Structural studies of odd-nylon crystals grown from solution

Lamellar crystals of nylons 5, 9 and 13 in-structure were grown from solution and systematically studied by electron microscopy and X-rays. Electron diffraction showed that they all consist of a twinned structure with hydrogen bonds running parallel to the long dimension of the crystals. Polyethylene decoration of such crystals revealed folding sectorization only in the case of nylon 9. Diffraction data from films both uniaxially and biaxially oriented were used to propose a unifying model for the-structure of odd nylons based on the antiparallel model given by Tadokoro. In this model successive sheets are progressively sheared along the chain axis by approximately twice the interatomic height. Such a model is able to account well for most of the structural observations we have made on solution grown crystals.     

Chain-folding and structures in nylon 6 oligoamide lamellar crystals

A series of carefully selected monodisperse nylon 6 oligomers, terminated with short alkane segments, have been crystallised from solution. This contribution contains an overview of the folding and structures in nylon oligomer crystals, recorded at room temperature, using transmission electron microscopy and X-ray diffraction (both wide- and small-angle); together with important conclusions drawn from studies of these molecules. The work is set in the context of much earlier studies by Professors Zahn and Keller and their co-workers. The oligomers contained 2, 3, 5, 9, 10 and 17 amide units, with a length range of 2 nm to 15 nm. The shorter oligomers (with 2, 3 and 5 amide units) crystallised with non-folded chains; the longer 9- and 10-amide oligomers were once-folded, following rapid crystallisation; whilst the 17-amide oligomer was found to fold twice. Thus chain folding sets in on rapid crystallisation, with the 9-amide molecule (length 8 nm). Studies of the once folded 9- and 10-amide oligomers show that both amide unit and alkane segment folds occur, in this instant the symmetry of the hairpin and the requirement for saturated hydrogen bonds between straight stems are the controlling factors; the nature of the fold is subservient. This shows that nylon 6 chains can stereochemically accommodate either alkane or amide folds. All the folded molecules were observed in the nylon 6 -structure, where chains are antiparallel within the hydrogen-bonded sheets, which have alternating chain (c)-axis shear alternately. In the shorter oligoamides (2-, 3- and 5-amide), where the molecules do not fold, the crystallisation is less restricted (e.g. the chains can form a parallel array) and two new crystal structures have been identified. In the 2- and 3-amide crystals, hydrogen bonds can occur in two directions between parallel chains and orthogonal to the molecular axis (c-axis), which is inclined at a substantial angle to the lamellar surface (the -structure). The 5-amide oligomer also does not fold. In this case, the hydrogen bonds are made between antiparallel chains, as in the nylon 6 -structure; however, these sheets progressively c-axis shear to give a new structure (the -structure). Chain-folded molecules can crystallise in the -structure although not in the -structure because it requires parallel chains.     

Kinetics of crystallization of trans-1, 4-polyisoprene crystallized in thin films

The lamellar thickness and crystal growth rates of both high melting form and low melting form trans-1,4-polyisoprene crystals growing from the same melt in thin films, have been determined by transmission electron microscopy. Values of the fold surface energy ( _e) have been determined and compare well with values for solution grown and bulk melt grown crystals. It is suggested that the crystals have a similar form in all three cases. The crystal growth rate data can be described by equations derived from secondary nucleation theory and the product of the surface energies ( _{e s}) is calculated. The value of the product is compared with recalculated values determined by using previously published optical growth rate data.     

Morphology of solution-grown poly(β-propiolactone) single crystals

Single crystals of poly(-propiolactone) (PPL) with different molecular weights (M _w = 70000 and M _w = 2300) were grown from four kinds of solvents under isothermal crystallization condition. The morphologies and crystal structures of PPL single crystals were investigated by means of transmission electron microscopy and atomic force microscopy. The single crystals of high-molecular-weight PPL (HMW-PPL) grown from cyclohexanone appeared elongated with dimensions of around 0.2–0.8 m along the short and 5–10 m along the long axes. Single crystals of low-molecular-weight PPL grown from cyclohexanone showed three to five elliptical-shaped lamellae, from central nucleus like petals. The long axes of both single crystals corresponded to the crystallographic b axis. The reciprocal lattice parameters: a* = 2.045 nm^–1, b* = 1.420 nm^–1 and * = 90° could be determined from electron diffractograms. Decoration of the crystals with polyethylene suqqested that the single crystals of HMW-PPL have regular chain-folding along their long axis in the [010] direction with consecutive folds in the [110] and [1 0] directions. Accordingly, it is deduced that HMW-PPL has the anti-parallel chain-packing structure.     

Electrical conductivity of α-H x V2O5 (X=0.00-0.27): Dependence of hydrogen concentration and orientation

The dependence of the electrical conductivity, , on the hydrogen concentration and crystallographic orientation has been investigated using single crystals of H_xV₂O₅, which were grown by the Bridgeman method and doped with hydrogen within the solid solubility in the phase by the spillover technique. The temperature dependence of showed the feature of diffusive hopping of thermally activated electrons above 180 K and variable range hopping below 180 K. The dependence of on the crystallographic orientation was little different from that of V₂O₅. The change in with the hydrogen concentration was not monotonic; increases withx up tox }-0.06, but decreases abovex }-0.06. This behaviour can be explained based on the competition between the increase in the carrier density and the depression of the mobility of carriers with increasingx.     

Hardness anisotropy and its dependence on composition in sodium tungsten bronzes and rhenium trioxide single crystals

Hardness anisotropy measurements using a Knoop diamond indenter on {0 0 1} and {0 0 1} surfaces of Na _x WO₃ (0.4<x<0.8) and ReO₃ single crystals with the cubic perovskite structure show that hardness is determined by slip on {1 1 0} 1 1 0. Room temperature slip is produced by Knoop and Vickers microhardness indentations on polished crystals and confirms the active slip system identified from a consideration of anisotropy. The hardness anisotropy is more pronounced as the sodium content of the crystals increases. The data suggests that hardness of Na _x WO₃ is dependent on both plane and direction.     

Electron spin resonance in single crystal yttria stabilized zirconia

Electron spin resonance has been observed at 35 GHz in 8 and 12 mol % yttria stabilized zirconia single crystals before and after blackening by current passage. Measurements were made between 293 and 77K. In both as-grown and blackened crystals the spectra showed anisotropic lines (type A) characterized by g =2.003±0.002 and g =1.880 ±0.002 with respect to a [1 1 1] symmetry axis and a broader, slightly anisotropic line (type B) centred near g=1.993. In current blackened crystals a weak isotropic line (type C) was also found near g=1.986. The type A lines are attributed to a charged complex formed by an electron trapped at an oxygen vacancy and associated with an yttrium ion. Tentative models are suggested to explain the type-B and type-C lines.     

Structure and Properties of LiGaO2Crystals

Structural evidence is presented that LiGaO₂crystals (sp. gr. Pna2₁, point gr. mm2) may have different polarities. X-ray structural analysis of nominally undoped and doped (Cr or V) LiGaO₂crystals grown by the Czochralski technique shows that LiGaO₂and LiGaO₂Cr crystals differ in polarity from LiGaO₂V. Defect LiGaO₂can be thought of as a twin constituted by individuals of different polarities. Polarimetry data indicate that both LiGaO₂and LiGaO₂V are optically active.     

Thermally Induced Deep Centers in Silicon Doped with Europium or Lanthanum

Melt-grown silicon crystals doped with europium or lanthanum were studied by capacitance spectroscopy and infrared absorption spectrophotometry. The Eu and La in the as-grown crystals were demonstrated to be electrically inactive. Annealing in the temperature range 900–1200°C produced a few deep levels in the band gap of both n-SiEu and n-SiLa. These levels are likely related to the activation of the Eu and La impurities: Eu seems to be responsible for the levels at E _c– 0.40 eV and E _c– 0.46 eV, and La for the level at E _v+ 0.32 eV. Infrared absorption measurements indicate that heat treatment increases the concentration of optically active (interstitial) oxygen in the crystals by 15–20%.     

Observations on whiskers and skeletal crystals in PbSe layers by scanning electron microscopy

Whiskers and skeletal crystals of PbSe are obtained on substrates of (100) germanium. The production conditions and peculiarities of their structures are discussed. SEM observations show that the whiskers are prism-shaped possessing rectangular or square cross-sections with lengths up to 190m and widths of the surrounding walls up to 26m. Hollow pyramidal crystals with their apexes connected to the layer surface by a whisker are described, as well as hollow crystals with an open or a closed surrounding surface. Certain aspects of the growth mechanism of skeletal crystals and whiskers are discussed.     

Self-diffusion of silicon-30 in α-SiC single crystals

The self-diffusion of³⁰Si in high purity and N-doped -SiC single crystals has been measured in the temperature range 2273 to 2573 K. The diffusion (D _Si ^* ) in N-doped crystals exceeds that in the pure crystals because of the increase in the concentration of the charged acceptor-type Si vacancies in the presence of the N species. A comparison ofD _C ^* andD _Si ^* shows that the former exceeds the latter by approximately 10², primarily because of the greater entropy of migration of C. Possible crystallographic paths of transport for both species are also discussed.     

Morphology of single crystals of poly (hydroxybutyrate) and copolymers of hydroxybuty rate and hydroxyvalerate

Single crystals of poly(hydroxybutyrate), and copolymers of hydroxybutyrate and hydroxyvalerate, have been grown from a variety of solvents and their morphology studied. In all cases the crystals appear elongated, with the crystallographic a axis along the long axis of the crystals. Screw dislocations (which act as branch points and can lead to large crystal aggregates) have been observed, and solvents, e.g. octanol in which more of these dislocations occur, have been identified. Decoration of the crystals with polyethylene shows that the fold plane is the (0 1 0) plane, while crystallographic and crystallinity arguments require successive adjacent folds to be along 1 1 0 directions. It is argued that folding is along [1 1 0] on the top, and [1 ¯1 0] on the bottom surface of a crystal.     

How and why CVD diamond is formed: a solution of the thermodynamic paradox

Diamond chemical vapour deposition (CVD) thought of as a crystal growth process, is a thermodynamic paradox because it violates fundamental principles of thermodynamics. The most astonishing violation is the experimental observation that CVD diamond can form in gaseous environments that are carbon under-saturated with respect to diamond. A new concept of CVD diamond formation that describes the process in terms of thermodynamics, without any violation of the latter, is presented. According to the present concept the diamond formation is a chemical process consisting in accretion of polymantane macromolecules. The process proceeds on surfaces of polymantane seed macromolecules which are identical to diamond crystals which have H-terminated surfaces. Chemical thermodynamics insist that the Gibb's energy of reaction describing the process, G, has a large negative value for the process to be able to proceed. However, under certain conditions, the diamond CVD may not proceed even if G 0, because the process may be kinetically hindered. Such a situation occurs at low temperatures at which the abstraction of hydrogen atoms from H-terminated diamond seed crystal surfaces by free hydrogen atom impact followed by the addition of new carbon atoms to the diamond lattice, is a rate-limiting step. The kinetic parameter determining the rate of this step is correlated with thermodynamic instability, TI, of H-terminated diamond seed crystal surfaces. Using G and TI functions, one can derive correlations between the film-phase composition as well as the growth-rate and process variables. The dependencies predicted by the present model are in excellent agreement with reported experimental data.     

High pressure melting and crystallization of Nylon-11

Differential thermal analysis (DTA), high pressure differential thermal analysis (HP-DTA), and high temperature X-ray studies are combined to elucidate the origin of the two melting peaks in Nylon-11. The results of the studies suggest that two species of crystals are involved in the melting of Nylon-11 for samples crystallized at atmospheric pressure or when the environmental pressure is below 4 kbar. At atmospheric pressure, the high melting species is predominant. However, under hydrostatic pressures, the high melting species undergoes phase transition to the low melting species before melting. The amount of the material involved in the transition depends on the pressure. At pressures of 4 kbar or greater, the entire high melting species transforms to the low melting species. The melting behaviour, at atmospheric pressure, of samples crystallized at high pressures also shows two melting peaks if the crystallization pressure is below 4 kbar. The amount of the low melting species increases with increasing pressure and, at 4 kbar or higher, only melting of the low melting species is observed. The X-ray photographs taken at room temperature suggest that samples crystallized between atmospheric pressure and 3 kbar contain both the-form and the-form crystals but the samples crystallized at 4 kbar and higher contain only the-form crystal. However, it appears from X-ray scans taken at high temperatures near melting that the low melting species is of the-form and the high melting species of the -form crystals for samples crystallized below 4 kbar. The-form crystals result from the- transition that occurs at 95° C. Moreover, the melting at high pressures (<4 kbar) of samples crystallized at atmospheric pressure also appears to involve a- transition. These results suggest that both the crystal forms, and, are stable at high temperatures, if the environmental pressure is below 4 kbar, and that only the-form crystals are stable up to melting at pressures greater than 4 kbar.     

Formation of a high-strength bioactive glass-ceramic in the system MgO-CaO-SiO2-P2O5

Formation of a high-strength bioactive glass-ceramic in the system MgO-CaO-SiO₂-P₂O₅ was investigated by observing the microstructure of the crystallized products. Crystallization of the parent glass in a bulk form led to the occurrence of large cracks in the crystallized product. This was attributed to the precipitation of fibrous-wollastonite crystals growing perpendicular to the outer surfaces of the glass after uniform precipitation of fine oxyapatite/fluoroapatite crystals. On the other hand, crystallization of the same glass in a powder compact led to the formation of a crack-free dense crystallized product due to uniform precipitation of both apatite and wollastonite fine crystals throughout the glass article. The uniform precipitation of the wollastonite crystals was attributed to the simultaneous formation of fine crystals in the individual glass particles.     

Vapor Growth of Pb1 – x In x Te Crystals

Vapor growth of In-doped PbTe crystals by the sublimation–condensation and vapor–liquid–solid (VLS) processes is examined. Well-faceted Pb_{1 – x} In _x Te crystals with x = 0.04–0.06 are prepared by the sublimation method. The effects of the charge composition on the facial development and growth rate in the range 0 x 0.02 are discussed. The growth process at x 0.02 is shown to follow the VLS mechanism. Bulk Pb_{1 – x} In _x Te crystals with x 0.05 are grown by a vertical VLS process. The crystal composition is shown to depend significantly on the rate of ampule translation through the temperature field of the furnace and the separation between the evaporation and condensation zones. The longitudinal indium profiles in the crystals are correlated with growth kinetics.     

Pulsed NMR study in nuclear ordered solid3He

The nuclear spin dynamics in nuclear spin ordered solid³He in low magnetic fields on the melting curve has been studied by pulsed NMR down to 0.6 mK. The free induction decay signals (FID) were measured in single crystals of solid³He at three operating frequencies of 920, 1380, and 1840 kHz. The FIDs were nonexponential and dependent on the rf pulse strength _p H ₁ t _w , where is the gyromagnetic ratio,H ₁ is the rf field strength, andt _w is the pulse width. At small _p they decayed almost linearly in time with a small exponential tail at the end. When _p was further increased they became shorter and neither exponential nor linear in time. At large _p they decayed very rapidly and sometimes could not be observed at all because of the dead time of the NMR detection system. Such behavior of the FID was observed in many different single crystals in the given temperature range at 920 kHz. Tsubota and Tsuneto have shown by solving the nonlinear equations of motion numerically that the motion of the nuclear spin becomes chaotic when the tipping angle exceeds a critical value. Comparing their result with our experimental results, we concluded that some of the results of the rapid decay of the FID at large _p might be attributed to the onset of the chaotic motion. At 1840 kHz it is expected that the nonlinear effects in the equations of motion become less effective than that at 920 kHz. In fact, at this operating frequency the FIDs even at large _p and the tipping angle-dependent frequency shift could be observed. These frequency shifts were in rather good agreement with Namaizawa's theory provided an effective tipping angle was taken into account.     

Stacking-faults in tellurium-doped gallium arsenide

Samples of bulk-grown gallium arsenide single crystals, taken from both static freeze, and Czochralski ingots doped to a high level with tellurium, have been examined using transmission electron microscopy. Observation of single and multiple stacking-fault layers which have fault vectors of the kindR=a/3111 is reported. It is shown by diffraction contrast experiments that the stacking-fault defects are extrinsic. They are thought to be rafts of tellurium substituting for arsenic in {111} planes. The prevalence of the observed layer defects correlates well with the increase in carrier concentration in certain regions of the crystals.     

Thermal conductivity of superconducting UPt3

The thermal conductivity of the heavy-fermion superconductor UPt₃ has been measured on two single crystals of different quality as a function of temperature and applied magnetic field for different orientations of the heat current relative to both the field and the crystalline axes. The temperature dependence of is far from exponential and nearly the same for both crystals, in which the heat current is, respectively, parallel and perpendicular to the hexagonalc axis, suggesting a gap structure with nodes in the basal plane and normal to it. The field dependence of is strongly anisotropic. In the best sample at low fields, where the scattering of heat carriers by vortices is thought to be important, (H) depends on the relative orientation of field and current. On the other hand, at high fields nearH _c2 (in both samples), (H) depends on the relative orientation of field and crystalline axes, reflecting an anisotropy in the gap structure and in the Fermi velocities.     

Piezoelectric Properties of TlIn1−x Nd x Se2 Crystals

The piezoelectric properties of TlIn_1–x Nd _x Se₂ crystals (0 x 0.05) have been investigated. It has been found that these crystals possess high coefficients of tensosensitivity which change on partial substitution of the indium atoms by neodymiun atoms and strongly depend on the intensity of the spectral composition of optical illumination.