situ XAFS technique to study the thermal expansion of CdSe QDs from room temperature to 700°C. We find that the thermal expansion of the CdSe QDs is about 2.0×10^-5/K, which is bigger than the bulk CdSe (3~8×10^-6/K). This can demonstrate that the thermal properties of CdSe QDs is related to the size of the dots, which means the thermal expansion is reducing as the size of the nanoparticles increasing. Keywords: CdSe quantum dots, thermal expansion, EXAFS technique

Abstract: Critical component of boiler that frequently experiencing failure is on the junction of boiler tube consist of two different materials. This failure mechanism would lead to tube rupture and would be followed by power plant shutdown. Failure analysis has been conducted on dissimilar metal weld (DMW) of Ferritic SA-213 T22 welded with Austenitic SA-213 TP 304H. This tube is used in Suralaya Steam Power Plant. Operating temperature and pressure of the steam inside the tube in normal condition are respectively 196.8kg/cm² and 540 °C.In order to understand the real cause of failure several tests are ran, which are chemical composition test, micro vickers, and metallographic test on based metal, heat effective zone, and filler area. The tests are conducted in a layered manner to clearly understand the cause of the failure. Causes of the failure are due to the contribution of carbon diffusion and disparity of expansion coefficient of two materials.

Abstract: To study ball bearing motor working principle, an ingenious experiment was self-designed, which results indicated that the nature for driving torque of the ball bearing motor was thermal expansion rather than electromagnetic force as generally considered. Furthermore, the threshold current and cutoff temperature for the ball bearing motor were found in our experiment.

Abstract: Increasing the functional parameters and endurance of the turbines, blades systems, rocket engines, high-performance metallurgical installations, power energy require the creation of new systems for integrated assessment, testing-investigating.From the ensemble of wear factors acting simultaneously on the multilayer ceramic materials, associated to the coatings structures, the quick thermal shock acts most disruptive.To illustrate the effect of gradients with high value for heating-cooling the authors have designed and developed a versatile installation with semi-automatic functioning and monitoring test parameters. The fundamental testing parameter that the installation uses is heating-cooling speed up to 100 ° C/s, which is superior to the performances of the known testing installations.The extreme thermal shock testing parameters generate in the multilayers ceramic structures, triplex type with intermediate layer, structural, morphological and composition modifications which initiate and develop the ceramic layer spallation.

Abstract: In addition to the good electrochemical performance criteria in solid oxide fuel cell (SOFC) applications, cathode material must match thermal expansion with other SOFC components. Thus, effects of Ag on thermal mismatch, chemical reactions, and microstructure are investigated. Ag (1 wt% to 5 wt. %) was mixed with La₀.₆Sr₀.₄Co₀.₂Fe₀.₈O_3-δ (LSCF6428) and Sm-doped ceria carbonate (SDCC) composite cathode powder. LSCF6428-SDCC-Ag samples were sintered at 600 °C for 2 h. The thermal expansion coefficients (TECs), which were determined using a dilatometer, indicated relatively less TEC mismatch between LSCF-SDCC-Ag cathodes composite and SDCC electrolyte. The average TEC value obtained from 20 °C to 600 °C implied that LSCF-SDCC-A5 (5 wt. % Ag) showed better thermal matching (13.18×10⁻⁶ K⁻¹) with SDCC electrolyte (12.84×10⁻⁶ K⁻¹) and achieved better compatibility. The X-ray diffraction patterns indicated that the LSCF6428-SDCC-Ag peak increased with the increase in the amount of Ag. Scanning electron microscopy analysis showed that Ag was capable of maintaining the porosity that is required for cathodes (20%–40%). Results showed that Ag exhibited desirable thermal and chemical compatibility with LSCF-SDCC. Thus, LSCF6428-SDCC-Ag can be used as a composite cathode for low-temperature SOFCs.

Abstract: Crystal structure of the mixed cobaltites-ferrites EuCo_1−xFe_xO₃, obtained by solid state reaction in air at 1573 K, has been studied by means of X-ray powder diffraction technique. It was found that all samples synthesized adopt orthorhombic perovskite structure. Obtained values of structural parameters indicate formation of continuous solid solution in the EuCoO₃−EuFeO₃ system. Thermal behaviour of the EuCoO₃ structure has been investigated in the temperature range of 298–1173 K by means of in situ high-resolution powder diffraction technique applying synchrotron radiation. Strong anomalies in the lattice expansion have been detected between 580 K and 800 K, which are evidently connected with the transitions of Co³⁺ ions to the higher spin states and further metal-insulator transition occurred in EuCoO₃ at about 600 K.

Abstract: Vermiculite has a structure consisting of layered silicates surfaces interspersed with layers of water. When the expansion occurs intra lamellar clay water is removed. This paper aims to characterize the Vermiculite clay of Santa Luzia, Paraíba (Brazil) in their natural form, and after thermogravimetric treatments by classic X-ray diffraction analysis, thermal analysis and scanning electron microscopy. The results show that the method of expansion, using the technique of microwave energy is possible, however, the expansion factor is lower when compared to the indirect heating by the thermal expansion of the clay.

Abstract: The paper presents a study about aluminum alloy, allied with Si, Cu, Mn, Mg. The Al-Si-Cu-Mg alloys for foundry are used for parts strongly required and which work at high temperatures, due to their good wear resistance. The industrial Al-Cu alloys contain 12 % cooper, are hipo-eutectic and may be for foundry or deformable. By alloying with magnesium, the Al-Cu alloys become with remarkable properties of resistance and plastic deformation processing. The improvement of mechanical characteristics for Al-Si alloys is realized with metals which forms the intermediate phases with silicon or aluminum, with variable solubility in solid state and which permits the structural hardening by heat treatments (quenching and ageing). From the analysis of dilatogramms, grouped for each sample, with the specific initial length, subjected to successive heating, from ambiance temperature up to 500°C, it is found that, with the appearance of ageing phenomena, on the samples aren’t significant modifications for elongation (few microns), only different may be the form of elongation-temperature curve. This analysis permits the determination of experimental data, regarding the behavior of Al-Si alloy subjected to heat treatments and repeated warming. Therefore, the Al-Si-Cu-Mg alloys, for foundry, are used for manufacture the parts strong required and which work at high temperatures, like pistons for engines with internal burn, parts for machines and reinforcements construction, due to their high usage resistance.

Abstract: A ball-screw system is widely used in high speed transmission platforms. High transmission speed brings high frictional force, heat and thermal expansion occurring on contact areas of ball, screw and nut. Positioning error of platform is affected by the thermal expansion during linear transmission. A vertical motion of ball-screw driving system was used for high speed transmission test. Thermal expansion of screw and positioning error were measured for discussion their effects. Cyclic vertical motion during a long transmission distance, 2 km, will integrate thermal heat to ball, screw and nut, thermal expansion of screw is also occurred. From experiments, temperature rising and thermal expansion of screw and nut were recorded and calculated for estimate elongation and positioning error of a vertical motion high speed ball-screw. Positioning error is not totally respected thermal elongation effects of screw and nut owing to contact forces caused by wear and thermal expansion at contact areas comparing to the heavy axial load is relative small in a vertical motion ball-screw system. Therefore, the structure stiffness is not affected significantly by thermal effect and wear.

Abstract: Ball screw is a high-precision and high performance linear drive of mechanical elements. The frictional heat of internal components what is very significant impact for platform transmission in high speed and the high axial load and causes the thermal expansion of element. In this research , the influence of different greases on ball screw is investigated in thermal rising of nut and driving torque in high speed and high axial load. A vertical motion platform was used for driving performance test. Thermal rising of nut of ball screw and the variance of transmission torque whose lubricated by high viscosity base oil grease is significant larger than the lower one. High viscosity grease is not easy to carry out the friction heat generated at ball and raceway contact area due to the bad flowing properties. It also has more serious wear occurred at contact area and high friction force, whose causes the large variance of transmission torque.