Thermal Aging Characteristics Research Articles

Ir-Based Schottky and Ohmic Contacts on n-GaN

Schottky contacts and ohmic contacts on n-type GaN were investigated as a function of annealing temperature and compared to their more common Ni-based counterparts. The ohmic contacts on n-type GaN with exhibited barrier heights of after annealing at and displayed less intermixing of the contact metals compared to . A minimum specific contact resistance of was obtained for the ohmic contacts on n-type GaN with after annealing at . The measurement temperature dependence of contact resistance was similar for both and , suggesting the same transport mechanism was present in both types of contacts. The Ir-based ohmic contacts displayed superior thermal aging characteristics at . Auger electron spectroscopy showed that Ir is superior to Ni as a diffusion barrier at these moderate temperatures.

Compatibilisation of Heterogeneous Styrene Butadiene Rubber/Poly(Ethylene-co-vinyl Acetate) Blends

The compatibilisation of styrene butadiene rubber (SBR) and poly(ethylene–covinyl acetate) (EVA) blends with maleic-anhydride grafted poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS-g-MA) triblock copolymer has been investigated. The introduction of SEBS-g-MA resulted in an improvement in the mechanical properties of the blends with a reduction in damping characteristics, indicating the presence of efficient interfacial bonding. The optimum loading of SEBS-g-MA for SBR/EVA blends, has been found to be 6 phr. The better mechanical performance has been observed to be achieved for the compatibilised blends with DCP as curing agent. The results have been complemented by the studies using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). It has also been found that the compatibilisation with SEBS-g-MA improved the thermal ageing characteristics of SBR/EVA systems.

A comparative study of thermal ageing characteristics of poly(ethylene‐co‐vinyl acetate) and poly(ethylene‐co‐vinyl acetate)/carbon black mixture

AbstractIn this comparative study, the effect of carbon black (CB) on the thermal ageing characteristics of poly(ethylene‐co‐vinyl acetate) (EVA) was investigated. EVA, containing 13% vinyl acetate (VA), and poly(ethylene‐co‐vinyl acetate)/carbon black mixture (EVA/CB) containing 13% VA and 1% CB were aged at 85°C in air up to 30 weeks. Sol‐gel analysis experiments were made to determine the percentage gelation of both virgin and aged samples. FT‐IR measurements were performed to follow the chemical changes which took place in the samples during ageing. Dynamic and isothermal thermogravimetric studies were performed for determination of the thermal stabilities of virgin and aged samples.Sol‐gel analysis results showed that EVA itself has a tendency to form a gel under thermal treatment, whereas EVA/CB never becomes a gel when being thermally aged under the same conditions. As a result of FT‐IR measurements, some oxidation products such as ketone, lactone and vinyl species were observed through thermal ageing of EVA. It is also clear that these kind of oxidation products did not appear to a considerable extent in EVA/CB. Thermal analysis experiments exhibit that thermal stability of EVA decreased through thermal ageing; whereas that of EVA/CB remained almost unchanged. Copyright © 2004 John Wiley & Sons, Ltd.

Effect of aging on glass transformation measurements by temperature modulated DSC

Differential scanning calorimetry (DSC) is a well established technique for studying the glass transformation behavior of glasses and for measuring the glass transition temperature T g and the relaxation enthalpy Δ H g. However, T g and Δ H g measurements on glasses from DSC heating scans depend on the thermal history and aging (annealing time at a given temperature below T g) which makes it difficult to compare measurements on a given glass from different laboratories and also examine small changes in T g and Δ H g due to small changes in the glass composition. We have performed both temperature modulated DSC (TMDSC) and conventional DSC experiments to study the dependence T g , complex heat capacity C p and relaxation enthalpy Δ H g on aging for vitreous Se alloyed with 0.5% As (a-Se:0.5%As). We have studied the behavior of the TMDSC total, reversing and nonreversing heat flows and the DSC heat flow as the glass is heated from 20 °C to a temperature above the glass transition temperature. We observe that TMDSC T g has a much smaller dependence on the aging time than DSC T g measurements and, therefore, TMDSC results are particularly useful for T g comparisons of glasses with unknown thermal history and aging characteristics. The change in TMDSC T g was almost ten times smaller than the DSC T g change over an aging time of 100 h. We observe that the apparent relaxation enthalpy Δ H g as measured by conventional DSC, reversing and total heat flows in TMDSC increases with the aging time. If we analyze the long time relaxation in terms of stretched exponential behavior we find β values in the range 0.39–0.43.

Thermal Aging and Low Cycle Fatigue Characteristics of CF8M in a Nuclear Reactor Coolant System

Thermal Aging and Low Cycle Fatigue Characteristics of CF8M in a Nuclear Reactor Coolant System

The Thermal Conductivity of Isocyanate-Based Rigid Cellular Plastics: A Technique for Predicting the 25 Year Values*This paper was first published in Cellular Polymers, Vol. 16, No. 2, 1997

This paper reviews the factors determining the thermal conductivity of isocyanate- based rigid cellular plastics. It provides research data obtained from a programme conducted at Salford University, to establish the basis of a method based on an accelerated procedure for the determination of the thermal conductivity of these products which can be assumed to apply, without taking into account production variation, to a product over its 50 year life. The latter is used to correspond to the term “reasonable economic life" used in the EU Construction Products Directive. The data is limited to products principally expanded with HCFC 141b and n-pentane blowing agents but the authors believe, although proof will be necessary, that the method has a good chance to be acceptable for all future products which are expanded principally with a gas which is shown to remain, for the most part, in the cells over its reasonable economic life and for which the gaseous composition of the cell gas is changed only by dilution, to varying degrees, by the ingress of air. In essence the ingress of air is accelerated by ageing the test specimen in air at 70°C for 25 weeks. The paper also discusses the merits of various other procedures claiming to produce "realistic aged thermal conductivity values". Finally it provides evidence that the products made with these new blowing agents have similar thermal conductivities and similar thermal conductivity ageing characteristics to their CFC blown counterparts.

Hydrogenated Hydroxy‐Terminated Polyisoprene as a Fuel Binder for composite solid propellants

AbstractThe synthesis and application of hydrogenated hydroxy‐terminated polyisoprene (HHTPI) to a fuel binder of composite solid propellants were attempted. An HHTPI prepolymer was synthesized through the hydrogenation for the hydroxy‐terminated polyisoprene (HTPI) in the presence of nickel and zirconium catalysts over kieselguhr in 2.0 MPa hydrogen and at 443 K – 453 K for 24h. A prepolymer of a number‐averaged molecular weight 2500–3800, provided a viscosity level required for the use of a fuel binder from which solid propellant can be possibly made by means of direct casting method. Thermal stability and aging characteristics of HHTPI elastomer against environmental attacks are superior to those of HTPB. Some plasticizers and bonding agents can bring about the acceptable mechanical properties to the propellant grains mainly composed of HHTPI, ammonium perchlorate and aluminium powder. The linear burning rates of HHTPI‐based propellants are at the same level with that of HTPB‐based propellants. However, the composition that gives the maximum performance with HHTPI‐based propellants, shifts to 1–2 wt% fuel‐rich side from the most adequate fuel content 12 wt% in HTPB/AP/Al. The HHPTI propellants indicated the similar burning rate as HTPB‐based propellants in the linear burning rates in spite of the comparatively poor ignitability. Nevertheless, the static tests of 100 mm dia. sounding rocket motors are successfully performed by an ignition operation at the pressurized condition. The ballistic performances are not inferior to those of the HTPB‐based propellants.

Characteristics and annealing behavior of polyethylenes designed for use as underground insulation

AbstractA series of polyethylenes developed for use as high voltage electric cable insulation have been studied. All crosslinked polyethylene materials show similar thermal aging characteristics, whereas linear low‐density polyethylene is susceptible to thermal aging at higher annealing temperatues. Tree‐retardant additives are soluble in xylene, but some are also attached to the gel network. They function both as enhanced antioxidants and as enhancers of electric breakdown strength. Biaxially oriented film shows the greatest resistance to electric breakdown.

Conductor insulation tests in oil: aramid vs. kraft

Developments in transformer insulation, leading to increased use of aramid insulation in oil-filled transformers, are briefly summarized. A series of tests designed to gather data on the dielectric strength of aramid paper in oil used as turn-to-turn insulation is presented. The test setup, sample configuration and preparation, and test procedures are described. Turn-to-turn and coil-to-coil results are presented and discussed. It is shown that, turn-to-turn, the aramid paper is stronger dielectrically than the kraft paper by 25% using a full-wave impulse and 20% using a 1 mu s chopped wave. There is little or no difference in the dielectric coil-to-coil strength between the aramid and kraft papers. Thus, it is possible to improve the turn-to-turn impulse strength of liquid-filled power transformers by using aramid paper in place of kraft paper. The aramid also has improved mechanical strength and thermal aging characteristics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermal aging and storage characteristics of rapidly solidified Nd-Fe-B powder (abstract)

Various magnetic products are now being produced from rapidly solidified Nd-Fe-B powder. Some types of magnets included injection molded, calendered, compression molded and hot pressed. A major factor leading to this increased use is the inherent stability of the powder. This paper reviews the test results of the following studies: (a) Rapidly solidified Nd-Fe-B powder stored at ambient conditions for a period of 11 months. Oxygen and nitrogen are monitored along with magnetic induction (Br). (b) Nd-Fe-B powder was tested for 6 months at 23 and 38 °C in humidity of 50% and 80%, respectively. Magnetic induction, Hci and Bd were monitored. (c) Nd-Fe-B powder was tested at elevated temperatures including 155 °C for 696 h. Magnetic induction and oxygen were monitored. The test results show the rapidly solidified Nd-Fe-B powder to be exceptionally stable requiring no special handling under normal conditions.

Rare-Earth-Iron-Boron permanent magnets by rapid solidification processing

The development of a light-rare-earth-iron-based permanent magnet has been a long standing goal of the permanent magnet industry. The first successful attempts at magnetically hardening these materials employed rapid solidification techniques. This work culminated in the discovery of the NdFeB-based permanent magnet materials, the properties of which have been shown to derive from the Nd 2Fe 14B intermetallic phase. This class of magnets has received considerable scientific and commercial interest stemming from the fact that energy products already significantly exceed those of the SmCo-based materials. Moreover, owing to the relative abundance of neodymium, the cost of these materials is potentially much lower. Processing these magnets is carried out by two distinctly different techniques. These include the conventional powder metal-sinter process and consolidation of rapidly solidified materials. This paper discusses the preparation of these materials by the latter method. Included are the properties of the rapidly solidified ribbons themselves, methods for consolidating these materials into monolithic magnet shapes, microstructural data, and long-term thermal aging characteristics.

Evaluation of Locomotive Cable Insulation Life Under Varying Temperature Loading

A method is described for evaluating the life of cable insulation subjected to the varying temperature and time electrical service conditions of a locomotive. This type of evaluation becomes more important as the current ratings of higher fuel efficiency locomotives are increased, subjecting cable insulation materials to higher operating temperatures. Insulations evaluated by this proposed technique are first tested at several elevated constant temperatures to determine their thermal aging characteristics. Based on the material performance data, the insulation life consumed by the thermal stresses imposed by the locomotive duty cycle is calculated. This method can be readily applied to measure the effects of temperature cycles on cable life, objectively compare the performance of different insulations, and assess the design margin of locomotive cables. It may also be used to establish Association of American Railroads (AAR) specification requirements for thermal aging tests based on the conditions of actual field application. A method for evaluating cable life through testing and duty cycle analysis has been developed and verified by examination of cables in service. A new high performance insulation developed by the General Electric Wire and Cable Business Department is shown to offer reliable service life under the varied and demanding operating conditions of today's higher performance locomotives.

The properties of equimolar copolycarbonates of bisphenol A with bisphenol S and 1,4-bis(hydroxymethyl)cyclohexane

Ranges of both alternating and random 1:1 copolycarbonates of Bisphenol-A (2,2-bis(4-hydroxyphenyl)propane) and Bisphenol-S (4,4′-dihydroxydiphenyl sulphone) and of Bisphenol-A and 1,4-dimethanoylcyclohexane have been synthesized in order to test the hypothesis that, other things being equal, a more random structured polymer should exhibit superior thermal ageing characteristics. Studies on these materials' mechanical and thermal properties establish that within each pair they are extremely similar and hence the original hypothesis is disproved.

N-(Morpholinothio)Imides as Curing Agents in Semiefficient Vulcanization Systems

Abstract It is well known that both the thermal and oxidative aging characteristics of vulcanizates can be improved by using an efficient vulcanization (EV) or semi-EV system. These desirable aging properties are attributed to the presence of mono- and disulfidic crosslinks in the vulcanizates cured by EV or semi-EV systems. In contrast to a polysulfide crosslink, mono- and disulfide crosslinks are more thermally stable and undergo minimal crosslink modification during aging. One way to achieve such desirable properties is to cure a stock with 2-benzothiazyl 4-morpholino disulfide (BMD) and low levels of sulfur. However, the BMD cure system lacks the scorch protection required for many applications. Moreover, neither N-(cyclohexylthio)phthalimide (CTP) nor N,N′-bis(cyclohexylthio)oxanilide (BCTO) shows an appreciable effect on the scorch time of BMD cure systems. This is true regardless of the level of sulfur used, as shown in Figure 1. The maximum increase in scorch time with 1.00 part CTP was only 35%, and this occurred only when three parts of BMD and no free sulfur was used. When 2.00 parts sulfur was used, scorch time was extended only 18%. Likewise, BCTO did not respond well in BMD cure systems. The poor performance of these retarders in BMD cure systems prompted us to design a cure system in which BMD can be formed during vulcanization.

A New Additive for Improving the Thermal Aging Characteristics of Kraft Insulating Paper

Kraft paper is used for insulation in oil-immersed equipment such as transformers. The reason is that Kraft paper has given satisfactory performance on electrical and mechanical properties. As for life, however, thermal degradation occurs due to temperature rise in operation. Examples of these kinds of the Kraft paper are cyanoethylated Kraft paper and amine modified Kraft paper. Kraft paper containing morpholine, oxyalkylamine, aromatic polyamine, para-toluene-sulfone, etc., are well known. This paper describes a method for thermally upgrading Kraft paper using carbamate compound. The results obtained are summarized as follows. 1) n-Butyl carbamate is effective for thermally upgrading Kraft paper. 2) n-Butyl carbamate modified paper does not contaminate transformer oil.

カーバメイト添加紙の耐熱性

カーバメイト添加紙の耐熱性

Thermal aging characteristics of bottom sands in acid open hearth furnaces

It may be that an understanding of the development of a liquid phase from the disordered silica upon the surface of new sand grains and the development of the liquid transition phase from crystalline quartz will lead to improved operation in acid open hearth furnaces. The existence of such liquid phase appears to have a relation to the formation of gas cavities in steel castings and possibly contributes to the phenomenon called metal penetration.

Aging of wire enamels in transformer oil

THERMAL aging characteristics of wire enamels aged under conditions similar to those which may be expected in sealed liquid-filled transformers have been studied and found to be quite different from results obtained by aging the same enamels in air. Enamels which have shown only fair thermal aging characteristics in air in comparison to other enamels may rate very differently when aged in a sealed system with a limited amount of oxygen present. For example, Formex (General Electric registered trademark) wire enamel which has a short life in comparison to a terephthalate polyester when aged thermally in a circulating air oven is equal or superior to the terephthalate polyester when aged in a closed system under oil.

Formex- and paper-insulated wires in transformer oil

WIRE INSULATION is probably the most important component of the insulation system used in oil-immersed transformers. Its strategic location — in contact with and separating turns — results in the insulation operating at temperatures very near that of the conductor which is the maximum temperature encountered in the transformer. Therefore, its thermal aging characteristics are of considerable interest.

Gas-insulated power transformers

Power transformers have reached such a degree of perfection that it seems unlikely that there is room for substantial improvement in any fundamental characteristic. However, the insulation of conventional transformers is in the main dependent upon a liquid which is not only inflammable but has other undesirable characteristics. The substitution of gas for oil as the insulating medium eliminates the undesirable characteristics of the latter, and in addition, gasinsulated units have other advantages.Of the various gases having desirable properties for use in modern equipment, sulphur hexafluoride is prominent. It is produced commercially in the United States.After reviewing the advantages of gas-insulated transformers, the paper examines the fundamental characteristics of this electro-negative gas, and experimental data are presented of its electric strength relative to oil. Of considerable interest to the designer is the peculiar behaviour of gases subjected to impulse tests. It is shown that, for the same low-frequency electric strength, gas-insulated units have lower impulse strength than oil-filled transformers. This deficiency, however, can be overcome with liberal margins of safety if gas-insulated transformers are protected with modern lightning arresters.The cooling of these new transformers is by forced circulation of the gas. The thermal capacity of gas-insulated units is, of course, somewhat lower than that of oil-filled units; theoretically, it would appear that, on this account, the overload capabilities of gas-filled transformers might be much lower than those of oil-filled units. This apparent deficiency is partially nullified by the fact that the thermal ageing characteristics of conventional class-A materials used in the construction of transformers is higher in gas than in oil. Consequently, the overload capabilities of gas-insulated units compare quite favourably with those of oil-filled units cooled by forced circulation of the liquid. Two units rated at 2000kVA at 69kV are in service in New York City; a third unit rated at 10 000kVA at 69 kV is in operation at Allentown, Pennsylvania. This unit is briefly described. Several other units, some of higher apparent power and voltage are now under construction.Development is proceeding, and it appears that in the near future some very large transformers may be built which could be installed next to, or built as an integral part of, the generators, and this at considerable overall cost reduction over present practices.