Improvement of the oxidation behavior of Ti1-xAlxN hard coatings by optimization of the Ti/Al ratio

The oxidation behavior of cubic Ti_1-xAl_xN films was improved by decreasing the Ti/Al ratio from 50/50 in the direction of the phase transition between cubic and hexagonal structure. Metastable, polycrystalline, single-phase Ti_1-xAl_xN films were deposited on high speed steel (HSS) substrates by reactive magnetron sputtering ion plating (MSIP). The composition of the bulk was determined by electron probe microanalysis (EPMA), the crystallographic structure by thin film X-ray diffraction (XRD). A Ti_1-xAl_xN film with a Ti/Al atomic ratio of 38/62 was deposited in cubic NaCl structure, whereas a further decrease of the Ti/Al ratio down to 27/73 led to a two-phase film with both cubic and hexagonal constituents. The Ti_0.38Al_0.62N film was oxidized in synthetic air for 1 h at 800?°C. The oxidic overlayer was analyzed by X-ray photoelectron spectroscopy (XPS) sputter depth profiling, EPMA crater edge linescan analysis, and secondary neutrals mass spectroscopy (SNMS). Scanning electron microscopy (SEM) micrographs of the cross sectional fracture were taken for morphological examination. With higher Ti content, the Ti_1-xAl_xN formed a TiO_2-x rich sublayer beneath an Al₂O₃ rich toplayer, whereas the oxide layer on the Ti_0.38Al_0.62N film consisted of pure Al₂O₃. The thickness of the oxide layer was determined to 60–80 nm, about a quarter of the oxide layer thickness detected on Ti_0.5Al_0.5N films. The absence of a TiO_2-x sublayer was also confirmed by XRD. The results show a distinct improvement of the oxidation resistance of cubic Ti_1-xAl_xN films by increasing the Al content from x = 0.5 to 0.62, whereas a further increase leads to the hexagonal structure, which is less suitable for tribological applications due to its tendency to form cracks during oxidation.     

Phase behaviour of the thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis-(4-n-octyloxybenzoyl)hydrazine, BABH(8), was investigated under pressures up to 200 MPa using a high pressure differential thermal analyser, wide-angle X-ray diffraction and a polarizing optical microscope equipped with a high pressure optical cell. The phase transition sequence, low temperature crystal (Cr₂)-high temperature crystal (Cr ₁)- cubic (Cub)-smectic C (SmC)-isotropic liquid (I) observed at atmospheric pressure, is seen in the low pressure region below about 30 MPa. The cubic phase disappears at high pressures above 30–40 MPa, in conjunction with the disappearance of the Cr₁ phase. The transition sequence changes to Cr₂-SmC-I in the high pressure region. Since only the Cub-SmC transition line among all the phase boundaries has a negative slope (dT/dP) in the temperature-pressure phase diagram, the temperature range for the cubic phase decreases rapidly with increasing pressure. As a result, a triple point was estimated approximately as 31.6 ±2.0 MPa, 147.0±1.0°C for the SmC, Cub and Cr₁ phases, indicating the upper limit of pressure for the observation of the cubic phase. Reversible changes in structure and optical texture between the Cub and SmC phases were observed from a spot-like X-ray pattern and dark field for the cubic phase to the Debye-Sherrer pattern and sand-like texture for the SmC phase both in isobaric and isothermal experiments.     

Visualisation-based analysis of structure and dynamics of liquid aluminosilicate under compression

Structure and dynamics of liquid aluminosilicate (Al₂O₃–2SiO₂, abbreviated as AS2) are investigated by molecular dynamics (MD) simulation and visualisation. The local structural characteristics are analysed via topology statistics of basic structural units TO_n and OT_m (T = Si, Al; n = 3, 4, 5, 6; m = 2, 3, 4, 5). The amount distribution as well as spatial distribution of the basic structural units under compression is also clarified. Regarding the intermediate range order, the amount and spatial distribution of all types of OT_m linkages as well as the bond statistics (corner-, edge-, and face-sharing) between two adjacent TO_n units are investigated in detail. The self-diffusion of Si, Al, and O is calculated via mean square displacement (MSD). The anomalous diffusion is explained in detail in relationship to structural characteristics. The structural and dynamical heterogeneities, micro-phase separation, and liquid–liquid phase transition are also discussed in this work.     

Molecular dynamics study on lattice defects and heat capacity of LiCr1/6Mn11/6O4-d

Heat capacity of spinel LiCr_1/6Mn_11/6O_4-d (d=0, 0.0184)was measured between 5 and 300 K. Both compounds showed no anomaly in the measured temperature range, especially around the room temperature where a structural phase transition is reported for the parent compound LiMn₂O₄. The non-stoichiometric compound LiCr_1/6Mn_11/6O_3.9816 has greater heat capacity than that of the stoichiometric LiCr_1/6Mn_11/6O₄. Molecular dynamics study on the vibrational property of LiMn₂O_4-d revealed that the lattice defects in the non-stoichiometric compound increase the low frequency phonons compared with the stoichiometric compound. It should be related to the greater heat capacity of the non-stoichiometric compound LiCr_1/6Mn_11/6O_3.9816. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ferroelectric phase transitions in Pb2xSn2(1-x)P2Se6 system

Heat capacities of the Pb_2xSn_2(1-x)P₂Se₆ crystals (x=0, 0.098, 0.251, 0.402 and 1.0) were measured using an adiabatic calorimeter at temperatures between 10 and 350 K. In the crystal of x=0, two heat capacity anomalies corresponding to the ferroelectric commensurate - intermediate incommensurate(C-IC) phase transition temperature T _i, and the incommensurate - paraelectric (IC-N) phase transition temperature T _c, were observed at 193.24±0.10 and 220.07±0.15 K, respectively. The phase transition temperatures decreased with an increase in Pb²⁺ concentration. The anomaly at Ti disappeared at x=0.251 in the mixed systems of the Pb_2xSn_2(1-x)P₂Se₆. In the crystal of Pb₂P₂Se₆ (x=1.0), no phase transition was observed. The normal heat capacities for the mixed crystals were determined by least squares fitting of the Debye and Einstein functions to the experimental data. The anomalous heat capacities gave the phase transition entropies of 8.5 and 1.5 J mol^-1 K^-1 for x=0. The large transition entropies are consistent with an order-disorder mechanism in the ferroelectric-paraelectric phase transitions in x=0. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterisation of Siliceous Extra‐Framework Species in DAY Zeolites by 29Si MAS NMR and IR Spectroscopic Measurements

Dealuminated Y zeolites (DAY) were obtained by steaming of NH₄NaY at temperatures between 450 °C and 700 °C. They were characterised by means of ²⁷Al and ²⁹Si MAS NMR, IR spectroscopic and XRD measurements. The Si/Al framework ratios of samples were calculated using the ²⁹Si MAS NMR signal intensities, the wave numbers of the double‐ring vibration band w_DR and the asymmetrical TOT valence vibration w_TOT of IR spectra as well as the XRD lattice constant a₀. In contrast to actual Si/Al ratio obtained from w_DR and a₀, the NMR spectroscopic and w_TOT values were determined to be too high because of the superposition of the signals coming from dealuminated zeolite framework and silica gel which forms in the zeolite as a result of steaming. The differently determined Si/Al ratios characterise the siliceous extra‐framework species.     

Ambient temperature thermotropic liquid crystalline viologen bis(triflimide) salts

Two dicationic salts with bis(triflimide) as counterions exhibited crystal-to-smectic liquid crystalline phase transitions (T _m=41 and 37°C) and smectic-to-isotropic liquid phase transitions (T _i=112, 136°C). They had a broad liquid crystalline phase range (71–99°C) and an excellent range of thermal stability (360–364°C). Their mixtures of various compositions also displayed liquid crystalline properties from r.t. to an extended range of temperatures. They exhibited fluorescence in 1, 2-dimethoxyethane and methanol.     

Decanuclear Ln10 Wheels and Vertex‐Shared Spirocyclic Ln5 Cores: Synthesis,Structure, SMM Behavior,and MCE Properties

The reaction of a Schiff base ligand (LH₃) with lanthanide salts, pivalic acid and triethylamine in 1:1:1:3 and 4:5:8:20 stoichiometric ratios results in the formation of decanuclear Ln₁₀ (Ln=Dy( 1 ), Tb( 2 ), and Gd ( 3 )) and pentanuclear Ln₅ complexes (Ln=Gd ( 4 ), Tb ( 5 ), and Dy ( 6 )), respectively. The formation of Ln₁₀ and Ln₅ complexes are fully governed by the stoichiometry of the reagents used. Detailed magnetic studies on these complexes ( 1 – 6 ) have been carried out. Complex 1 shows a SMM behavior with an effective energy barrier for the reversal of the magnetization (U_eff)=16.12(8) K and relaxation time (τ_o)=3.3×10^?5 s under 4000 Oe direct current (dc) field. Complex 6 shows the frequency dependent maxima in the out‐of‐phase signal under zero dc field, without achieving maxima above 2 K. Complexes 3 and 4 show a large magnetocaloric effect with the following characteristic values: ?ΔS_m=26.6 J kg^?1 K^?1 at T=2.2 K for 3 and ?ΔS_m=27.1 J kg^?1 K^?1 at T=2.4 K for 4 , both for an applied field change of 7 T.     

Spreading and compression of n-hexadecyl acrylate at liquid-air interface

Usingn-hexadecyl acrylate, surface pressure-area (F-A) curves and equilibrium spreading pressuresF ^e were measured at various temperatures (5.7°–46°C) by the Langmuir balance (F-A) and the Wilhelmy-plate method (F ^e). At low temperatures (T<13 °C) condensed films and the liquid-condensed/solid condensed transition can be observed. At high temperatures (T>30 °C) liquid-expanded films occur. In the intermediate range the compression curves have two transition points. The transition pressureF ₁ between liquid-expanded and condensed film has a marked temperature dependence. The transition enthalpiesH ₁ decrease with increasing temperature and become zero at 29.2 °C. The second transition is related to a transition between the condensed films (F ₂). The slight temperature dependence of this transition is accompanied by an increasing change of area as well as by increasing transition enthalpiesH ₂.TheF ^e-T curve has two distinct breaks, at the melting pointT _m and atT=30 °C. The break atT _m is due to the melting process and the break atT=30 °C is caused by a phase transition between a liquid-expanded film and a condensed film.The phase diagram was constructed from the transition pressures. It can be demonstrated that the highest pressures of the thermodynamic stable film occurs atT _m. At temperaturesT>T _m equilibrium spreading pressure and equilibrium collapse pressure are identical whereas atT _m supercompression of the monolayer occurs. The film in this state behaves like a supercooled liquid. Obviously, rupture and collapse of such a film lead to a thermodynamically metastable bulk phase.     

Study on mineral surface reacted with water at temperatures above 300��C and 23?MPa

In this work, we carried out experiments on silicate mineral dissolution using a flow-through reactor from 20 to 400°C at 23 MPa. The dissolution of silicate minerals such as actinolite, diopside, and albite in water may require the breaking of more than one metal?Coxygen bond type. Different metal elements in silicate minerals have different release rates and the dissolution product is often non-stoichiometric. Na, Mg, Fe, and Ca dissolve faster than Si at T < 300°C. At T ?? 300°C, the release rate of Si is higher than that of the other metals. The molar concentration ratios of the dissolving metal Mi versus Si such as Mg/Si, Ca/Si and Al/Si, which are the release ratios in the effluent solutions, are often different from the molar ratios of these elements in the minerals. The results show that the incongruent dissolution of minerals is related to surface chemical modifications. Non-stoichiometric dissolution is caused by the formation of a non-stoichiometric leaching layer at the surface or by the presence of a secondary mineral at the surface. Our experiments indicate that the dissolution of most silicate minerals is close to stoichiometric at 200 and 300°C, e.g., for actinolite and albite at 300°C. The surfaces after reaction with aqueous solutions were investigated using SEM and TEM. At T < 300°C, the mineral surfaces (e.g., for actinolite) after the reaction with water are slightly Si-rich and slightly Fe (and/or Mg, Ca) deficient. In contrast, at T ?? 300°C, the surfaces after reaction with water are slightly Fe-rich and somewhat Si deficient.     

Hydrate formation in the tetraethylammonium hydroxide-water system

A phase diagram of the tetraethylammonium hydroxide-water binary system was studied by differential thermal analysis. Seven crystal phases of tetraethylammonium hydroxide were found. Two of them are stable: tetrahydrate (T _m = 47.0°C), two phases of a compound with the hydration number 7.5 (T _m = −18.1 and −23.2°C); five are metastable phases: two polymorphs of pentahydrate (T _m = 39.4 and 0.9°C), hexahydrate (T _m = −70.1°C), and a compound with hydrate number 10–11 (T _m = −112°C).     

Characterization of phase transition of Ba2-xSrxIn2O5 by thermal analysis and high temperature X-ray diffraction

The phase transitions of Ba_2-xSr_xIn₂O₅ were investigated with various thermal analyses and high-temperature X-ray diffraction. It was clarified that crystal structure of Ba_2-xSr_xIn₂O₅ with x=0.0~0.4 varies from brownmillerite through distorted perovskite to another distorted perovskite with increase of temperature. The phase transition from brownmillerite to distorted perovskite was revealed to be first order, whereas transition from distorted perovskite to another one was second order. The specimen with x≥0.5 showed only one first order phase transition from brownmillerite to distorted perovskite. The phase diagram of Ba_2-xSr_xIn₂O₅ was established and existence of tricritical point at ~1100°C with x=0.4~0.5 was suggested. This revised version was published online in August 2006 with corrections to the Cover Date.     

Calorimetric study and thermal analysis of crystalline nicotinic acid

As one primary component of Vitamin B₃, nicotinic acid [pyridine 3-carboxylic acid] was synthesized, and calorimetric study and thermal analysis for this compound were performed. The low-temperature heat capacity of nicotinic acid was measured with a precise automated adiabatic calorimeter over the temperature rang from 79 to 368 K. No thermal anomaly or phase transition was observed in this temperature range. A solid-to-solid transition at T _trs=451.4 K, a solid-to-liquid transition at T _fus=509.1 K and a thermal decomposition at T _d=538.8 K were found through the DSC and TG-DTG techniques. The molar enthalpies of these transitions were determined to be Δ_trs H _m=0.81 kJ mol^-1, Δ_fus H _m=27.57 kJ mol^-1 and Δ_d H _m=62.38 kJ mol^-1, respectively, by the integrals of the peak areas of the DSC curves. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hydrogen‐Atom Abstraction from Methane by Stoichiometric Vanadium–Silicon Heteronuclear Oxide Cluster Cations

Vanadium–silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O₂ background. Reactions of the heteronuclear oxide cations with methane in a fast‐flow reactor were studied with a time‐of‐flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V₂O₅)_n(SiO₂)_m]⁺ (n=1, m=1–4; n=2, m=1), and the estimated first‐order rate constants for the reactions were close to that of the homonuclear oxide cluster V₄O₁₀⁺ with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal‐oxygen‐centered radicals (O_t ^. ) were found in all of the stable isomers. These O_t ^. radicals are active sites of the clusters in reaction with CH₄. The O_t ^. radicals in [V₂O₅(SiO₂)_1–4]⁺ clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH₄ activation on silica‐supported vanadium oxide catalysts.     

Synthesis and characterization of thermotropic liquid crystalline poly(ester imide)s

Two closely series of poly(ester imide)s had been synthesized by solution polycondensation of p‐phenylenebis(trimellitate) dianhydride with aliphatic diamines. The differential scanning calorimetry (DSC) traces of the most poly(ester imide)s exhibited two endotherms representing the solid state to anisotropic phase transition (T_m1) and the anisotropic to isotropic melt transition (T_m2), respectively. Observation under polarizing microscope and wide‐angle X‐ray diffraction (WAXD) measurements suggested that the anisotropic phase formed above the melting points (T_m1) had a smectic character. The thermogravimetric analyses (TGA) revealed that the thermal stabilities of the poly(ester imide)s were up to 350°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 211–218, 1999     

Molecular dynamics simulation of the phase behavior of AgI<Subscript>1-x</Subscript>Cl<Subscript>x</Subscript>

Summary The temperature and pressure dependencies of the structure and transport properties of AgI_1-xCl_x are studied by means of molecular dynamics. From the results of the calculations, the P-T phase diagram for the AgI_0.8Cl_0.2 system has been obtained. The phase diagram indicates that the superionic phase transition temperature increase with the application of pressure. It is also pointed out that AgI_1-xCl_x has a tendency of phase separation.     

Molecular structure and the twist-bend nematic phase: the role of terminal chains

ABSTRACT

The synthesis and characterisation of two homologous series of non-symmetric dimers are reported, the 1-(4-methoxybiphenyl-4?-yl)-6-(4-alkylanilinebenzylidene-4?-oxy)hexanes (MeOB6O.m, m = 1–10) and 1-(4-methoxybiphenyl-4?-yl)-6-(4-alkyloxyanilinebenzylidene-4?-oxy)hexanes (MeOB6O.Om, m = 1–9). All 10 members of the MeOB6O.m series exhibit the conventional nematic phase. At lower temperatures, the members with m = 1–7 formed the twist-bend nematic phase, N_TB, whereas for m = 8–10 smectic behaviour replaced the N_TB phase. All nine members of the MeOB6O.Om series also show the conventional nematic phase and for m = 1–3, a strongly monotropic N_TB phase is also observed. The alkyloxy terminated dimers show the higher values of T_NI and T_{NTB N} . For both series, the values of T_NI and T_{NTB N} show a modest alternation and in the same sense as m is increased. These observations suggest that the spatial uniformity of molecular curvature is important in driving the formation of the N_TB phase. The observation of smectic behaviour is attributed to the molecular inhomogeneity arising from the long terminal alkyl chain driving microphase separation. The transitional behaviour of these series is compared to those of the corresponding cyanobiphenyl-based series and overarching observations discussed.     

Mechano-chemical synthesis K2MF6 (M = Mn,Ni) by cation-exchange reaction at room temperature

In order to establish the power of mechanochemistry to produce industrially important phosphors, synthesis of K₂MnF₆ has been attempted by the successive grinding reactions of manganese (II) acetate with ammonium fluoride and potassium fluoride. The progress of reaction was followed by ex-situ characterization after periodic intervals of time. Cubic symmetry of K₂MnF₆ was evident from its powder X-ray diffraction pattern which was refined successfully in cubic space group (Fm-3m) with a = 8.4658 (20) Å. Stretching and bending vibration modes of MnF₆²⁻ octahedral units appeared at 740 and 482 cm⁻¹ in the fourier transformed infrared spectrum. Bands at 405 and 652 cm⁻¹ appeared in the Raman spectrum and they were finger-print positions of cubic K₂MnF₆. Other than the ligand to metal charge transfer transition at 242 nm, transitions from ⁴A_2g to ⁴T_1g, ⁴T_2g and ²T_2g of Mn⁴⁺-ion appeared at 352, 429, 474 and 569 nm in the UV–visible diffuse reflectance spectrum of the sample. Red emission due to Mn⁴⁺ was observed in the photoluminescence spectrum with a decay time of 0.22 ms. Following the success in forming cubic K₂MnF₆, this approach has been extended to synthesize cubic K₂NiF₆ at room temperature. All these results confirmed the susceptibility of acetate salts of transition metals belonging to first-row of the periodic table to facile fluorination at room temperature aided by mechanical forces.     

Crystal Structure and Dimorphism of Silicon Tetraisocyanate Si(NCO)4

Silicon tetraisocyanate Si(NCO)₄ was obtained by reacting SiCl₄ and AgNCO in boiling toluene. The colourless liquid was analyzed by Raman and NMR spectroscopy. Structural studies on solid Si(NCO)₄ (melting point: +26 °C) have revealed it to exist in two polymorphic modifications. According to the results of single‐crystal X‐ray diffraction, at T = –173 °C, α‐Si(NCO)₄ exhibits triclinic symmetry (P$\bar{1}$ ; a = 10.05(5), b = 10.50(2), c = 14.32(1) Å, α = 91.62(1)°, β = 92, 32(1)°, γ = 99.68(1)°; V = 1488.56(3) ų; Z = 8). Above T = –33 °C, a monoclinic phase evolves, β‐Si(NCO)₄ (P2₁/c; a = 10.78(3), b = 7.11(1), c = 10.27(5) Å, β = 99, 06(9)°; V = 777.39(1) ų; Z = 4). The charge distribution was studied for both polymorphs. In the solid state, Si(NCO)₄ is a tetrahedral molecule with the Si–N=C=O linkages bent at the nitrogen atoms.     

The non-stoichiometry of tetracyanocomplexes

Tetracyanocomplexes can be non-stoichiometric in three different ways: 1. The preparation of complexes such as M(NH₃)₂M'(CN)₄.nG and M(en)_m H'(CN)₄.nG can result in a non-stoichiometric product i.e. n is non-integer (refs. 1 and 2). 2. Heating a fully stoichiometric compound such as M(NH₃)₂M'(CN)₄.2G, can result in the loss of some of the guest molecules (ref. 3). 3. A stoichiometric or non-stoichiometric compound can absorb an aromatic molecule of the appropriate size (refs. 4–6) when heated in the presence of the molecule.