共查询到20条相似文献，搜索用时 31 毫秒
1.
The mineral ammonioborite is monoclinic with unit cell dimensions (in angstroms): a = 25.27, b = 9.65, and c = 11.56; beta = 94 degrees 17', and the space group is C 2/c. Analysis of the crystal structure revealed the crystallo-chemical formula (NH(4))(3)B(15)O(20)(OH)(8).4H(2)O, with four such formula units in the unit cell. The basic structural unit is the double ring consisting of one BO(4) tetrahedron and four BO(3) triangles: in ammonioborite three of these units are connected to give trimeric ions [B(15)O(20)(OH)(8)](3-). 相似文献
2.
Aizenberg J Weaver JC Thanawala MS Sundar VC Morse DE Fratzl P 《Science (New York, N.Y.)》2005,309(5732):275-278
Structural materials in nature exhibit remarkable designs with building blocks, often hierarchically arranged from the nanometer to the macroscopic length scales. We report on the structural properties of biosilica observed in the hexactinellid sponge Euplectella sp. Consolidated, nanometer-scaled silica spheres are arranged in well-defined microscopic concentric rings glued together by organic matrix to form laminated spicules. The assembly of these spicules into bundles, effected by the laminated silica-based cement, results in the formation of a macroscopic cylindrical square-lattice cagelike structure reinforced by diagonal ridges. The ensuing design overcomes the brittleness of its constituent material, glass, and shows outstanding mechanical rigidity and stability. The mechanical benefits of each of seven identified hierarchical levels and their comparison with common mechanical engineering strategies are discussed. 相似文献
3.
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
4.
喷施钙硼对龙眼叶片和果实矿质营养状况的影响 总被引：3，自引：0，他引：3
果园土壤低硼条件下,于龙眼假种皮发育期间,树冠分别喷施1?C l2、0.2%H3BO3、1?C l2 0.2%H3BO3溶液及清水3次,探讨Ca、B营养对龙眼叶片、果实组织矿质营养状况的影响。结果表明,喷施1?C l2明显提高叶片和果实组织Ca水平,降低叶片、果皮和果肉的B水平,但对叶片N、P、K、Mg的影响不明显;喷施0.2%H3BO3显著提高了叶片和果实组织B水平,促进了叶片和果实对N、P、K、Mg的积累,明显降低了果皮的Ca含量,但对叶片Ca含量无明显影响;喷施1?C l2 0.2%H3BO3明显提高了叶片和果实组织Ca、B水平,促进了叶片和果实组织对N、K、P、Mg的积累,但Ca、B增加的幅度分别低于单独喷施1?C l2、0.2%H3BO3的;此外,在叶片和果皮组织中,Ca与B易产生拮抗作用。 相似文献
5.
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
6.
果园土壤低硼条件下,于龙眼假种皮发育期间,树冠分别喷施1?Cl2、0.2%H3BO3、1?-Cl2 0.2%H3BO3溶液及清水3次,探讨Ca、B营养对龙眼果实品质及采后耐贮性的影响。结果表明,0.2%H3BO3、1?Cl2 0.2%H3BO3处理显著提高了果实组织B含量,对增大龙眼果型、提高单果重、增加果实的可食率和果肉糖含量有明显效应,但0.2%H3BO3处理的效果优于1?Cl2 0.2%H3BO3处理,两处理均降低了贮藏期间果实失重率、提高好果率,减缓了果肉营养成分的变化;1?Cl2处理显著提高了果实组织Ca含量,也提高了龙眼果实的耐贮藏性,但对果实品质的影响不明显。 相似文献
7.
[目的]研究滴灌施硼对香蕉生长及矿质养分吸收的影响,为香蕉生产中高效合理利用硼肥提供理论依据。[方法]在盆栽试验条件下,通过滴灌施肥系统滴施H3BO3,探讨硼对香蕉生长、农艺性状、根系参数和矿质养分吸收的影响。[结果]滴施不同硼处理对香蕉叶片数和叶片宽度的影响差异不显著,对香蕉叶片长度、假茎高度和假茎围长度的影响表现出一定的差异性,但规律性不明显;适宜的硼施用量有利于提高香蕉植株的生物量,增加根系的根长和根表面积;滴灌施硼处理对香蕉叶片、假茎和根系等各部分对氮、磷吸收的影响差异不显著,但能显著增加叶片和假茎中的硼含量。[结论]在滴灌施肥条件下,适宜的硼施用量有利于香蕉生长,增加根系的根长和根表面积,显著增加叶片和假茎中的硼含量。 相似文献
8.
Three crystalline phases of xenon hexafluoride are based on tetrameric association of XeF(5)(+) and F(-) ions into eight-membered rings. Phase I (monoclinic, 8 XeF(6) units per cell) transforms at approximately 10 degrees C to phase II (orthorhombic, 16 XeF(6) units per cell), which in turn transforms at approximately -25 degrees C to phase 111 (monoclinic, 64 XeF(6) units per doubly primitive cell). The transformation from phase I to phase II requires gross reorientation of half of the tetramers in the structure. The transformation from phase II to phase III involves only an ordering of right-handed and left-handed configurations. 相似文献
9.
Pham KN Puertas AM Bergenholtz J Egelhaaf SU Moussaïd A Pusey PN Schofield AB Cates ME Fuchs M Poon WC 《Science (New York, N.Y.)》2002,296(5565):104-106
Experiments, theory, and simulation were used to study glass formation in a simple model system composed of hard spheres with short-range attraction ("sticky hard spheres"). The experiments, using well-characterized colloids, revealed a reentrant glass transition line. Mode-coupling theory calculations and molecular dynamics simulations suggest that the reentrance is due to the existence of two qualitatively different glassy states: one dominated by repulsion (with structural arrest due to caging) and the other by attraction (with structural arrest due to bonding). This picture is consistent with a study of the particle dynamics in the colloid using dynamic light scattering. 相似文献
10.
Otero R Lukas M Kelly RE Xu W Laegsgaard E Stensgaard I Kantorovich LN Besenbacher F 《Science (New York, N.Y.)》2008,319(5861):312-315
Nonsymmetrical organic molecules adsorbed on solid surfaces may assemble into random networks, thereby providing model systems for organic glasses that can be directly observed by scanning tunneling microscopy (STM). We investigated the structure of a disordered cytosine network on a gold(111) surface created by thermal quenching, to temperatures below 150 K, of the two-dimensional fluid present on the surface at room temperature. Comparison of STM images to density functional theory calculations allowed us to identify three elementary structural motifs (zigzag filaments and five- and six-membered rings) that underlie the whole supramolecular random network. The identification of elementary structural motifs may provide a new framework for understanding medium-range order in amorphous and glassy systems. 相似文献
11.
以枳橙砧木为试验材料,设置根施与叶施2种施硼方式,研究不同施硼方式对枳橙砧木的农艺性状、干物质积累量、不同器官各元素(B、K、Ca、Mg)含量以及叶片中糖类、丙二醛(MDA)、脯氨酸(Pro)和叶片伤害率的影响。结果表明:叶施硼显著增加枳橙幼苗茎和叶的干物质质量、总干物质质量和株高,并促进根系的生长;叶片中的B含量提高126.5%,1.3%的B在茎中转移积累,但叶施硼使枳橙根部B含量降低11.1%;叶片中K、Ca、Mg含量均显著高于对照。根施硼各元素(B、K、Ca、Mg)含量均显著高于叶施硼处理(除叶片外);叶片中糖类、MDA、Pro含量和叶片伤害率在施硼后均下降。因此,叶施硼在维持叶片各元素平衡、糖类物质合成和提高叶片抗逆性方面有较好的效果,但叶施硼会降低根系硼含量、加重根系缺硼。 相似文献
12.
Elucidation of the chain conformation in a glassy polyester, PET, by two-dimensional NMR 总被引：1，自引：0，他引：1
The chain conformation of glassy poly(ethylene terephthalate) (PET) was characterized by two-dimensional double-quantum nuclear magnetic resonance (NMR). In amorphous carbon-13-labeled PET, the statistics of the O-13CH2-13CH2-O torsion angle were determined, on the basis of the distinct shapes of the two-dimensional NMR patterns of trans and gauche conformations. In crystalline PET, the trans content is 100 percent, but in the amorphous PET it is only 14 percent (+/-5 percent). An average gauche torsion angle of 70 degrees (+/-9 degrees) was obtained. Implications for materials properties of polyesters are discussed. 相似文献
13.
硒减轻油菜幼苗硼毒害机理的研究 总被引：1，自引：1，他引：0
【目的】硼是植物生长发育所必需的微量元素,但在土壤中过量存在会对植物产生毒害。在干旱半干旱地区硼毒害是限制作物生长的重要因子,此外,硼矿开采以及硼肥用量过大或施用不均匀均会导致硼浓度过高,从而限制作物生长。适量硒能提高作物抗性,减轻作物因逆境产生的伤害,本文旨在通过适量硒处理,研究硒对高硼条件下油菜生物量及油菜活性氧代谢的变化,探讨硒缓解高硼对油菜生长抑制作用的机理,进而提高作物对硼的适宜范围,减轻硼毒害对农业生产的危害。【方法】本文以油菜（湘农油571）为试验材料,设置硼和硒2因素2水平（硼水平为：50、500 μmol•L-1,分别用B50、B500表示,硒水平为：0、1 μmol•L-1,分别用Se0、Se1表示）完全随机试验,共4个处理,通过苗期溶液培养,研究硒对高硼（500 µmol•L-1）胁迫条件下油菜幼苗生长、硼吸收累积和抗氧化酶及非酶系统的影响。【结果】高硼胁迫条件下油菜幼苗地上部和地下部干物质重显著降低,与正常硼（50 μmol•L-1）处理相比分别降低20.1%和32.0%;硼含量和累积量显著增加,与正常硼处理相比分别增加2.95和2.97倍;油菜幼苗因硼毒害导致叶片内抗氧化酶（过氧化氢酶（CAT）、过氧化物酶（POD）和抗坏血酸过氧化物酶（APX））活性和非酶抗氧化物（谷胱甘肽（GSH））含量显著降低,与正常硼处理相比,CAT、POD和APX活性分别降低19.7%、11.0%和15.0%;而过氧化氢（H2O2）和丙二醛（MDA）含量显著增加,比正常硼处理分别增加19.0%和18.5%。高硼条件下,经1 μmol?L-1硒处理,与无硒处理相比,地上部和地下部干物质重分别增加22.8%和28.6%,硼含量和累积量分别降低38.6%和31.9%;油菜幼苗叶片内抗氧化酶活性和非酶抗氧化物含量显著增加,CAT、POD和APX活性分别增加12.8%、15.1%和15.3%,GSH和ASA含量分别增加9.7%和21.0%;而过氧化氢和丙二醛含量分别降低25.1%和21.2%。【结论】适量硒可降低高硼胁迫下油菜幼苗硼的吸收累积,增强抗氧化酶和非酶系统抗氧化作用,降低活性氧累积,减轻细胞质膜因过氧化作用而产生的损伤,从而显著减轻过量硼对油菜生长的影响,提高油菜幼苗对硼过量的适应性。 相似文献
14.
Khalifah P Osborn R Huang Q Zandbergen HW Jin R Liu Y Mandrus D Cava RJ 《Science (New York, N.Y.)》2002,297(5590):2237-2240
We report experimental evidence for a full orbital ordering transition in the two-dimensional lanthanum ruthenate La4Ru2O10. The observable consequences of this orbital ordering include the loss of the Ru local moment, a structural distortion which partitions Ru-O bonds into axially oriented short and long sets, a sharp jump in electrical resistivity, and the opening of a spin gap that is visible in neutron scattering experiments. This is a rare example of a discrete orbital ordering transition in a 4d transition metal oxide and demonstrates that orbital effects can have an influence on the properties of layered ruthenates, a family of compounds that notably includes the p-wave superconductor Sr2RuO4 and the field-tuned quantum critical metamagnet Sr3Ru2O7. 相似文献
15.
差式扫描量热仪(DSC)、核磁共振波谱仪(NMR)及其衍生的核磁共振成像技术(MRI)作为水合物不同尺度研究的重要手段,可用于水合物成核、生长、分解过程热流曲线的获取,结构信息识别及多相态表征,日益受到国内外学者的广泛关注。阐述了3种方法在水合物研究中的应用现状,介绍了DSC恒压变温和恒压恒温宏观实验方法在水合物热力学和动力学方面的研究进展,总结了MRI在水合物相变过程中微观可视化的研究成果,综述了NMR在水合物结构识别、混合气体水合物中客体分子组成、孔穴占有率及水合数等分子尺度信息获取的应用情况。针对3种方法的优劣和特点,提出了将DSC与NMR结合,进行水合物生成和分解过程多尺度系统研究的建议。 相似文献
16.
为探究甜菜幼苗对硼毒害的响应,采用水培试验的方法,研究了不同硼浓度（0.05、0.25、0.50、2.50、5.00 mmol·L-1 H3BO3）条件下,高硼胁迫对甜菜幼苗叶片光合性能、渗透物质调节及活性氧代谢的影响。结果表明：高硼胁迫影响甜菜幼苗生长发育进程,硼处理第15~20 d时对幼苗影响最大。随着硼浓度的增加,叶绿素a、叶绿素b含量呈下降趋势,与对照相比差异显著,净光合速率与叶绿素含量规律表现一致,叶绿素a/b在0.50 mmol·L-1时达到最大值;果糖及淀粉含量在高硼胁迫下增加,在2.50 mmol·L-1时与对照相比均差异显著,分别比对照增加了114.5%、78.2%。蔗糖含量显著下降,与对照相比降低了31.8%~54.1%;当硼浓度达到0.50 mmol·L-1时,丙二醛（MDA）、脯氨酸（Pro）含量显著增加。染色观察发现,随着硼浓度的增加,叶片中超氧阴离子与过氧化氢积累也逐渐增加,均在5.00 mmol·L-1时累积最严重。研究表明,硼毒害使甜菜叶片光合能力下降,阻碍光合产物的运输,细胞内活性氧大量积累,对细胞产生氧化胁迫,进而抑制甜菜植株生长。本试验条件下,硼浓度0.50 mmol·L-1可以作为甜菜幼苗高硼胁迫的临界值,超过该浓度,植株生长受到显著抑制。 相似文献
17.
Hayward MA Cussen EJ Claridge JB Bieringer M Rosseinsky MJ Kiely CJ Blundell SJ Marshall IM Pratt FL 《Science (New York, N.Y.)》2002,295(5561):1882-1884
We present the synthesis and structural characterization of a transition metal oxide hydride, LaSrCoO3H0.7, which adopts an unprecedented structure in which oxide chains are bridged by hydride anions to form a two-dimensional extended network. The metal centers are strongly coupled by their bonding with both oxide and hydride ligands to produce magnetic ordering at temperatures up to at least 350 kelvin. The synthetic route is sufficiently general to allow the prediction of a new class of transition metal--containing electronic and magnetic materials. 相似文献
18.
Swallen SF Kearns KL Mapes MK Kim YS McMahon RJ Ediger MD Wu T Yu L Satija S 《Science (New York, N.Y.)》2007,315(5810):353-356
Vapor deposition has been used to create glassy materials with extraordinary thermodynamic and kinetic stability and high density. For glasses prepared from indomethacin or 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene, stability is optimized when deposition occurs on substrates at a temperature of 50 K below the conventional glass transition temperature. We attribute the substantial improvement in thermodynamic and kinetic properties to enhanced mobility within a few nanometers of the glass surface during deposition. This technique provides an efficient means of producing glassy materials that are low on the energy landscape and could affect technologies such as amorphous pharmaceuticals. 相似文献
19.
P Yang T Deng D Zhao P Feng D Pine BF Chmelka GM Whitesides GD Stucky 《Science (New York, N.Y.)》1998,282(5397):2244-2246
Porous silica, niobia, and titania with three-dimensional structures patterned over multiple length scales were prepared by combining micromolding, polystyrene sphere templating, and cooperative assembly of inorganic sol-gel species with amphiphilic triblock copolymers. The resulting materials show hierarchical ordering over several discrete and tunable length scales ranging from 10 nanometers to several micrometers. The respective ordered structures can be independently modified by choosing different mold patterns, latex spheres, and block copolymers. The examples presented demonstrate the compositional and structural diversities that are possible with this simple approach. 相似文献
20.
Raman microprobe spectra from the secondary wall of earlywood tissue from Picea mariana (black spruce) reveal evidence of the orientation of lignin relative to the plane of the cell wall. In most instances, the aromatic rings of the phenyl propane structural units are parallel to the plane of the cell-wall surface. 相似文献