Thermal expansion of metals

Inserts can be incorporated into the mould before injection or placed after demoulding into a moulded hole. For neat resins, the first solution is generally avoided because of the big difference between the coefficients of thermal expansion of metals and plastics. In both cases, inserts and embossing must obey some general rules. Among these, some, but not all, are recalled below ... 

Steam soaking involves allowing intimate contact between steam and the surface to be cleaned. The technology depends on the penetration of the deposit by steam and hot condensate, in addition to the effects of the differences in thermal expansion of metal and deposit, which produces spalling of the deposit. 

Table 5.2 Linear coefficients of thermal expansion of metals and oxides ...

Thermal Expansion of Graphite Magnesium Castings Linear Thermal Expansion of Metals and Alloys Thermal Expansion of Ceramics... 

Concentrations of Schottky defect can be measured with experiment of thermal expansion of metals, namely the determination of thermal expansion coefficient of both the whole crystal and lattice parameters, respectively. The thermal expansion coefficient of the whole crystal includes not only the thermal expansion of crystal lattice itself, but the formation of Schottky defect. Therefore, the difference of two results can reflect both the existence and concentration of Schottky defect. For instance, at conditions near to the melting point, the concentration of Schottky vacant for alumina is about 1 x 10, and formation energy of its vacant is about 0.6eV (leV = 1.60 x 10 J) while that of NaCl is 10 -10 and formation energy is 2 eV, respectively. 

In nanoparticle-polymer composites, thermal stability is one of the most important property enhancements. Recently, some theoretical efforts have been made to predict the thermal stability of such composites. For example, FEM and the theory of Chow have been used to predict the thermal expansion of clay-polymer nanocomposites. The results indicate that it is possible to considerably reduce and eventually match the thermal expansion of metal and polymer parts by dispersing a small amount of exfoliated muscovite mica platelets into a polymer matrix. Moreover, reduction is controlled by the product of aspect ratio and volume fraction of the platelets. 

Table 1 compares the thermal properties of metals, minerals and plastics. The thermal expansion of plastics is up to nine times greater than that of metals. This becomes important when plastic parts are used in combination with metals. For example, the chassis and fi amework of an automobile are made from steel. If a large part - a door, trunk lid or hood -made of plastic is set in a metal fi ame, problems will occur as the automobile imdergoes its service temperature cycle of 140°F in summer in the Texas sim to -60°F in Alaskan winters. Between these temperature extremes, the differential expansion can be as high as 0.01 inch per inch. In a part that is 50 inches wide, this corresponds to one-half inch. If the door fits its opening in winter, it will jam in summer if it fits in summer, it will rattle in winter. As Table 1 shows, minerals have one-half to one-third the thermal expansion of metals. Compounding plastics with minerals brings their thermal expansion closer to that of metals and reduces the problem. 

Holzer, H. and Dunand, D. Processing, structure and thermal expansion of metal matrix composites containing zirconium tungstate. Fourth International Conference on Composite... 

The coefficient of linear thermal expansion of metals also varies slightly with temperature. Coefficients for commonly used metals at 21°C and 149°C are presented in Table 4.2 [5]. 

Thermal expansion mismatch between the reinforcement and the matrix is an important consideration. Thermal mismatch is something that is difficult to avoid ia any composite, however, the overall thermal expansion characteristics of a composite can be controlled by controlling the proportion of reinforcement and matrix and the distribution of the reinforcement ia the matrix. Many models have been proposed to predict the coefficients of thermal expansion of composites, determine these coefficients experimentally, and analy2e the general thermal expansion characteristics of metal-matrix composites (29-33). 

Cases can be classified as either hermetic or nonhermetic, based on their permeabiUty to moisture. Ceramics and metals are usually used for hermetic cases, whereas plastic materials are used for nonhermetic appHcations. Cases should have good electrical insulation properties. The coefficient of thermal expansion of a particular case should closely match those of the substrate, die, and sealing materials to avoid excessive residual stresses and fatigue damage under thermal cycling loads. Moreover, since cases must provide a path for heat dissipation, high thermal conductivity is also desirable. 

Cross-linked polyester composites have a relatively low coefficient of thermal conductivity that can provide beneficial property retention in thick laminates at high temperatures as well as remove the need for secondary insulation. The coefficient of thermal expansion of glass-reinforced composites is similar to aluminum but higher than most common metals. 

Thermal Properties. Refractories, like most other soHds, expand upon heating, but much less than most metals. The degree of expansion depends on the chemical composition. A diagram of the thermal expansion of the most common refractories is shown in Figure 1. 

The specific heats of polymers are large - typically 5 times more than those of metals when measured per kg. When measured per m, however, they are about the same because of the large differences in density. The coefficients of thermal expansion of polymers are enormous, 10 to 100 times larger than those of metals. This can lead to problems of thermal stress when polymers and metals are joined. And the thermal conductivities are small, 100 to 1000 times smaller than those of metals. This makes polymers attractive for thermal insulation, particularly when foamed. 

The allowable dimensional variation (the tolerance) of a polymer part can be larger than one made of metal - and specifying moulds with needlessly high tolerance raises costs greatly. This latitude is possible because of the low modulus the resilience of the components allows elastic deflections to accommodate misfitting parts. And the thermal expansion of polymers is almost ten times greater than metals there is no point in specifying dimensions to a tolerance which exceeds the thermal strains. 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

Dressing Sharp edges must be removed. Thermoplastics have a greater coefficient of thermal expansion than metals. They therefore shrink onto the metal and if sharp edges are present then these will cut through the coating and become exposed. These exposed edges will start to corrode and this will inevitably result in underfilm creep corrosion. 

Where a chrect fusion of glass-to-metal or metal oxide is obtained (sometimes termed the wetting of metal by glass) two types of seal can result—one matched, the other mismatched, depending on how the relative coefficients of thermal expansion of the glass and the metal compare. 

The uncertainty of the expansion data was evaluated to be less than 5%. The relative thermal expansion AL/L versus T is shown in Fig. 13.3. Curves for other materials are also reported for the sake of comparison. Torlon shows a linear thermal expansion lower than most polymers. As Stycast 2850FT, Torlon thermal contraction closely matches that of some common metals (i.e. aluminium [53] and brass [54]). Smoothed values for the thermal expansion of Torlon relative to 4.2K are reported in Table 13.1. The data... 

A typical measurement was performed as follows. The feeder was lowered into the crucible and the sample solution (seawater) was allowed to flow under an inert atmosphere with the suction on. A constant current was applied for a predetermined time. When the pre-electrolysis was over, the flow was changed from the sample to the ammonium acetate washing solution, while the deposited metals were maintained under cathodic protection. Ammonium acetate was selected for its low decomposition temperature, and a 0.2 ml 1 1 concentration was used to ensure sufficient conductivity. At this point the feeder tip was raised to the highest position and the usual steps for an electrothermal atomic absorption spectrometry measurement were followed drying for 30 s at 900 C, ashing for 30 s at 700 °C, and atomization for 8 s at 1700 °C, with measurement at 283.3 nm. The baseline increases smoothly with time as a consequence of an upward lift of the crucible caused by thermal expansion of the material. 

It is important to consider thermal effects in the design of the grid-to-shell seal. Bypassing of the grid at the seal point is a common problem caused by situations such as uneven expansion of metal and ceramic parts, a cold plenum and hot solids in contact with the grid plate at the same time, and start-up and shutdown scenarios. When the atmosphere in the bed is sufficiently benign, a sparger-type... 

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