High-density amorphous structures water

When partially hydrated samples are cooled down to 77 K, no crystallization peak is detected by differential thermal analysis. The x-ray and neutrons show that an amorphous form is obtained and its structure is different from those of low-and high-density amorphous ices already known [5]. Samples with lower levels of hydration corresponding to one monolayer coverage of water molecules are under investigation. This phenomenon looks similar in both hydrophilic and hydrophobic model systems. However, in order to characterize more precisely the nature of the amorphous phase, the site-site partial correlation functions need to be experimentally obtained and compared with those deduced from molecular dynamic simulations. 

Ice films condensed from the water vapour on a cold substrate (T<30 K) has been characterized as a high-density amorphous form of ice, which could be a denser variant of the low-density phase obtained by deposition above 30 K. Condensation from the background pressure also leads to ice films that are highly porous at a nanoscale.This porosity is lost by warming or by direct deposition of water at T>90 K. Warming ice at 150 K induces the crystallization, whatever the initial structure is. 

It is well known that irradiation alters the structure of ice. It is amorphized at T < 80 K by protons, ions, photons and electrons. The structures of the irradiated ices have only been determined in the case of intense electron irradiation, where the high-density amorph was detected. Recent molecular dynamic simulations have also shown a densification of the ASW ice when irradiated with 35 eV water molecules,but these simulations also questioned the existence of the high-density phase as the initial structure of ice films deposited at low temperature. 

Narten et al. also deduced from their data that one fifth of the water molecules are located at this additional distance. In a RDF picture, this corresponds to 4 nearest-neighbours at 2.76 A and 1 second neighbour at 3.3 A. This matches well the atomic surrounding depicted by the cluster corresponding to the structure of ice after the irradiation (cluster 2). The local order before the irradiation is better described by the 4-coordinated tetrahedron found in the normal amorphous low-density ice and in the crystalline ice (cluster 1). Thus we conclude that the structure of the ice film before the irradiation is not that of the high-density phase but that of the normal low-density phase. In addition, since the irradiated ice has a local order similar to what expected in the high-density phase, we also conclude that the photolysis at 20 K has induced the phase transition from the low-density to the high-density amorph. 

The possibility of the existence of a second liquid-liquid phase transition in water was discussed following the discovery [42] of an even higher density amorphous state, named very high density amorphous (VHDA). However, unlike the LDA-HDA transition, this has since been widely accepted to be a continuous change in the structure [74]. 

There exist different types of ice (see also Fig. 1.12). The ice we know from everyday live (also snow) has a hexagonal structure. At higher temperatures and pressures ice can also form a cubic structure 4). Other forms of ice are called II, III, V, VI, VII, VIII, IX and X. The difference between these forms is their crystalline structure. One also speaks of low-density amorphous ice (LDA), high-density amorphous ice (HDA), very high-density amorphous ice (VHDA) and hyperquenched glassy water (HGW). 

More recently, simulation studies focused on surface melting [198] and on the molecular-scale growth kinetics and its anisotropy at ice-water interfaces [199-204]. Essmann and Geiger [202] compared the simulated structure of vapor-deposited amorphous ice with neutron scattering data and found that the simulated structure is between the structures of high and low density amorphous ice. Nada and Furukawa [204] observed different growth mechanisms for different surfaces, namely layer-by-layer growth kinetics for the basal face and what the authors call a collected-molecule process for the prismatic system. 

Raman spectra for the sample were conducted in a compression-decompression cycle. In this experiment, the crystalline diffraction began to disappear above 7-8 GPa during compression, and pressure-induced amorphization was indicated by the Raman spectra above 13 GPa (Fig. 14). The resultant HDA Si exhibits the Raman spectrum that differs from the spectrum of normal -Si (LDA Si). Rather, the characteristics of the spectrum for HDA Si resemble those of the (3-tin crystal, which indicates that HDA Si has a (locally) analogous structure to the (3-tin structure. The synthesis of the HDA form of Si by Deb et al. [263] has a strong resemblance to that of water (ice) by Mishima et al. [149, 196]. Whereas compression induced amorphization that was almost completed at 13-15 GPa, decompression induced an HDA-LDA transition below 10 GPa, which is clearly shown in the Raman spectra (Fig. 14). This is the first direct observation of an amorphous-amorphous transition in Si. The spectrum at 0 GPa after the pressure release exhibits the characteristic bands of tetrahedrally coordinated -Si (LDA Si). Based on their experimental findings Deb et al. [263] discussed the possible existence of liquid-liquid transition in Si by invoking a bond-excitation model [258, 259]. They have predicted a first-order transition between high-density liquid (HDL) and low-density liquid... 

Barrier Properties. Vinylidene chloride polymers are more impermeable to a wider variety of gases and liquids than other polymers. This is a consequence of the combination of high density and high crystallinity in the polymer. An increase in either tends to reduce permeability. A more subde factor maybe the symmetry of the polymer structure. It has been shown that both polyisobutylene and PVDC have unusually low permeabilities to water compared to their monosubstituted counterparts, polypropylene and PVC (88). The values listed in Table 8 include estimates for the completely amorphous polymers. The estimated value for highly crystalline PVDC was obtained by extrapolating data for copolymers. 

Therefore, experiments are performed on immobilized liquids , or in other words on amorphous water (also called vitreous water or glassy water). Currently, three structurally distinct amorphous states of water are known low- (LDA) , high- (HDA) and very high- (VHDA) density amorphous ice We emphasize that HDA is not a well defined state but rather comprises a number of substates. It has been suggested to use the nomenclature uHDA ( unrelaxed HDA ) ", eHDA ( expanded HDA ) " and/or rHDA ( relaxed HDA ) to account for this. Even though no signs of micro-crystallinity have been found in neutron or X-ray diffraction studies, it is unclear whether... 

PMMA is the synthetic polymer of methyl methacrylate, which is widely known by a variety of trade names like Lucite, Oroglas, Perspex, Plexiglas, and so on. PMMA is a glassy polymer with an amorphous structure. It has a density of 1.19 g/cm and has very low water absorption. The refractive index ranges from 1.49 to 1.51 depending on the type. PMMA shows high... 

Neutron scattering methods have been used in the past primarily to explore both the structural and dynamic properties of bulk water. One example is a study in which the two phases of the water polymorphism were described, that is, the LDL and the HDL [42]. These experiments were on compressed water in a temperature regime in which the anomalous properties of water are most visible, that is, close to the ice I/ice III triple point (T = 251K, P = 209 MPa). The 00, OH, and HH partial structure factors and the site site radial distribution function between distinct atoms were extracted from the diffraction data. If we assume that the structure of water can be represented as a linear combination of the structures of the end points, that Is, the HDL and LDL structures, we obtain two values for the densities /Ohdl = L20 g cm (0.0402 molecules A ) and pldl = 0.88 g cm (0.0295 molecules/A ). These values are close to the reported densities of high-density and low-density amorphous ice [97]. 

Rust is the electrochemical corrosion product of iron and steel. In the initial periods of exposure, it forms in a highly dispersed state, containing excess of water, with a mainly amorphous structure and is subjected to dehydration and a ciystalli tion mechanism modifying its density. The composition of rust depends on climate and the nature and concentration of pollutants in the atmosphere. 

Exists in two adotropic modifications. Crystalline sihcon is made up of grayish-black lustrous needle-hke crystals or octahedral platelets cubic structure Amorphous sdicon is a brown powder. Other physical properties are density 2.33g/cm3 at 25°C melts at 1,414°C high purity liquid silicon has density 2.533 g/cm at its melting point vaporizes at 3,265°C vapor pressure 0.76 torr at 2,067°C Mohs hardness 6.5. Brinell hardness 250 poor conductor of electricity dielectiric constant 13 critical temperature 4°C calculated critical pressure 530 atm magnetic susceptibility (containing 0.085%Fe) 0.13x10 insoluble in water dissolves in hydrofluoric acid or a mixture of hydrofluoric and nitric acids soluble in molten alkalies. 

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