High Temperature Modulus Research Articles

First-principles study of high temperature and high-pressure behavior of carbides and nitrides of group IVB elements

Full potential linearized augmented plane wave method combined with quasi-harmonic approximation, has been used to perform the calculations of thermophysical properties of carbides and nitrides of the group IVB elements at high temperature and pressure. Relative accuracy of linear density approximation (LDA) and generalized gradient approximation (GGA) exchange correlation potentials have been tested. Specific heat () obtained through LDA and GGA agrees with experimental data up to 1500 K. Above 1500 K, GGA gives better agreement whereas LDA under-estimates the specific heat. LDA overestimates the bulk modulus, GGA gives better agreement with the experimental data. High-temperature bulk modulus follows the Wachtman formula. Calculated isotherms agree with published experimental results. The transformation pressures () from NaCl-type structure (B phase) to CsCl-type structure (B phase), and collapsed volumes () at () have been predicted. The stability and hardness of these compounds are related with the calculated density of states.

High Tg Bio-Based Aliphatic Polyesters from Bicyclicd-Mannitol

The carbohydrate-based diol 2,4:3,5-di-O-methylene-d-mannitol (Manx) has been used to obtain aliphatic polyesters. Manx is a symmetric bicyclic compound consisting of two fused 1,3-dioxane rings and bearing two primary hydroxyl groups. In terms of stiffness, it is comparable to the widely known isosorbide, but it affords the additional advantages of being much more reactive in polycondensation and capable of producing stereoregular polymers with fairly high molecular weights. A fully bio-based homopolyester (PManxS) has been synthesized by polycondensation in the melt from dimethyl succinate and Manx. The high thermal stability of PManxS, its relatively high glass transition temperature (Tg = 68 °C) and elastic modulus, and its enhanced sensitivity to the action of lipases point to PManxS as a polyester of exceptional interest for those applications where biodegradability and molecular stiffness are priority requirements. In addition, random copolyesters (PBxManxyS) covering a broad range of compositions have been obtained using mixtures of Manx and 1,4-butanediol in the reaction with dimethyl succinate. All PBxManxyS were semicrystalline and displayed Tg values from -29 to +51 °C steadily increasing with the content in Manx units. The stress-strain behavior of these copolyesters largely depended on their content in Manx and they were enzymatically degraded faster than PBS.

Synthesis and characterization of calcium-containing polyurethane using calcium lactate as a chain extender

In this study, a calcium-containing monomer, namely calcium lactate, was used for the synthesis of calcium-containing polyurethane for use in biomedical applications. Ether-based polyurethane was prepared using poly(oxytetramethylene)glycol, hexamethylene diisocyanate and calcium lactate. The prepared polymer was characterized by FTIR spectroscopy, which confirmed the presence of ionic linkages in the main chain. The thermal behavior, mechanical properties and viscosity of the polymer were studied, and the properties were compared with those of a control polyurethane sample without any metal. Because of ionic clustering in calcium lactate-incorporated ionic polyurethane, the mechanical properties and storage modulus were enhanced compared with those of nonionic polyurethane. The prepared calcium-containing polyurethane was evaluated for its blood compatibility. An evaluation of blood–material interactions revealed that the material is blood compatible and that the polymer does not induce any hemolysis. Segmented polyurethane prepared using a novel chain extender, calcium lactate along with polytetramethylene glycol and hexamethylene diisocyanate. The properties of the synthesized polyurethane compared with that of polyurethane without any calcium. The prepared polymer has high glass transition temperature and storage modulus, and has improved mechanical properties. In vitro hemocompatibility evaluation reveals that the material is blood compatible and does not induce any hemolysis to blood.

Elastic Properties of Solid 4He

The shear modulus of solid 4He increases below 200 mK, with the same dependence on temperature, amplitude and 3He concentration as the frequency changes recently seen in torsional oscillator (TO) experiments. These have been interpreted as mass decoupling in a supersolid but the shear modulus behavior has a natural explanation in terms of dislocations. This paper summarizes early ultrasonic and elastic experiments which established the basic properties of dislocations in solid helium. It then describes the results of our experiments on the low temperature shear modulus of solid helium. The modulus changes can be explained in terms of dislocations which are mobile above 200 mK but are pinned by 3He impurities at low temperature. The changes we observe when we anneal or stress our crystals confirm that defects are involved. They also make it clear that the shear modulus measured at the lowest temperatures is the intrinsic value—it is the high temperature modulus which is reduced by defects. By measuring the shear modulus at different frequencies, we show that the amplitude dependence depends on stress in the crystal, rather than reflecting a superfluid-like critical velocity. The shear modulus changes shift to lower temperatures as the frequency decreases, showing that they arise from a crossover in a thermally activated relaxation process rather than from a true phase transition. The activation energy for this process is about 0.7 K but a wide distribution of energies is needed to fit the broad crossover. Although the shear modulus behavior can be explained in terms of dislocations, it is clearly related to the TO behavior. However, we made measurements on hcp 3He which show essentially the same modulus stiffening but there is no corresponding TO anomaly. This implies that the TO frequency changes are not simply due to mechanical stiffening of the oscillator—they only occur in the Bose solid. We conclude by pointing out some of the open questions involving the elastic and TO behavior of solid helium.

Thermo-Mechanical Properties and Microstructure of Al<sub>2</sub>O<sub>3</sub>-SiC-SiAlON Composites

The composite specimens have been prepared using fused corundum, silicon and bauxite-based β-SiAlON powder as starting materials. The physical properties, high temperature modulus of rupture (HMOR), thermal shock resistance (TSR), phase composition and microstructure of the specimens after firing at 1500°C have been studied. The results show: (1) Al2O3-SiC-SiAlON composites are sintered well at 1500°C. (2) HMOR and TSR of the specimens increase with increase of Si content, high temperature properties are further increased with appropriate β-SiAlON addition. (3) The in-situ formed SiC and O’-SiAlON fill in the interstices of corundum skeleton structure, forming interlocking network structure, thus creating strengthening and toughening effects.

Intrinsic and dislocation-induced elastic behavior of solid helium

Recent experiments showed that the shear modulus of solid 4He stiffens in the same temperature range (below 200 mK) where mass decoupling and supersolidity have been inferred from torsional oscillator measurements. The two phenomena are clearly related and crystal defects, particularly dislocations, appear to be involved in both. We have studied the effects of annealing and the effects of applying large stresses on the elastic properties of solid 4He, using both acoustic resonances and direct low-frequency and low-amplitude measurements of the shear modulus. Both annealing and stressing affect the shear modulus, as expected if dislocations are responsible. However, it is the high temperature modulus which is affected; the low temperature behavior is unchanged and appears to reflect the intrinsic modulus of solid helium. We interpret this behavior in terms of dislocations which are pinned by isotopic 3He impurities at low temperatures and so have no effect on the shear modulus. At higher temperatures they become mobile and weaken the solid. Stressing the crystal at low temperatures appears to introduce new defects or additional pinning sites for the dislocation network but these effects can be reversed by heating the crystal above 500 mK. This is in contrast to dislocations produced during crystal growth, which are only annealed at temperatures close to melting.

Copper pillar bump design optimization for lead free flip-chip packaging

The interfacial delamination between the under bump metallurgy and the aluminum (Al) pad was observed in a copper (Cu) pillar bump evaluation in the work. The finite element analysis was employed to investigate the failure mechanism and the cu pillar bump geometry designed optimization. The finite element simulation result shows that the Cu pillar bump sustains the largest tensile stress at the temperature cycling test. The larger diameter Cu pillar bump can reduce the tensile stress significantly at the high temperature stress environment. Besides, an experiment confirms the finite element simulation result. Furthermore, an underfill with high glass transition temperature (T g) and high modulus also improve the flip-chip ball grid array packaging reliability.

Photopolymerization and characteristics of reactive organoclay–polyurethane nanocomposites

AbstractPolymerizable ammonium surfactants with two methacrylate alkyl chains were successfully synthesized via Michael‐Addition and quaternization reaction. Organoclays containing reactive methacrylate groups were prepared by cationic exchange process. Intercalated nanocomposites have been produced by UV initiated polymerization with different organophilic clay loading. X‐ray diffraction (XRD) spectroscopy showed that the d spacing of organoclay increased with increment of methacrylate alkyl chains length. When organoclay were dispersed in urethane acrylate resin, and photopolymerized, the d spacing increased, but the increasing extent decreased with increment of the methacrylate alky chains length. Thermogravimetric analysis indicated that incorporation of reactive organoclay had better thermal stablity than cetyltrimethylammonium bromide (CTMA)‐modified organoclay and dynamic mechanical analysis (DMA) showed nanocomposites containing reactive organoclay had high glass transition temperature and storage modulus. Tensile analysis implied that incorporation of reactive organoclay could enhance mechanical and tensile properties dramatically. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Molecular relaxation phenomena during accelerated weathering of a polyurethane coating

Results from a polyester-urethane (PU) coating system under accelerated weathering showed that crosslink density, obtained from high-temperature modulus data, diminished due to chain scission. However, T g and room-temperature tensile modulus both increased with weathering. Molecular relaxation phenomena in polymers have long been investigated to explore changes occurring in a polymer at temperatures below its glass transition and were explored for an explanation to supplement chemical degradation for these observations. Relaxation was quantified using “enthalpy recovery” which first increased with exposure, then diminished. The concurrent physical and chemical aging effects were characterized by tracking nonexponentiality in the spectrum of relaxation times, and the size of “co-operatively relaxing regions” deduced from relaxation around the glass transition. Mechanical relaxation in this coating extended longer than cycle periods typical of accelerated weathering, suggesting that frequency effects might be important when comparing accelerated to natural weathering.

ELECTRON-BEAM CURING CHARACTERIZATION OF EPOXY RESIN SYSTEMS CONTAINING CO-INITIATOR

Electron beam (EB) curing of polymer matrix composites offers many particular advantages compared to conventional thermal curing.There has been a considerable development about the EB curing advanced composites for the use of aerospace and other industries in recent years.Most variety of epoxy resin has been confirmed to possess radiation reactivity and have already become chief resin matrices used in EB curable composites.The influences of initiator variety and concentration on the curing characterizations of epoxy resin system containing co-initiator benzophenone and isopropanol were analyzed.The variation regularities of temperature during the radiation,the gel fraction,damping factor and dynamic modulus of samples were studied.The experimental results indicate that under the same condition of epoxy resin content and absorbed dose,the addition of co-initiator can enhance the gel fraction of system,but the enhanced extent of gel fraction does not increase with the increase co-initiator content,there exists an optimal content of the co-initiator.With the increase of onium salt initiator,the enhanced extent of gel fraction induced by co-initiator will decrease.A temperature peak appears during the radiation,and the influences of co-initiator on the peak temperature and the glass transition temperature are the same as those on the gel fraction.The room temperature modulus and high temperature modulus of cured samples with low content of onium salt initiator have little change before and after the addition of co-initiator.When the diphenyliodonium hexafluorophosphate initiator content is high,the room temperature modulus of samples with co-initiator increases,while the high temperature modulus has little changes.

Reinforcement of Poly(ethylene terephthalate) Fibers with Polyhedral Oligomeric Silsesquioxanes (POSS)

Poly(ethylene terephthalate) (PET)-based composite fibers were prepared by melt spinning three types of PET/polyhedral oligomeric silsesquioxane (POSS) composites. These composites were made by either melt blending POSS with PET at 5 wt% loading level (non-reactive POSS and silanol POSS) or by in-situ polymerization with 2.5 wt% reactive POSS. Significant increases in tensile modulus and tensile strengths were achieved in PET fibers with non-reactive POSS at room temperature. The hightemperature modulus retention was found to be much better for PET/silanol POSS fiber when compared to that of control PET. Although other PET/POSS nancomposite fibers tested did not show this high retention of modulus at elevated temperatures, PET/isooctylPOSS nanocomposite fibers did show increased modulus at elevated temperature compared to that of PET. Higher compressive strengths, compared to PET fibers, were observed for all three nanocomposite fibers. Gel permeation chromatography measurement suggested that there is no significant change in molecular weight during preparation of PET/POSS nanocomposites. SEM observations suggest that there is no obvious phase separation in any of the three PET/POSS systems. Crystallization behavior and thermal stability of the composite were also studied. The fiber spinning and mechanical performance with 10 and 20 wt% of trisilanolisooctyl POSS2 were also investigated1 the composites with higher concentrations of this nanofiller can be spun without any difficulty. At room temperature, the fiber tensile modulus increased steadily with the POSS concentration while fiber tensile strength showed no significant change. The elongation at break decreased significantly with increasing of POSS concentration. The high-temperature moduli of PET/POSS nanocomposite fibers were found to be rather variable, likely due to the modest compatibility between filler and polymers, which can lead to structural anisotropy within the composite.

Evaluating high-temperature modulus and elastic recovery of Ti3SiC2 and Ti3AlC2 ceramics

Abstract Elastic modulus and elastic recovery of both Ti3SiC2 and Ti3AlC2 at various temperatures were evaluated using cyclic loading–unloading in three-point bending tests and comparing relative slopes. It was found that the elastic moduli of Ti3SiC2 and Ti3AlC2, 310 and 288 GPa, were nearly constants in the temperature range from 20 to 1000 °C and then dramatically declined with further increasing of temperature. Elastic recovery as a function of temperature exhibited quite similar curve as the elastic modulus. The simple testing method was demonstrated convenient for estimating the elastic modulus and plastic deformation of ceramics at high temperature.

Dynamic mechanical studies on epoxy resins cured by electron beam radiation

Dynamic mechanical analyses on electron beam (EB)-cured epoxy resins were made in the paper. Through the studies on variation rules of gel fraction, tan δ and storage modulus for varied samples, the important effects of EB radiation dosage, initiator dosage, chemical structure, molecular weight and distribution, and heat treatment on curing reaction and properties of epoxy resin systems have been obtained. Under low radiation doses, the gel fraction, glass transition temperature (Tg) and high temperature modulus of cured epoxy resin increase with increasing radiation dose and initiator dosage. The crosslinking density of epoxy resin decreases slightly with increasing molecular weight. When radiation doses increase, the molecular weight has a little influence on the increasing of curing level and an optimal dosage of initiator appears. The experimental results indicate that the radiation reactivity of epoxy resins is directly associated with their chemical structures. Under the same radiation dose, the reaction extent in sample with high polydispersity is higher than that in low polydispersity sample, but the degree of homogeneity in crosslinking structure is lower. When the EB-cured epoxy resin is heated, the crosslinking density is enhanced. If the temperature of heating treatment exceeds the thermal-initiating temperature of initiator, the local thermal-crosslinking network can be formed in resin system.

Organic–inorganic networks in foams from high internal phase emulsion polymerizations

Hybrid foams, which combine an inorganic polysilsesquioxane network with an organic polystyrene network, were successfully synthesized using high internal phase emulsions (HIPE). Methacryloxypropyltrimethoxysilane (MPS) was copolymerized radically with styrene (S) and divinylbenzene (DVB) in the organic phase of the emulsion. The hydrolytic condensation of the trimethoxysilyl groups formed an inorganic polysilsesquioxane network, which significantly increased the high temperature modulus and thermal stability of the foam. The hybrid foam had an open-cell structure typical of polymer foams based on HIPE (polyHIPE), with a significantly smaller cell diameter than S/DVB polyHIPEs. In the absence of DVB, there is no crosslinked network to restrict the mobility of MPS and the polysilsesquioxane network is more fully developed. At low MPS contents, the thermal stability of the foams is higher for foams with DVB, reflecting their higher crosslinking density. At high MPS contents, the thermal stability is higher for foams without DVB, reflecting the more fully developed inorganic network.

Structural properties of recycled plastic/sawdust lumber decking planks

Abstract Plastic lumber is being used to replace wooden lumber in some construction applications, especially in outdoor applications where the plastic lumber is presumed to weather better than the wood. However, the structural properties of the plastic lumber are not well understood, and the use of plastic lumber in structural applications is not authorized in the common building codes. In this research effort, standard 2×6 plastic lumber planks were tested for many different structural properties. The plastic lumber tested was a blend of recycled plastic and sawdust. The tests were conducted at −23.3°C to simulate winter conditions, and at 40.6°C to simulate summer conditions. In all cases the high temperature strength and stiffness was lower than at low temperature, so the high temperature values would determine the allowable strength and stiffness for design. The high temperature modulus of the plastic lumber was 5.79, 1.03, and 1.12 GPa in compression, flexure and tension respectively. High temperature strength values were 16.8, 12.0, and 1.45 MPa in compression, flexure and tension respectively. The high temperature shear strength of the plastic lumber was 5.31 MPa. Strength tests were also performed for nail and screw connections typically used with lumber, and the pull-out and lateral load were comparable to wooden lumber. The plastic lumber performed well under sustained load tests at high temperature. Slip resistance tests were performed, and it was found that the plastic lumber is more slippery than wooden lumber, but probably does not represent a safety hazard. The conclusion was that the plastic lumber is a good structural material, but it is not appropriate to simply substitute plastic lumber for wooden lumber pieces of the same dimension in structural applications. Plastic lumber structures must be designed using the structural properties of the plastic lumber.

Investigation of readily processable thermoplastic-toughened thermosets. II. Epoxy toughened using a reactive solvent approach

The fracture toughness of epoxy thermosets was increased by up to 220% using very low-molecular-weight (∼ 1000 g/mol) imide thermoplastic. The objective was to produce a low-viscosity prepolymer that could be easily autoclave-processed to give a tough thermoset. Here, an homogenous epoxy prepolymer was prepared by first synthesizing very low-molecular-weight linear aromatic imide (∼ 1000 g/mol) directly in a liquid allyl phenol reactive solvent, followed by dissolution of the epoxy (Epon® 825) and the cure agent (DDS) directly in the thermoplastic solution. The allyl phenol both cures into the epoxy network, through phenol functional groups, and accelerates the cure. The viscosity of the pure epoxy was 1.4 Pa · S at 30°C. The prepolymer formulations ranged from ∼ 5–33 Pa · S at 30°C, but all reduced to less than 1 Pa · S at 90°C. The onset of cure is well above 90°C so the prepolymer viscosity is within the range for autoclave processing. The cured resin plaques were not transparent, but phase-separated domains were not found by scanning electron microscopy, indicating that the domain size is below the detection limit of the instrument. The reactive solvent causes a decrease in both the Tg and the high temperature modulus of the thermoset. Introduction of the thermoplastic results in partial recovery of the Tg and modulus. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 935–942, 1998

Investigation of readily processable thermoplastic-toughened thermosets. III. Toughening BMIs and epoxy with a comb-shaped imide oligomer

This is the third in a five-part series describing the preparation of tough, high-performance thermosets from low viscosity, autoclave-processable prepolymers. The first 2 articles described toughening of bismaleimides (BMI) and epoxy with linear imide thermoplastics of ∼ 1000 g/mol. Highly processable prepolymers were obtained, which resulted in increases in fracture toughness for BMI of ∼ 75–100%, while the fracture toughness of epoxy was increased by up to 220%. This article describes the preparation of a low-molecular-weight comb-shaped imide oligomer (∼ 4100 g/mol) and the effect of the oligomer architecture and end-group on BMI and epoxy prepolymer viscosity and fracture toughness. When an unreactive comb-shaped oligomer was incorporated in a BMI prepolymer (10% thermoplastic loading in the thermoset), the fracture toughness increased by 67% over that of an untoughed control, while a reactive oligomer increased the fracture toughness by 150% over an untoughened control. At 55°C, the viscosity of the solution of the reactive comb-shaped imide in B was only 6.2 Pa · S. When the oligomer was dissolved in epoxy resin, the viscosity was less than 0.2 Pa · S at 90°C, and the fracture toughness increased by 110 and 133% (at ∼ 13% loading in the thermoset), relative to an untoughened control, depending on the reactivity of the end group. The Tg and high-temperature modulus of BMI and epoxy remained approximately the same relative to the untoughened controls. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 943–951, 1998

Effects of thermal exposure on the properties of two γ alloys

Abstract TiAl alloys with various alloying additions show promise as high temperature materials because of their high temperature strength and modulus. Recent strides in processing and microstructural control produced alloys which exhibit an acceptable balance of properties for aerospace applications. However, little is known about the influence of the high temperature exposures, which these materials will experience in service, on the mechanical properties. Therefore, a study was undertaken to examine the effects of thermal exposure on the tensile properties of two wrought alloys: Ti-47Al-2Mn-2Nb and Ti-47Al-1.6Cr-0.9V-2.4Nb. The effects of microstructure and exposure at temperatures spanning the anticipated use range were investigated.

High temperature young's modulus of a fine-grained nuclear graphite oxidized or prestressed to various levels

Abstract Effects of oxidation and compressive prestress on high temperature Young's modulus were examined on a fine-grained isotropic graphite, IG-110. Oxidation was performed in air at 500°C up to a burn-off level of 10%. Prestress level was either 60% or 80% of the mean compressive strength. Both oxidation and prestressing caused decreases both in the room temperature and high temperature moduli, which were explained by a crack opening and growth model. An empirical equation by which the high temperature modulus of oxidized specimens can be estimated from the room temperature modulus of the unoxidized specimen was obtained as E (T, d) E(RT, d o ) = ƒ(T)( d d o ) n(T) . Here, E(T, d) and E(RT, do) are the high temperature modulus of the oxidized specimen with apparent density d, and the room temperature modulus of the unoxidized specimen with apparent density do, ƒ(T) is a polynomial in the third order of T, and n(T), a linear function of T. Annealing treatment or heat-up and cool-down process caused a partial recovery of the modulus of the oxidized specimen. This recovery was explained with the aid of a model: The internal stress generated as a result of the anisotropy of the thermal expansion of crystallites becomes smaller for the oxidized material because of the preferential oxidation of the binder region.

Studies on crosslinking of EPDM-PE blends by thermoanalytical techniques

The effect of blend ratio and peroxide concentration on crosslinking characteristics of EPDM-PE blends were studied by Differential Scanning Calorimetry, Brabender plasticorder and Rheometer. Crosslinking of EPDM-PE blends follows first order reaction kinetics. The curing exotherm increases but activation energy decreases with increase in EPDM content in the blends. The same however increases with the increase in concentration of DCP upto a certain level, while the activation energy is almost independent of peroxide concentration. The cure rate increases whereas optimum cure time and energy consumption for curing decrease with increase in the EPDM-PE ratio. A method for determination of crosslinking efficiency in the case of blend systems was developed from high temperature modulus to predict the properties and the curing behaviour of the blends.