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High‐nitrogen‐content compounds 6‐azido‐1,2,4‐triazolo[4,3‐b]pyridazine (C5H3N7) and its 3‐methyl derivative (C6H5N7) have been in situ crystallized in a diamond‐anvil cell and their structures determined by single‐crystal X‐ray diffraction. Under ambient and high‐pressure conditions the crystallizations yield the same phases: the C5H3N7 anhydrate and C6H5N7 hydrated clathrate. In both the structures there are clearly distinguished regions of short CH...N and N...N intermolecular contacts, the latter involving exclusively the azide groups. High pressure initially increases the contents of water in the channel pores of the clathrate. [ABSTRACT FROM AUTHOR]

Distortions of the directional requirements in halogen…halogen contacts between hexachloroethane (HCE), C2Cl6, molecules lead to a loose crystal under ambient conditions. Single-crystal X-ray diffraction shows that the orthorhombic HCE phase of space group Pnma, with the molecules in the staggered conformation, extends at least from 85 to 305 K and from 0.1 MPa to 5.42 GPa. At ambient pressure, all intermolecular distances are longer than the sum of van der Waals radii, reached only at the pressure of ca. 1.2 GPa. [ABSTRACT FROM AUTHOR]

The structures of a series of the simplest aliphatic amines determined at high pressure by single crystal X-ray diffraction reveal systematic structure property relations involving intermolecular interactions and molecular symmetries. Five new polymorphs have been found: one of methylamine (phase II, space group Fdd2), two of dimethylamine (phase I, space group C2/c and phase II, P21/c), and two of trimethylamine (phase II, space group P21/n and phase III, P21/m). The methylamine and dimethylamine molecules are arranged according to the NH...N hydrogen bonds. The phase diagrams of methylamines have been outlined up to 6 GPa. [ABSTRACT FROM AUTHOR]

In pentachloroethane, C2HCl5, high pressure initially strongly compresses the C–H…Cl bonds in phase I; however, in phase II which is stable above 0.62 GPa the role of hydrogen bonds is diminished and molecular aggregation is dominated by halogen bonds Cl…Cl. Both phases have been determined by X-ray diffraction and the phase diagram of C2HCl5 has been outlined. The transition between phases I and II retains some relation between their structures and reduces the symmetry from class mmm (space group Pnma) to 2/m (space group P21/n11). The discontinuous transition, with the shear strain exceeding 21°, is so strong that its progress can be visually observed even for powdered samples. The single-crystal growth suggests a surface-roughening transition, as phase I grows in the form of dendrites combining into oval plates, while phase II forms prisms with sharp edges. [ABSTRACT FROM AUTHOR]

Ti(C 0.5 ,N 0.5)-reinforced alumina matrix composites with an addition of 2 wt% ZrO 2 were tested to determine residual stresses of Al 2 O 3 and Ti(C 0.5 ,N 0.5) phases. The advanced sintering technique (spark plasma sintering ―SPS) at various temperatures of 1600°C and 1700°C was used. Vickers hardness HV1, Young's modulus E, apparent density ρ and indentation fracture toughness K IC(HV) were evaluated. An indirectly residual stress measurement by the XRD method using the sin2 ψ technique was applied. Compressive residual stresses in both phases: α-Al 2 O 3 and Ti(C 0.5 ,N 0.5) were observed. Residual stresses of α-Al 2 O 3(226) are in the range between −204 ± 20 MPa and −120 ± 20 MPa (for 1600 °C and 1700 °C respectively) are lower compared to Ti(C 0.5 ,N 0.5) (420), for which the stresses are in the range of between −292 ± 20 MPa and −256 ± 20 MPa (for 1600 °C and 1700 °C respectively). The results exhibit the influence of the sintering temperature on the residual stresses of the tested phases. The residual stresses revealed at 1700°C are lower by about 40% for α-Al 2 O 3(226) and much less for Ti(C 0.5 ,N 0.5) (420) , by only about 15%. Microstructure studies using scanning electron microscopy, X-ray and electron diffraction phase analysis were used. [ABSTRACT FROM AUTHOR]

In this paper, Al2O3–ZrO2 composites with an addition of 20 wt% TiN and 10 wt% TiC were modified. The addition of zirconia in a range from 2 to 5 wt% of the monoclinic phase and 10 wt% of Y2O3 stabilised ZrO2 affected the mechanical properties of the composites. A new type of sintering technique—the spark plasma sintering (SPS) method—within a temperature range from 1575 °C to 1675 °C, was used. Vickers hardness, apparent density, wear resistance and indentation fracture toughness KIC(HV) were evaluated at room temperature. An increase of the sintering temperature resulted in an improvement of Vickers hardness and an increase of the fracture toughness of the tested composites. The tribological properties of the samples were tested using the ball-on-disc method. The friction coefficient was in a range from 0.31 to 0.55, depending on the sintering temperature. An enhancement of the specific wear rate was dependent on the sintering temperature. The mechanical properties of the samples sintered by pressureless sintering (PS) were compared. X-ray diffraction patterns were presented in order to determine the phase composition. SEM microstructure of the tested composites sintered at different temperatures was observed. [ABSTRACT FROM AUTHOR]

• A novel composite ablator based on silicone resin and B 4 C ceramics is shown. • Methylpolysiloxane ablation rates significantly reduced by formed B 2 O 3 on surface. • Methylpolysiloxane pyrolyses to SiOC and SiO 2. • The presence of B 4 C promotes β-SiC crystallisation during pyrolysis and ablation. Before siloxane resin matrix composites could be successfully used as ablative materials, their mechanism of high-temperature degradation must be investigated in depth. In this study, degradation mechanism, ablation resistance and degradation kinetics of both methylpolysiloxane and the methylpolysiloxane - B 4 C (10 wt%) composite were investigated.. The presence of B 4 C resulted in a higher final sample mass in thermogravimetric tests in synthetic air up to 1500C (85% vs 81%), while during ablation tests it reduced linear and mass ablation rates by 45% and 54%, respectively. The XRD and SEM/EDS studies provided an estimate of the phase contributions formed as a result of exposure to corrosive conditions. The effect of the presence of powders on the amount of SiO 2 glassy phase, β-SiC crystalline phase and free carbon precipitates formed is apparent. [ABSTRACT FROM AUTHOR]