共查询到20条相似文献,搜索用时 703 毫秒
1.
J. M. Dereppe B. F. Borisov E. V. Charnaya A. V. Shelyapin M. M. Nassar Yu. A. Kumzerov 《Physics of the Solid State》2000,42(1):193-196
An acoustic study is reported of the crystallization and melting of gallium embedded in an opal-like matrix. The variations of the velocity and absorption of longitudinal ultrasonic waves during phase transitions in the α and β modifications have been found to be hysteretic in nature. It is shown that acoustic methods do not detect gallium melting and crystallization in the tetragonal phase forming in a restricted geometry. Experimental evidence for heterogeneous crystallization of gallium in pores has been obtained. 相似文献
2.
A.L. Pirozerski O.I. Smirnova A.I. Nedbai O.L. Pirozerskaya N.A. Grunina V.M. Mikushev 《Physics letters. A》2019,383(30):125872
Melting and crystallization of n-decane embedded into porous glass with the mean pore size of about 6.4 nm were studied using acoustic and DSC methods. Smearing of the phase transitions, decrease of melting and freezing temperatures, pronounced hysteresis between melting and crystallization were revealed by both methods. In DSC measurements for the pore filling factors 70% and higher double peaks were observed upon cooling while only single peaks were present upon heating. Also a high reduction of the corresponding phase transition heats was revealed. Melting and freezing intervals determined by acoustic and DSC methods strongly differed from each other. A model which qualitatively explains the observed anomalies is proposed. It supposes the formation of liquid layers on the surface of the pores. 相似文献
3.
A. L. Pirozerskii E. V. Charnaya E. N. Latysheva A. I. Nedbai Yu. A. Kumzerov A. S. Bugaev 《Acoustical Physics》2011,57(5):637-641
Temperature dependences of the velocity of the longitudinal ultrasonic waves in a nanocomposite on the basis of porous glass
filled with gallium-indium alloy have been measured. Acoustic anomalies due to crystallization and melting of the alloy in
nanopores have been revealed for the complete and partial cooling-heating cycles. A two-step temperature hysteresis loop between
the curves of the velocity change upon cooling and heating has been found, the existence of which is related to the formation
in pores of two types of mixtures, with α- and β-Ga. Stabilization of β-Ga in nanopores has been observed. It was shown that
the conditions of confined geometry lead to a shift to low temperature of the melting regions for both mixtures in comparison
with the regions of the coexistence of the liquid and solid phases in bulk alloy. 相似文献
4.
5.
C. Faivre D. Bellet G. Dolino 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(1):19-36
The phase transitions of non-polar organic fluids and of water, confined in the pores of porous silicon samples, were investigated
by Differential Scanning Calorimetry (DSC). Two types of PS samples (p- and p+ type) with different pore size and morphology were used (with spherical pores with a radius of about 1.5 nm and cylindrical
shape with a radius of about 4 nm respectively). The DSC results clearly show that the smaller the pores are, the larger is
the decrease in the transition temperature. Moreover, a larger hysteresis between melting and freezing is observed for p+ type than for p- type samples. A critical review of the thermodynamical properties of small particles and confined fluids is presented and
used to interpret and discuss our DSC results. The effects of the chemical dissolution as well as the influence of anodization
time are presented, showing that thick p+ type porous silicon layers are non-homogeneous. The DSC technique which was used for the first time to investigate fluids
confined in porous silicon, enables us to deduce original information, such as the pore size distribution, the decrease in
the freezing temperature of confined water, and the thickness of non-freezing liquid layer at the pore wall surface.
Received: 11 May 1998 / Revised and Accepted: 29 July 1998 相似文献
6.
Xiao Meng Chen Xing Feng Li Kang Zheng Li De Zhang 《Journal of Physics and Chemistry of Solids》2010,71(6):918-7995
In this paper, submicrometer-sized Ga-In eutectic alloy particles were dispersed into polymethylmethacrylate (PMMA) matrix by ultrasonic vibration and sedimentation method. The solidification and melting processes of Ga-In eutectic alloy particles were studied by differential scanning calorimeter (DSC). Four endothermal peaks with the onset temperature located at 16, −11, −22, and −27 °C were observed in DSC heating curves, which corresponded to the melting process of the stable Ga-In phase α-Ga(In) and three metastable phases of β-Ga(In), δ-Ga(In) and γ-Ga(In), respectively. The stable phase α-Ga(In) can only be formed when the size of alloy particle was larger than 0.58 μm. Conversely, metastable phases β-Ga(In), δ-Ga(In) and γ-Ga(In) are mainly formed. The result shows that phase structures in Ga-In eutectic alloy are size dependent. 相似文献
7.
S.?V.?Baryshnikov B.?F.?Borisov A.?V.?Gartvik A.?G.?Gorchakov E.?V.?Charnaya W.?B?hlmann D.?Michel 《Acoustical Physics》2009,55(1):55-60
A conventional phase-pulse acoustic method was used to study melting and crystallization of sodium nitrite embedded in the pores of mesoporous silicate matrices. The pore diameter was 20, 37, and 52 Å. The measurements were performed at a frequency of 3–8 MHz in the temperature interval 290–560 K. The temperature dependence of ultrasonic velocity was found to exhibit anomalies corresponding to phase transitions of sodium nitrite. The transitions were smeared in temperature and shifted to lower temperatures from the melting point T b of bulk sodium nitrite; the shift in crystallization temperature was greater than that of the melting temperature. The irreversible character of melting was revealed. The size dependence of the melting temperature of sodium nitrite was obtained. Phenomena observed in the experiments were discussed with the use of different size effect models. 相似文献
8.
9.
A. L. Pirozerskii E. V. Charnaya M. K. Lee L. J. Chang A. I. Nedbai Yu. A. Kumzerov A. V. Fokin M. I. Samoilovich E. L. Lebedeva A. S. Bugaev 《Acoustical Physics》2016,62(3):306-312
The paper presents the results of studying the crystallization and melting processes of Ga–In eutectic alloys, which are embedded in opal matrices, using acoustic and NMR methods. The indium concentrations in the alloys were 4, 6, 9, and 15 at %. Measurements were performed upon cooling from room temperature to complete crystallization of the alloys and subsequent heating. It is revealed how the size effects and alloy composition influence the formation of phases with α- and β-Ga structures and on changes in the melting-temperature ranges. A difference was observed between the results obtained using acoustic and NMR methods, which was attributed to different temperature measurement conditions. 相似文献
10.
β-Ga2O3是一种宽带隙半导体材料,能带宽度Eg≈5.0eV,在光学和光电子学领域有广泛的应用.用射频磁控溅射方法在Si衬底和远紫外光学石英玻璃衬底制备了本征β-Ga2O3薄膜及Zn掺杂β-Ga2O3薄膜,用紫外-可见分光光度计、X射线衍射仪、荧光分光光度计对本征β-Ga2O3薄膜及Zn掺杂β-Ga2O3薄膜的光学透过、光学吸收、结构和光致发光进行了测量,研究了Zn掺杂和热退火对薄膜结构和光学性质的影响.退火后的β-Ga2O3薄膜为多晶结构,与本征β-Ga2O3薄膜相比,Zn掺杂β-Ga2O3薄膜的β-Ga2O3(111)衍射峰强度变小,结晶性变差,衍射峰位从35.69°减小至35.66°.退火后的Zn掺杂β-Ga2O3薄膜的光学带隙变窄,光学透过降低,光学吸收增强,出现了近边吸收,薄膜的紫外、蓝光及绿光发射增强.表明退火后Zn掺杂β-Ga2O3薄膜中的Zn原子被激活充当受主. 相似文献
11.
R. Berwanger Ch. Schumacher P. Huber R. Pelster 《The European physical journal. Special topics》2010,189(1):239-249
We present a study on the phase behavior of several linear n-alcohols (heptanol, nonanol and undecanol) in their bulk state
as well as confined in mesoporous silicon. We were able to vary the mean pore radii of the nanochannels from r = 3.5 nm to 7 nm and to determine the respective temperatures of the freezing and melting transitions using infrared and
dielectric spectroscopy. The smaller the chain length the lower the freezing point, both in the bulk and in the confined state.
Under confinement the freezing temperature decreases by up to 28 K compared to the bulk value. In accordance with the Gibbs-Thompson
model the lowering is proportional to the inverse pore radius, ΔT
fr
∝ 1/r. Moreover, the ratio of freezing temperature depression to melting temperature depression is close to the theoretical value
of ΔT
fr
/ΔT
melt
= 3/2. The spectra also indicate a structural change: while the solid bulk alcohols are a polycrystalline mixture of the
orthorhombic β- and monoclinic γ-form, geometrical confinement forces the alcohol-chains into the more simple orthorhombic
structure. In addition, a part of the material does not crystallize. Such an additional amorphous phase seems to be a logical
consequence of the size mismatch between molecular crystals and irregular shaped pores. 相似文献
12.
The melting of ice in porous glass having different distribution of pores sizes is analyzed in details. One shows that confined water crystallizes only partially and that an interface layer, between the ice crystallites and the surface of the pore, remains liquid. Properties of this non crystalline interface at low temperature is studied by NMR and DSC. Both methods lead to an interface thickness h of the order of 0.5 nm, this explains why water do not crystallize when the dimension of confinement is less than a critical length
nm. The variation of the melting enthalpy per gram of total amount of water with the confinement length is explained taking into account two effects: a) the presence of this layer of water at the interface and b) the linear variation of the melting enthalpy
with the melting temperature Tm. From the data of the literature one draws the same conclusions concerning other solvents in similar porous materials. Also one points out the important role of the glass temperature Tg in preventing the crystallization of the liquids confined in small pores and/or between the crystallites and the surface of the pores.Received: 20 September 2003, Published online: 30 January 2004PACS:
64.70.Dv Solid-liquid transitions - 64.70.Pf Glass transitions - 81.05.Rm Porous materials; granular materials 相似文献
13.
S. P. Zhvavyi 《Technical Physics》2000,45(8):1014-1018
Numerical simulation of the melting and crystallization processes of monocrystalline silicon exposed to the nanosecond radiation of a ruby laser was carried out with the kinetics of the phase transformations accounted for on the basis of Kolmogorov equations. A two-dimensional mechanism of nucleation and growth of the new phase was invoked to describe the phase transitions. It was shown that the temporal dependences of monocrystal overheating and liquid phase supercooling in the melting and crystallization stages, respectively, are nonmonotonic and determined by the kinetics of the phase transitions. The maximum values of the overheating and supercooling were ∼100 K. 相似文献
14.
The temperature dependences of the integrated intensity and of the Knight shift of 199Hg NMR signals are measured for liquid and solid mercury introduced into porous carbon and silica gel. A decrease in the temperature of completion of crystallization and a small temperature hysteresis (from 4 to 9 K) between melting and crystallization are observed. The melting temperature of mercury in pores coincides with that in the bulk. The 199Hg NMR signal from crystalline mercury under the condition of restricted geometry is observed for the first time. It is established that the Knight shift for liquid and crystalline mercury in pores is smaller than in the bulk. 相似文献
15.
Hoyt JJ 《Physical review letters》2006,96(4):045702
A thermodynamic treatment of the freezing of fluids confined to nanosized closed pores is presented. The model includes the effects of pressure in the liquid, the volume change on solidification, and the strain energy in both the solidifying material and the wall material. When applied to the system of Pb droplets in Al, the model predicts an elevation of the melting point, in agreement with experiment. 相似文献
16.
ABSTRACTSolid, liquid and alloyed phases of gallium play a role in a variety of important technological applications. While many of the gallium phases involved in these applications are metallic, some have been proposed or are known to contain covalently bound Ga dimers. Thus, understanding the nature of bonding in Ga is crucial to the development of Ga-based materials. The solid phase of gallium at ambient conditions, α-Ga, is metallic and composed of molecular dimers, and can serve as a testing ground for studying gallium bonding with electronic structure calculations. We use density functional theory-based molecular dynamics simulations in conjunction with maximally localised Wannier functions to examine the nature of chemical bonding in α-Ga. We propose a geometric criterion for defining various bonding environments, which enables the quantification of covalent and weak bonds in solid gallium. We additionally connect the bonding structure of α-Ga to its phonon density of states and discuss similarities and differences with diatomic halogen crystals. 相似文献
17.
《Journal of Macromolecular Science: Physics》2013,52(3-4):875-888
Amorphous poly(l-lactide) (PLLA) was annealed in two different ways: amorphous samples were heated at a given temperature to induce crystallization (one-step annealing); and amorphous samples were first crystallized at a low temperature and subsequently annealed at a higher temperature than the crystallization temperature. Samples thus prepared were measured by DSC. The original amorphous sample exhibited an exothermic peak at about 100°C (exothermic peak I), an exothermic peak just below the melting point (exothermic peak II), and an endothermic peak when it was melted. Exothermic peak I was caused by cold crystallization. When the melting points of PLLA samples, heat-treated in various ways, were plotted as a function of annealing temperature, there was discontinuity at about 120°C. From analyses of wide-angle X-ray diffraction patterns, it was found that when amorphous PLLA was crystallized at a temperature below 120°C, crystallites of the β-form formed, and when annealed at a temperature above 120°C, crystallites of the α-form grew. Thus, exothermic peak I was attributed to cold crystallization of the β-form, and peak II was caused by the phase transition of the β-form to a more stable form. 相似文献
18.
Zh. M. Wang Yu. I. Mazur K. A. Sablon T. D. Mishima M. B. Johnson G. J. Salamo 《固体物理学:研究快报》2008,2(6):281-283
The structural complexity of GaAs quantum‐dot pairs has been revealed by cross‐sectional transmission electron microscopy. As a result of high‐temperature droplet epitaxy, the AlGaAs substrate beneath the quantum‐dot pairs is no longer immobile and its reconstruction is observed to define the crystallization of gallium droplets under an arsenic flux. The GaAs quantum‐dot pairs are immersed into the substrate and further confined by the re‐distributed AlGaAs materials above the substrate plane. There are two underlying mechanisms responsible for the final nanostructure configuration, melt‐back etching by the gallium droplets and preferential crystallization of gallium around reconstructed sidewalls. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
19.
The thermodynamic and kinetic behaviors of gold nanoparticles confined between two-layer graphene nanosheets (two-layer-GNSs) are examined and investigated during heating and cooling processes via molecular dynamics (MD) simulation technique. An EAM potential is applied to represent the gold–gold interactions while a Lennard–Jones (L–J) potential is used to describe the gold–GNS interactions. The MD melting temperature of 1345 K for bulk gold is close to the experimental value (1337 K), confirming that the EAM potential used to describe gold–gold interactions is reliable. On the other hand, the melting temperatures of gold clusters supported on graphite bilayer are corrected to the corresponding experimental values by adjusting the εAu–C value. Therefore, the subsequent results from current work are reliable. The gold nanoparticles confined within two-layer GNSs exhibit face center cubic structures, which is similar to those of free gold clusters and bulk gold. The melting points, heats of fusion, and heat capacities of the confined gold nanoparticles are predicted based on the plots of total energies against temperature. The density distribution perpendicular to GNS suggests that the freezing of confined gold nanoparticles starts from outermost layers. The confined gold clusters exhibit layering phenomenon even in liquid state. The transition of order–disorder in each layer is an essential characteristic in structure for the freezing phase transition of the confined gold clusters. Additionally, some vital kinetic data are obtained in terms of classical nucleation theory. 相似文献
20.
A novel multicollision induced dissociation scheme is employed to determine the energy content for mass-selected gallium cluster ions as a function of their temperature. Measurements were performed for Ga(+)(n) (n=17 39, and 40) over a 90-720 K temperature range. For Ga+39 and Ga+40 a broad maximum in the heat capacity-a signature of a melting transition for a small cluster-occurs at around 550 K. Thus small gallium clusters melt at substantially above the 302.9 K melting point of bulk gallium, in conflict with expectations that they will remain liquid to below 150 K. No melting transition is observed for Ga+17. 相似文献