Microscopical study of carbon/carbon composites obtained by chemical vapor infiltration of 0°/0°/90°/90° carbon fiber preforms

The microstructure of carbon/carbon composites obtained by isothermal, isobaric chemical vapor infiltration (CVI) of carbon fiber preforms consisting of aligned fiber bundles separated by fiber fleeces was studied comparatively by polarized light microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED). Deposition rate as well as matrix microstructure do not differ in the aligned fiber bundles and fiber fleeces exhibiting different local surface area/volume ratios. The matrices which are homogeneously textured according to PLM exhibit pronounced spatial texture gradients at the sub-μm-scale if investigated by SAED. The texture gradients appear to be independent on the infiltration time, distance between fibers but evidently depend on the total methane pressure. TEM and SEM observations show a thin high-textured layer between the fiber and the medium-textured transitional layer below the high-textured matrix layer containing columnar grains. This thin layer replicates the surface unevenness of the fiber surface while it is absent at the initial carbon fiber surface before infiltration.     

The thermal degradation of an amine-cured epoxide resin at temperatures between 200°C. and 310°C.

This paper describes the nonoxidative thermal degradation of an epoxide resin based on the diglycidyl ether of bisphenol A crosslinked with p,p′-diaminodiphenyl methane. Temperatures of degradation lay between 200 and 310°C. and the process was followed concurrently by three means: changes in dielectric properties, changes in infrared spectra, and weight loss. Dielectric properties support the contention that there is a dehydration step during degradation. It is proposed that vacuum curing at high temperature can produce optimum crosslinking. Evidence of phenol and N-methyl aniline as degradation products is advanced, and possible degradation mechanisms are discussed.     

Studies in dyeing. II. Effect of crystal modification of disperse dyes on polyester dyeing at 130°C

Four commercial disperse dyes were purified and their different crystal forms were prepared by crystallizing from different solvents or precipitating from their solutions in glacial acetic acid by dilution with water. These forms were found to have different melting points. They were dyed on polyester fibers at 130°C to fiber saturation values by changing the dyebath every 15 min. The effect of pretreatment of the dyes in an aqueous environment at different temperatures (60°, 100°, and 130°C) for 15–60 min in the presence and absence of a dispersing agent on the dye uptake values was also studied. Such treatments are shown to reduce the dye uptake. The implication of these treatments in practical dyeing are pointed out.     

Wear of zirconia-dispersed alumina at ambient temperature, 140 °C and 250 °C

Commercial grade alumina along with 5, 10, 15 and 20 wt.% zirconia-dispersed aluminas were tested for their wear resistance at ambient temperature, 140 °C and 250 °C using a pin on disk tribotester fitted with a hot stage. The sample suite was investigated for physical characteristics including hardness, fracture toughness, bulk density, alumina grain size and zirconia grain size. The wear track and wear debris were investigated using profilometry, SEM as well as TEM.

The 5 wt.% zirconia-dispersed aluminas had the lowest wear volume loss over the temperature range. The alumina sample exhibited a wear dependence with relative humidity which is attributed to the formation of a tribochemical layer. Investigation of the tribochemical layer using SEM/EDS and TEM electron diffraction showed the tribochemical layer to be aluminium hydroxide. The major wear mechanism for all samples was brittle fracture.     

Thermal degradation of poly(methyl methacrylate) at 50°C to 125°C

Small but significant numbers of chain scissions occur in a commercial poly(methyl methacrylate) sheet exposed to temperatures between 50 and 125°C. The scission rate is initially high and then levels off to a constant rate. The short-time rate of chain scissions is temperature dependent, while the long-time rate of chain scissions appears to be temperature independent. Four possible sources of random chain scission initiation were considered: (1) the presence of unreacted initiators of polymerization, (2) free radicals generated from additives in the commercial film, (3) weak links in the polymer chain, and (4) free radicals generated from the thermal decomposition of an oxidation product of methyl methacrylate (MMA) monomer. The source most consistent with our results is the one involving free radicals generated from the oxidation product of MMA monomer.     

The solubility of ettringite at 25°C

Available solubility constants indicate that ettringite should be the stable form of calcium aluminate sulfate hydrate with respect to monosulfate in cement porewater. However, monosulfate is generally present in mature cement pastes, usually in the absence of ettringite. The objectives of this study were to determine the solubility product of ettringite under equilibrium conditions and to examine the solubility data used in predictive thermodynamic models. Solubility products were calculated for ettringite prepared from both supersaturated and undersaturated solutions with a pH range between 10.4 and 13.7. The mean solubility product for ettringite dissolution: Ca₆Al₂O₆(SO₄)₃ · 32H₂O → 6Ca²⁺ + 2Al(OH)₄⁻ + 3SO₄²⁻ + 4OH⁻ + 26H₂O was 10^−44.91, i.e. Log K_sp = -44.91 ± 1.06 (2 S.D.). Activity coefficients were calculated using the specific ion interaction approach. The mean solubility product was close to other values calculated from concentrations reported elsewhere for the solubility of ettringite. As is the case for all solubility products, this value cannot be inserted directly into the databases of other thermodynamic models because of differences in the methods used to calculate activity coefficients and the manner in which ion-pairing is handled by different models. However, raw solubility data are provided for recalculation of the solubility product for use in other models.     

The Solubilization of Disperse Dyes by Dispersing Agents at 127°C

The solubilities in water at 127°C of eight disperse dyes of various structures have been determined, as well as the effect of eleven different dispersing agents on the solubilization of these dyes. A special cell was designed for these experiments. While the solubilities of six of the eight dyes are similar and those of the remaining two larger by approximately one order, solubilization by dispersing agents covers a wide range of values. No clear pattern of dye-dispersing agent interaction has emerged from the work, especially with regard to the effects of dye structure, and a more detailed investigation is therefore being undertaken.     

The three successive stages of creep of PMMA between 55°c and 90°c

After cooling from 140°C, the physical aging of poly(methyl methacrylate), PMMA, has been studied at 55, 70, 84, and 90°C by means of temporary microcreep tests. The systematic analysis of recoveries reveals that at 90°C, creep follows successively three different stages. After stress application, creep starts by a logarithmic function of time, while recovery follows exactly the same kinetics. An Andrade creep begins after the logarithmic creep: It is in fact the beginning of the Kohlrausch-Williams-Watts creep with an exponent of 0.36. This creep is fully recoverable, but the duration of this recovery is ～3 times the creep duration. The third stage of creep is superimposed on the Andrade creep and follows a power law in t^0.8, but it is not recoverable in the usual duration of experiments. By comparison with the recoverable creep of PMMA studied by Plazek above T_g, which can be fitted by a KWW law with an exponent 0.8, this third creep is interpreted as the beginning of the rubbery deformation.     

Thermal Decomposition of Energetic Materials 69. Analysis of the kinetics of nitrocellulose at 50 °C–500 °C

Kinetic constants for decomposition of nitrocellulose in the 50 °C to 500°C range are analyzed. At T < 100°C, three processes (depolymerization, peroxide formation, and hydrolysis) are consistent with the reported kinetics. For T = 100°C–200°C, 28 of 30 previously reported kinetic measurements can be organized clearly into two categories by the use of the kinetic compensation effect. These two groups fit the first-order and autocatalytic processes. Conflicting interpretations are reconciled by this approach. At T > 200°C, the kinetics are consistent with the existence of the first-order step and desorption of the products as two parallel processes which, together, control the rate. Time-to-exotherm and mass burning rate kinetics are compared as temperature-dependent reaction-desorption events.     

Stabilizer reactions in cast double base rocket propellants. Part II: Formation and subsequent reactions of N-nitroso derivatives of para-nitro-N-methylaniline and 2-nitrodiphenylamine in mixed-stabilizer propellants aged at 80°C and 90°C

The HPLC determination of the stabilizers para-Nitro-N-methylaniline (pNMA) and 2-Nitrodiphenylamine (2NDPA) and their derivatives in two mixed-stabilizer cast double base rocket propellants undergoing accelerated aging at 80°C and 90°C has led to the conclusion that N-nitroso derivatives play a critical role in the stabilizer depletion processes in this type of propellant. In order to account for the principal stabilizer products in the aged propellants it is suggested that pNMA enters into an equilibrium with its N-nitroso derivative (formed during the initial stages of aging) and that this reacts with 2NDPA in a transnitrosation reaction, to give mainly N-nitroso-2NDPA from which the observed C-nitro derivatives of 2NDPA are in part generated by rearrangement and subsequent oxidation.     

Reactivity of heat-treated coals in air at 500 °C

Twenty one US coals, of widely ranging rank, have been carbonized under controlled conditions to 1000 °C, and the reactivity in air at 500 °C of the resulting chars or cokes has been measured by a gravimetric method. The reactivities lie within a well-defined band when plotted against rank of the parent coal. The lower-rank coal chars are more reactive than those prepared from high-rank coals. In extreme cases, the reactivity found for a Montana lignite char is some 100 times as great as that obtained for a char produced from a Pennsylvania low-volatile coal. Variation of reactivity with heat-treatment temperature (600 to 1000 °C) has been studied for three coals. As heat-treatment temperature increases, there is a decrease in reactivity. Some results are reported on the effects which mineral matter and pore structure have on the reactivity parameter. Chars containing high concentrations of magnesium and calcium impurities are most reactive. The amount of macro and transitional porosity in a char has a marked influence on reactivity.     

An investigation into the effect of fibre protective agents on the physical properties of wool dyed at 130°C

The effect of dyeing at elevated temperature on wool fabric strength is investigated.A range of fibre protective agents are evaluated and the formation of dehydroalanine examined.     

Thermal shock behavior of Ti2AlC from 200°C to 1400°C

The thermal shock behavior of Ti₂AlC synthesized by means of self‐propagating high‐temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti₂AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al₂O₃. Although the flexural strength decreases quickly in the temperature range of 300°C‐500°C, no discontinuous decrease in the retained strength is observed in Ti₂AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti₂AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared with the first quench. Quenching at 300°C and above, voids after the pullout of grains and cracks are present, which however are absent in the un‐quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock.     

Sintering of aluminum nitride by using alumina crucible and MoSi2 heating element at temperatures of 1650 °C and 1700 °C

Aluminum nitride (AlN) ceramics, prepared with Y₂O₃ and CaO sintering additives, have been densified in an Al₂O₃ crucible at temperatures of up to 1650 °C and 1700 °C using a conventional MoSi₂ heating element furnace. The results of this study show that relative densities in excess of 99% of theoretical and a relatively high-thermal conductivity of 147 W m⁻¹ K⁻¹ have been achieved for feedstock materials prepared with combined addition of 1 wt.% Y₂O₃ and 1 wt.% CaO. All of the phases in sintered samples have been shown to be crystalline AlN and minor amount of secondary phases, were detected such as enriched Y- and Ca-aluminates by the XRD patterns, back-scattered imagery and microprobe analysis. The advantage of using the particular experimental system and sintering condition is considered to be amenable to lower production cost and enhance the feasibility of mass production. Critical temperature for AlN densification to obtain the highest density is about 1650 °C.     

Solubility of potassium ferrate in 12 M alkaline solutions between 20°C and 60°C

The solubility of potassium ferrate (K₂FeO₄) was measured in aqueous solutions of NaOH and KOH of total concentration 12 M containing various molar ratios of KOH:NaOH in the range 12:0 to 3:9. Several analytical methods were tested for the determination of ferrate concentration. The final method chosen consisted of potentiometric titration of the ferrate sample with an alkaline solution of As₂O₃. The assumption was made that ferrate dissociates in concentrated KOH solutions predominantly to KFeO₄⁻. The solubility constant, S, defined as the product of the molar concentration of the potassium ion, K⁺, and the ferrate anion, KFeO₄⁻, was found to be 0·044 ± 0·006 mol² dm⁻⁶ for 20°C, 0·093 ± 0·004 mol² dm⁻⁶ for 40°C and 0·15 ± 0·09 mol² dm⁻⁶ for 60°C. From these results the heat of dissolution of K₂FeO₄ was calculated as −14·3 kJ mol⁻¹. At 60°C the enhanced decomposition of the ferrate at the higher temperature led to a greater deviation in solubility values compared with data for either 20°C or 40°C.     

Structural characterization of vulcanizates. Part VIII. The N-cyclohexylbenzothiazole-2-sulfenamide-accelerated sulfur vulcanization of natural rubber at 140–180°C. and of synthetic cis-1,4-polyisoprene at 140°C

Vulcanization of natural rubber at 140°C. with a CBS-accelerated sulfur system of conventional type gives rise to a structurally complex network in which the number of sulfur atoms combined per chemical crosslink present increases from 12 to 21 with increasing reaction time. The complexity of the network increases with increasing temperature of vulcanization. Crosslinking of a purified synthetic cis-1,4-polyisoprene proceeds more slowly and yields a slightly more complex network. Despite this overall similarity the natural rubber vulcanizates exhibit considerably higher tensile strengths.     

Stabilizer reactions in cast double base rocket propellants. Part I: HPLC Determination of Stabilizers and their derivatives in a propellant containing the stabilizer mixture para-nitro-N-methylaniline and 2-nitrodiphenylamine aged at 80°C and 90°C

Reverse phase high performance liquid chromatography (HPLC) employing an acctonitrile/methanol/water (45/10/45) mobile phase has been used to separate and quantitate the stabilizaers para-nitro-N-methylaniline, (pNMA) and 2-nitrodiphenylamine, (2NDPA) and their derivatives in a mixed-stabilizer cast double hase rocket propellant composition undergiong acceterated aging at 80°C adn 90°C. Quantitative measurements show that pNMA is converted almost cntirely into N-nitroso-pNMA during the carly stages of aging and that this derivative persists for the remainder of the aging period. 2NDPA, on the other hand, reacts more slowly to give both N-nitroso and C-nitro derivatives.     

Electric Field Assisted Sintering of Cubic Zirconia at 390°C

Using the flash sintering technique, cubic yttria‐stabilized zirconia is shown to sinter at 390°C, more than 1000°C below nominal sintering temperatures, by using a DC electric field of 2250 V/cm. Furthermore, flash sintering experiments performed with electric fields between 60 and 2250 V/cm were used to show that the relationship of the temperature at the onset of flash sintering (T_Onset) and the applied field (E) follows the power relationship T_Onset (K) = 2440 E^?1/5.85(V/cm). Using this relationship, and considering the sintering of the sample in the absence of an electric field, the critical electric field to enter the flash sintering regime is shown to be 24.5 V/cm. For electric fields between this critical electric field and 2250 V/cm, the onset of flash sintering occurs in the same range of critical volumetric power dissipation, between 1 and 10 mW/mm³, suggesting this is a material property. Despite the volumetric power dissipation being the critical value for the onset of flash sintering behavior, the current density achieved during sintering appears to be more critical for densification rather than maximizing power dissipation.     

Reactivity of heat-treated coals in carbon dioxide at 900 °C

Reactivities of sixteen 40 × 100 (U.S.) mesh U.S. coals charred to 1000 °C were measured in carbon dioxide at 900 °C. Chars derived from coals with less than 80% carbon, on a dry-ash-free basis, were the most reactive. These chars also gave the widest spread in reactivity. Plots of inorganic element content in the chars versus reactivity showed that magnesium and calcium are important to char reactivity. Six coals were acid-washed with hydrochloric acid and four coals were further demineralized with hydrofluoric acid. Most acid-treated coals showed a decrease in reactivity; but two coals of high rank increased in reactivity. This increase in reactivity is attributed to the creation of additional porosity as a result of mineral matter removal and thus a reduction in resistance to carbon dioxide diffusion to reactive sites. Two demineralized and two original coals were divided into four size ranges and chars were produced from each size of each coal. Gasification rates increased monotonically with decreasing particle size reacted.