共查询到20条相似文献，搜索用时 31 毫秒
1.
Youngman RE Haubrich ST Zwanziger JW Janicke MT Chmelka BF 《Science (New York, N.Y.)》1995,269(5229):1416-1420
Ordering at short-length scales is a universal feature of the glassy state. Experiments on boron oxide and other materials indicate that ordering on mesoscopic-length scales may also be universal. The high-resolution nuclear magnetic resonance (NMR) measurements of oxygen in boron oxide glass presented here provide evidence for structural units responsible for ordering on short- and intermediate-length scales. At the molecular level, planar BO(3/2) units accounted for the local ordering. Oxygen-17 NMR spectra resolved detailed features of the inclusion of these units in boroxol rings, oxygen bridging two rings, and oxygen shared between two nonring BO(3/2) units. On the basis of these and corroborative boron-11 NMR and scattering results, boron oxide glass consists of domains that are rich or poor in boroxol rings; these domains are proposed to be the structural basis of intermediate-range order in glassy boron oxide. 相似文献
2.
落叶松、杨木热处理材及压缩材热动态力学特性分析 总被引：1，自引：0，他引：1
对落叶松、杨木未压缩材和压缩材进行高温热处理,利用DMA-242型热分析仪对处理后的压缩材和未压缩材进行热动态力学分析,并与其各自素材进行对比。通过分析模量、损耗角正切等热动态力学参数的变化规律,及热处理时间对压缩材和未压缩材中半纤维素、木质素等木材组成物质的影响,探讨其变化趋势和在变形固定中的内在作用机理。结果表明,在玻璃态区域损耗模量和损耗角正切随热处理时间延长而呈递减的趋势(尺寸稳定性和压缩变形固定效果增强),压缩材中热处理24h者玻璃化转变起始温度大幅降低,即木质素大量低分子化木材蠕变恢复丧失而形成压缩材变形固定。 相似文献
3.
Viscoelasticity describes the ability of a material to possess both elasticity and viscosity. Viscoelastic materials, such as rubbers, possess a limited operational temperature range (for example, for silicone rubber it is -55° to 300°C), above which the material breaks down and below which the material undergoes a glass transition and hardens. We created a viscoelastic material composed from a random network of long interconnected carbon nanotubes that exhibited an operational temperature range from -196° to 1000°C. The storage and loss moduli, frequency stability, reversible deformation level, and fatigue resistance were invariant from -140° to 600°C. We interpret that the thermal stability stems from energy dissipation through the zipping and unzipping of carbon nanotubes at contacts. 相似文献
4.
Isolated single crystals of polyethylene have been crystallized from the glassy state by annealing uniformly thick films of amorphous linear polyethylene just above their glass transition temperature. In agreement with previous results for polycarbonate, substantial molecular mobility at the glass transition temperature is. 相似文献
5.
When a liquid is cooled below its melting temperature, it usually crystallizes. However, if the quenching rate is fast enough, the system may remain in a disordered state, progressively losing its fluidity upon further cooling. When the time needed for the rearrangement of the local atomic structure reaches approximately 100 seconds, the system becomes "solid" for any practical purpose, and this defines the glass transition temperature Tg. Approaching this transition from the liquid side, different systems show qualitatively different temperature dependencies of the viscosity, and accordingly they have been classified by introducing the concept of "fragility." We report experimental observations that relate the microscopic properties of the glassy phase to the fragility. We find that the vibrational properties of the glass well below Tg are correlated with the fragility value. Consequently, we extend the fragility concept to the glassy state and indicate how to determine the fragility uniquely from glass properties well below Tg. 相似文献
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7.
该研究选取了北京山地人工油松林、人工白皮松林、天然油松林和人工落叶松林4种针叶林型下0～0.2 m和0.2～0.4 m土层的土壤,通过测定不同的底物浓度、培养时间及培养温度下的放氨量,计算出脲酶的各项动力学和热力学参数,探讨了北京山地4种针叶林型下土壤脲酶酶促反应特征。结果表明,4种林型下土壤酶促反应的Vmax值随土层深度的增加而减小,随温度的升高而增大;Km值能在一定程度上反映土壤脲酶酶促反应的变化规律;土壤酶促反应K值和Ea值均表现出随温度和土壤深度变化而变化的规律,但变化特点因林型而异;酶促反应ΔH和ΔG值随温度和土壤深度的变化而呈规律性变化;ΔS为负值,说明土壤脲酶酶促反应体系中有序性增大。 相似文献
8.
Kohara S Suzuya K Takeuchi K Loong CK Grimsditch M Weber JK Tangeman JA Key TS 《Science (New York, N.Y.)》2004,303(5664):1649-1652
Inorganic glasses normally exhibit a network of interconnected, covalent-bonded, structural elements that has no long-range order. In silicate glasses, the network formers are based on SiO4 tetrahedra interconnected through oxygen atoms at the corners. Conventional wisdom implies that alkaline and alkaline-earth orthosilicate materials cannot be vitrified, because they do not contain sufficient network-forming SiO2 to establish the needed interconnectivity. We studied a bulk magnesium orthosilicate glass obtained by containerless melting and cooling. We found that the role of network former was largely taken on by corner and edge sharing of highly distorted, ionic Mg-O species that adopt 4-, 5-, and 6-coordination with oxygen. The results suggest that similar glassy phases may be found in the containerless environment of interstellar space. 相似文献
9.
The magnitude and temperature dependence of most of the properties of amorphous solids are anomalous at very low temperatures ( less, similar1 Kelvin). Phonon-assisted tunneling of a distribution of glassy bistable configurations, or two-level systems, can account for these anomalies. A unified understanding of the low-temperature properties is required for an understanding of the glassy state. Persistent nonphotochemical hole burning of impurity optical transitions allows a glass state to be produced that is thermally inaccessible to the preburn state, and that allows the probing of tunneling dynamics on time scales that range between picoseconds and days. These data combined with recently obtained distribution functions for the two-level systems offer new insights into the tunneling dynamics. 相似文献
10.
Swisher CC Grajales-Nishimura JM Montanari A Margolis SV Claeys P Alvarez W Renne P Cedillo-Pardoa E Maurrasse FJ Curtis GH Smit J McWilliams MO 《Science (New York, N.Y.)》1992,257(5072):954-958
(40)Ar/(39)Ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180-kilometer subsurface Chicxulub crater in Yucatán, Mexico, has yielded well-behaved incremental heating spectra with a mean plateau age of 64.98 +/- 0.05 million years ago (Ma). The glassy melt rock of andesitic composition was obtained from core 9 (1390 to 1393 meters) in the Chicxulub 1 well. The age of the melt rock is virtually indistinguishable from (40)Ar/(39)Ar ages obtained on tektite glass from Beloc, Haiti, and Arroyo el Mimbral, northeastern Mexico, of 65.01 +/- 0.08 Ma (mean plateau age for Beloc) and 65.07 +/- 0.10 Ma (mean total fusion age for both sites). The (40)Ar/(39)Ar ages, in conjunction with geochemical and petrological similarities, strengthen the recent suggestion that the Chicxulub structure is the source for the Haitian and Mexican tektites and is a viable candidate for the Cretaceous-Tertiary boundary impact site. 相似文献
11.
A specially designed solid-state deuterium nuclear magnetic resonance probe was used to examine the effect of uniaxial elongation on the chain mobility in the amorphous region of semicrystalline nylon 6. In measurements conducted near the glass transition temperature, there was measurable deformation-induced enhancement of the mobility of the amorphous chains up to the yield point. This enhanced mobility decayed once deformation was stopped. Enhanced mobility was not observed in deformation at room temperature. The mechanics of deformation can be explained by the Robertson model for glassy polymers near the glass transition temperature, which states that applied stress induces liquid-like behavior in the polymer chains. 相似文献
12.
The structure of a silicate liquid changes with temperature, and this substantially affects its thermodynamic and transport properties. Models used by geochemists, geophysicists, and glass scientists need to include such effects. In situ, high-temperature nuclear magnetic resonance (NMR) spectroscopy on (23)Na, (27)A1, and (29)Si was used to help determine the time-averaged structure of a series of alkali aluminosilicate liquids at temperatures to 1320 degrees C. Isotropic chemical shifts for (29)Si increase (to higher frequencies) with increasing temperature, probably in response to intermediate-range structural changes such as the expansion of bonds between nonbridging oxygens and alkali cations. In contrast, isotropic chemical shifts for (27)Al decrease with increasing temperature, indicating that more significant short-range structural changes take place for aluminum, such as an increase in mean coordination number. The spectrum of a sodium aluminosilicate glass clearly indicates that at least a few percent of six-coordinated aluminum was present in the liquid at high temperature. 相似文献
13.
Schreiner PR Reisenauer HP Ley D Gerbig D Wu CH Allen WD 《Science (New York, N.Y.)》2011,332(6035):1300-1303
Chemical reactivity is conventionally understood in broad terms of kinetic versus thermodynamic control, wherein the decisive factor is the lowest activation barrier among the various reaction paths or the lowest free energy of the final products, respectively. We demonstrate that quantum-mechanical tunneling can supersede traditional kinetic control and direct a reaction exclusively to a product whose reaction path has a higher barrier. Specifically, we prepared methylhydroxycarbene (H(3)C-C-OH) via vacuum pyrolysis of pyruvic acid at about 1200 kelvin (K), followed by argon matrix trapping at 11 K. The previously elusive carbene, characterized by ultraviolet and infrared spectroscopy as well as exacting quantum-mechanical computations, undergoes a facile [1,2]hydrogen shift to acetaldehyde via tunneling under a barrier of 28.0 kilocalories per mole (kcal mol(-1)), with a half-life of around 1 hour. The analogous isomerization to vinyl alcohol has a substantially lower barrier of 22.6 kcal mol(-1) but is precluded at low temperature by the greater width of the potential energy profile for tunneling. 相似文献
14.
[目的]研究热岛效应造成的城市边界层热力稳定度变化特征及其对降水的影响。[方法]从热流量方程出发,探讨了因城市加热扰动、热岛效应平均态所引起的城市边界层热力稳定度的变化特征,分析了城市边界层热力稳定度变化对降水的影响,并结合西安地区降水个例进行了验证。[结果]在扰动温度与扰动气流相互作用下,边界层热力扰动稳定度(θ′/z)有利于城区及城市上下游边界层的层结趋于稳定,但中心城区的稳定性弱于郊区,有利于短历时对流降水天气的发生与发展。平均气流(盛行风)、平均水平扰动温度相互作用对热力不稳定分布的影响:当边界层环境气流为上升运动时,边界层平均热力扰动稳定度(θ′/z)表现为上游不稳定、下游稳定的特征,强降水易发生在上游或城市上风方;当边界层环境气流为弱下沉运动时,下游不稳定趋于发展、上游趋于稳定,局地强降水易发生在下游或城市下风方。[结论]该研究在制作城市精细化预报中有一定的参考价值。 相似文献
15.
利用紫外吸收光谱法研究了七元瓜环与三环唑在水溶液中的分子识别作用，考察了温度对该作用体系的影响；并探讨了瓜环对三环唑抑制稻瘟病菌抑菌活性的影响。结果表明：七元瓜环与三环唑可形成1：1配合物；其稳定常数（K）为（9．23±0．05）×10^3L／mol，热力学参数为△G（-2．26×10^4J／mo1），△H0（-43．79J／mol），△S0（-72．78J／K·mol）。并且七元瓜环与三环唑形成的包结配合物对三环唑抑制稻瘟病菌活性具有增效作用，抑菌率提高16％。 相似文献
16.
Knowing the thermodynamic stability of transition metal oxide nanoparticles is important for understanding and controlling their role in a variety of industrial and environmental systems. Using calorimetric data on surface energies for cobalt, iron, manganese, and nickel oxide systems, we show that surface energy strongly influences their redox equilibria and phase stability. Spinels (M(3)O(4)) commonly have lower surface energies than metals (M), rocksalt oxides (MO), and trivalent oxides (M(2)O(3)) of the same metal; thus, the contraction of the stability field of the divalent oxide and expansion of the spinel field appear to be general phenomena. Using tabulated thermodynamic data for bulk phases to calculate redox phase equilibria at the nanoscale can lead to errors of several orders of magnitude in oxygen fugacity and of 100 to 200 kelvin in temperature. 相似文献
17.
Computer-aided molecular design 总被引：6，自引：0，他引：6
J A McCammon 《Science (New York, N.Y.)》1987,238(4826):486-491
Theoretical chemistry, as implemented on fast computers, is beginning to yield accurate predictions of the thermodynamic and kinetic properties of large molecular assemblies. In addition to providing detailed insights into the origins of molecular activity, theoretical calculations can be used to design new molecules with specific properties. This article describes two types of calculations that show special promise as design tools, the thermodynamic cycle-perturbation method and the Brownian reactive dynamics method. These methods can be applied to calculate equilibrium and rate constants that describe many aspects of molecular recognition, stability, and reactivity. 相似文献
18.
利用各种大气探测资料,采用动力学和热力学原理,从环流背景、主要影响系统、物理量等方面对2008年7月23～28日广东省揭阳市一次高温过程进行分析。结果表明,广东省夏季出现的高温天气多是由副高加强西伸造成的,此次高温并不仅仅是因为副高主体控制,揭阳市高温过程中台风外围下沉增温的影响也起主要作用;前期受大陆副高的控制,吹弱的西南风,有下沉运动,后期受副高和热带气旋外围偏北下沉气流的共同影响,也是下沉运动。日照时数维持在较大的数值而又没有明显的降水,也是导致此次高温形成的原因。高温结束于台风“凤凰”的外围环流和西南季风作用的降水。 相似文献
19.
以精刨竹碎料和酚醛树脂为原料,采用高温真空炭化烧结工艺制备了竹材陶瓷,研究了烧结工艺对竹材陶瓷性能、物相组成和微观结构等的影响。结果表明:①烧结温度对竹材陶瓷的尺寸收缩率有较大影响,当烧结温度为600~1 200 ℃时,竹材陶瓷的尺寸收缩率大于20%范围内;②竹材陶瓷的密度减少率在600~1 000 ℃范围内,随着烧结温度的升高逐渐降低,在1 000~1 200 ℃范围内,随着烧结温度的升高逐渐增加;③随着烧结温度的升高,竹材陶瓷的炭得率降低、静曲强度和弹性模量升高、石墨化程度逐渐增强;④酚醛树脂经高温炭化后形成的硬质玻璃碳对竹材细胞起填充、强化作用;⑤竹材陶瓷的较佳烧结温度范围为800~1 000 ℃。图7参15 相似文献
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