Large Difference In Thermal Properties Research Articles

Experimental investigation of mechanical properties of GGBS-FA-SF blended geopolymer concrete at elevated temperatures

Geopolymer has excellent mechanical properties at elevated temperatures, but geopolymer concrete may not be so because of the large difference in thermal properties between geopolymer and aggregate which could lead to substantial thermal stresses when they are in a high temperature environment. In this paper we present an experimental investigation on the mechanical properties of GGBS-FA-SF blended geopolymer concrete with and without steel fibres at elevated temperatures. The influences of exposure temperature, coarse aggregate and steel fibre on the failure mode, compressive strength, elastic modulus, peak strain, and ductility of the geopolymer mortar and geopolymer concrete are examined. Based on the experimentally obtained data, empirical temperature-dependent stress-strain constitutive equations are also proposed, which can be used for the fire safety analysis and design of geopolymer concrete with and without steel fibres.

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

Thermal performance analysis of multiple borehole heat exchangers in multilayer geotechnical media

Based on the finite line source (FLS) model, A heat transfer model considering both multilayer soil and multiple Borehole Heat Exchangers (BHEs) is proposed. The accuracy of the model is verified by experiments. The model is used to study the effect of thermal physical properties of multilayer geological media on the overall thermal efficiency of multiple BHEs. The dimensionless regional thermal efficiency (E) is introduced as an evaluation index of heat transfer characteristics of multiple BHEs. Under a certain geological structure, the heat transfer efficiency of multiple BHEs in homogeneous soils and multilayer soils were compared with the analytical model. The results show that the E of the multilayer multiple BHEs model is less than that of the homogeneous multiple BHEs model, with the maximum difference being 11.43% in 2,000 h. With these thermal properties and soil layer structure, the rate at which E decrease for the multilayer model is a factor of 4 higher than that for the homogeneous model. In the soil layer where the thermal interaction is the most intense, the unit Heat Transfer Rate (HTR) decreased by 25.3%. At a fixed spacing, the thermal diffusivity is the key to determining the degree of thermal interaction. Under the 4 × 4 BHEs layout, the dynamic performance loss increases by about 4% for every 1 × 10−7 m2/s increase in soil thermal diffusivity. When the multiple BHEs are in a multilayer geological media with a large difference in thermal properties (especially thermal diffusivity), a comprehensive multilayer analysis is needed to obtain the thermal efficiency of the overall multiple BHE area accurately. The multilayer multiple finite line source model can be used during the engineering design stages of a BHE field to predict the regional thermal efficiency with borehole spacing and the number of boreholes.

Laser offset welding of AZ31B magnesium alloy to 316 stainless steel

In this paper, the feasibility of using a fiber laser to perform a dissimilar metal joining was explored. AZ31B magnesium and 316 stainless steel were autogenously joined in butt configuration. The weldability between different materials is often compromised by a large difference in thermal properties and poor metallurgical compatibility. Thus, the beam was focused onto the top surface of the magnesium plate, at a certain distance from the interfaces (offset), and without using any interlayer or groove preparation. Such a method was called laser offset welding (LOW). Results proved a very good capability. The ultimate tensile strength exceeded the value of 100MPa, since a resistant and thin layer of hard intermetallic compounds is formed within the fusion zone. The rupture was observed within the magnesium side, far from the centerline. The metallurgy of fusion zone indicated the effectiveness of phases coalescence, without mixing at liquid states. LOW was demonstrated to be a promising technique to join dissimilar metal welds, being capable to produce an effective bonding with good tensile strength.

Thick tungsten coating on ferritic–martensitic steel applied with a vacuum plasma spray coating method

The characteristics of thick tungsten layers coated onto ferritic–martensitic (FM) steel blocks using a vacuum plasma spray system were investigated. Because of the large differences in thermal properties between tungsten and FM steel, it is difficult to coat a thick tungsten layer onto FM steel without delamination or detachment of the coated layer from the substrate. To avoid detachment of the tungsten layer from the FM steel surface, the optimization of the coating process was studied in this work. The tungsten layer having a 3.7mm thickness was coated using 43kW DC torch power at 16kPa of chamber pressure with a mixture of argon, hydrogen, and helium. The adhesion between the coating layer and the FM steel was enhanced by inserting pre-heating process before coating process, which increased the penetration of molten tungsten droplets into the FM substrate. The hardness of the coating layer was about 380–400HV, which was 80 to 95% of the hardness levels for commercial bulk tungsten monoblocks, and the porosity, purity, and morphology properties of the tungsten coating layer were also discussed. To evaluate the homogeneity of the layer bonding between the FM steel and the tungsten coating layer, an ultrasonic test was also performed.

The Effect of Laser Cutting Parameters on the Aerospace Structure Panel of CFRP Composite Material

This paper presents a research progress on the laser cutting characteristic of the carbon fiber reinforced plastic (CFRP) composites material for the aerospace structure panel. The CFRP is a high performance material and become one of the most important materials in the aerospace industry. The current machining techniques used by the aerospace composites manufacturing for cutting and trimming process have created some quality issues such as fiber pulled out and delamination. The use of laser cutting technology has shows promising advantages. However, the Laser cutting represents the interaction between laser beam and the CFRP composites, that produce heat affected zone (HAZ), kerf width and taper angle. Thermal damage is a direct consequence of the large difference in thermal properties of the carbon fiber and the polymer matrix. Therefore, the effect of laser cutting parameters such as pulse energy, pulse repetition rate, cutting speed and pulse duration need to be taken into consideration. A 300W Pulsed Nd:YAG laser machine was used in the experiment and successfully cut or trim the 1.5mm thickness of the CFRP component. The results also showed that the pulse energy and pulse repetition rate gave the most significant effect on the cutting characteristic in particular of kerf width, HAZ and taper angle.

Laser cladding of copper with molybdenum for wear resistance enhancement in electrical contacts

Laser cladding of Mo on Cu has been attempted with the aim of enhancing the wear resistance and hence increasing the service life of electrical contacts made of Cu. In order to overcome the difficulties arising from the large difference in thermal properties and the low mutual solubility between Cu and Mo, Ni was introduced as an intermediate layer between Mo and Cu. The Ni and Mo layers were laser clad one after the other to form a sandwich layer of Mo/Ni/Cu. Excellent bonding between the clad layer and the Cu substrate was ensured by strong metallurgical bonding. The hardness of the surface of the clad layer is seven times higher than that of the Cu substrate. Pin-on-disc wear tests consistently showed that the abrasive wear resistance of the clad layer was also improved by a factor of seven as compared with untreated Cu substrate. The specific electrical contact resistance of the clad surface was about 5.6 × 10 −7 Ω cm 2.

Resin Bonded CBN Grinding Wheel Dressing by Laser — Simulation and Experiment

Due to the large difference in thermal properties of the resin and the abrasive grits, the laser-assisted dressing technology is of great interest for grinding wheel preparation. From a viewpoint of thermal induced material removal mechanism, a numerical method was presented to simulate the laser dressing process. The numerical simulation results could reveal the relations between the laser parameters and the groove formation. The overlap coefficient was introduced for practical application. Simulation for the variation of the number of active grinding points was also made for non-uniform wheel topography. Based on the numerical modeling, the suitable grinding^wheel surface topography can be achieved through the laser-assisted dressing technology. The comprehensive researches on the laser-assisted dressing process control, grinding wheel topography reconfiguration by 3D laser scanning technology and analyses of grinding temperature were made. A series of grinding tests with the laser-assisted dressed grinding wheel and mechanically dressed grinding wheel were conducted for comparison. The results proved the feasibility of laser-assisted dressing for resin bonded superabrasive grinding wheels and revealed the importance of choosing appropriate laser dressing parameters.

Numerical study for enhancing the thermal conductivity of phase change material (PCM) storage using high thermal conductivity porous matrix

In this paper, the melting process inside an irregular geometry filled with high thermal conductivity porous matrix saturated with phase change material PCM is investigated numerically. The numerical model is resting on solving the volume averaged conservation equations for mass, momentum and energy with phase change (melting) in the porous medium. The convection motion of the liquid phase inside the porous matrix is solved considering the Darcy, Brinkman and Forchiemer effects. A local thermal non-equilibrium assumption is considered due to the large difference in thermal properties between the solid matrix and PCM by applying a two energy equation model. The numerical code shows good agreement for pure PCM melting with another published numerical work. Through this study it is found that the presence of the porous matrix has a great effect on the heat transfer and melting rate of the PCM energy storage. Decreasing the porosity of the matrix increases the melting rate, but it also damps the convection motion. It is also found that the best technique to enhance the response of the PCM storage is to use a solid matrix with high porosity and high thermal conductivity.

Analysis and Control of Ice Crystal Structure in Frozen Food and Their Application to Food Processing.

The large differences in thermal properties between water and ice cause thermal properties of frozen food to be strongly dependent on ice fraction, which is a function of solute concentration and temperature. Effective thermal conductivity of frozen food is described well by the dispersed model with ice phase dispersed in consideration of ice fraction. Ice crystal structure size in frozen food is determined by the advance rate of ice front reflecting the important role of the molecular diffusion of water in the mechanism of ice crystal growth. Supercooling causes the formation of very fine ice structure through the very high freezing rate at the moment of the cease of supercooling but the similar effect is realized by the pressure-shift freezing as a thermodynamically controllable process. By controlling the ice crystal structure size very large, the progressive freeze-concentration becomes possible. This method is very effective to make the freeze concentration system much simpler as compared with the conventional method of suspension crystallization to reduce the cost of freeze concentration substantially.

Welding of dissimilar aluminium alloys

The results of GTA-welding of dissimilar aluminium alloys of the series 5xxx and 6xxx for plate thickness of 1.5, 3 and 5 mm are presented in this paper. Specific attention was given to the asymmetry of the weld and melting behaviour of the different weld grooves. Practical solutions are given for realising optimal weld penetration for materials with a large difference in thermal properties. Due to welding, the strength of the material in the HAZ is reduced. This reduction in strength is smaller for solution hardened and strain hardened alloys, than for precipitation hardened alloys. For the precipitation hardened alloys the reduction in strength due to welding is smaller for the naturally aged condition than for the artificially aged situation. The reduction in strength for the strain hardened alloys is independent of material thickness for the specified range of material thickness.

The effect of change in thermal properties on the propagation of a periodic thermal wave: Application to a snow‐buried rocky outcrop

The propagation of a periodic thermal wave into snow is significantly altered by the presense of a shallow rock interface because of the large difference in thermal properties of the two media. The temperature distribution is modeled using classical heat conduction equations subject to a periodic diurnal or seasonal surface heat flux condition, jump conditions at the interface, and insulating conditions in the far‐field. The natural length scale (skin depth) over which order unity changes in temperature occur is proportional to the square root of the timescale of the surface temperature variations. If the interface lies close to or within the skin depth then large temperature gradients can be sustained in the snow before temperature oscillations are forced through to the underlying rock. These features are explained by an analytic one‐dimensional periodic solution. A numerical algorithm is constructed to solve for the temperature around plane two‐dimensional rock geometries. The results show that during a period of atmospheric cooling the presence of a buried rocky outcrop increases the snow temperature and temperature gradients simultaneously to produce very favorable conditions for crystal growth and avalanche formation.

Analysis of the friction and wear mechanisms of multilayered plasma-sprayed ceramic coatings

Wear tests were conducted on a rotor-vane-disk adaptor where three rotating vanes were pressed against a disk. Vanes were coated by WC and used as the upper specimen while the disk was coated by Cr 2O 3 and used as the lower specimen. A buffer layer of various thicknesses and contents was placed between the top coating and the bulk steel of the disk to alleviate the effects of the large difference in thermal properties of the two materials. The experimental results reveal that correct placement of a buffer layer can indeed improve the wear resistance. Factors such as the temperature to which the specimen was heated before testing, the proportion by weight of each individual constituent in the buffer layer, and the thickness of each coating layer, were also important for the volume of wear of the lower specimen. As the specimens were heated to higher temperatures, the wear volume decreased with increasing proportions of Cr 2O 3 in the buffer layer. Elevating the preheating temperature of the specimens can diminish the wear volume but increases the friction coefficient. The steady-state wear rate is not much influenced by the constituents of the buffer layer and the coating thickness. Brittle fracture, abrasion, adhesion and oxidation were found to be the primary wear mechanisms in the tests.

Scanning thermal profiler

A new high-resolution profilometer has been demonstrated based upon a noncontacting near-field thermal probe. The thermal probe consists of a thermocouple sensor with dimensions approaching 100 nm. Profiling is achieved by scanning the heated sensor above but close to the surface of a solid. The conduction of heat between tip and sample via the air provides a means for maintaining the sample spacing constant during the lateral scan. The large difference in thermal properties between air and solids makes the profiling technique essentially independent of the material properties of the solid. Noncontact profiling of resist and metal films has shown a lateral resolution of 100 nm and a depth solution of 3 nm. The basic theory of the new probe is described and the results presented.