首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   530篇
  免费   67篇
  国内免费   198篇
测绘学   5篇
大气科学   65篇
地球物理   65篇
地质学   519篇
海洋学   89篇
天文学   11篇
综合类   23篇
自然地理   18篇
  2024年   1篇
  2023年   4篇
  2022年   14篇
  2021年   11篇
  2020年   15篇
  2019年   22篇
  2018年   12篇
  2017年   8篇
  2016年   15篇
  2015年   12篇
  2014年   27篇
  2013年   29篇
  2012年   24篇
  2011年   24篇
  2010年   25篇
  2009年   44篇
  2008年   39篇
  2007年   40篇
  2006年   40篇
  2005年   36篇
  2004年   24篇
  2003年   24篇
  2002年   31篇
  2001年   28篇
  2000年   21篇
  1999年   36篇
  1998年   21篇
  1997年   23篇
  1996年   34篇
  1995年   23篇
  1994年   21篇
  1993年   13篇
  1992年   12篇
  1991年   9篇
  1990年   9篇
  1989年   8篇
  1988年   5篇
  1987年   1篇
  1986年   5篇
  1985年   1篇
  1984年   2篇
  1983年   1篇
  1981年   1篇
排序方式: 共有795条查询结果,搜索用时 171 毫秒
541.
张春牛  郑云法 《岩矿测试》2006,25(3):249-251
报道了1-(4-硝基苯基)-3-(3-硝基-5-氯吡啶)三氮烯(NPNCPDT)的合成及其与锌的显色反应研究。在Triton X-100存在下,pH 10.5的Na2B4O7-NaOH缓冲溶液中,该试剂能与锌发生显色反应,锌与NPNCPDT形成摩尔比为1∶2的黄色配合物,在450 nm处有一最大正吸收,540 nm处有一最大负吸收。以450 nm为参比波长、540 nm为测量波长进行双波长测定,表观摩尔吸光系数为2.50×105L.mol-1.cm-1。锌量在0~480μg/L符合比尔定律。用拟定方法测定人发中的微量锌,方法精密度小于2.5%(RSD,n=6)。  相似文献   
542.
地球化学模式在确定砂岩型铀矿矿化地段研究中的应用   总被引:1,自引:0,他引:1  
刘金辉 《矿物岩石》2004,24(1):65-70
在分析和讨论与成矿有关的水文地球化学参数和新疆准噶尔盆地北部地质背景的基础上,运用地球化学模式通过计算饱和指数和反应条件指数将第三系含矿层水化学成分与其所处的水文地球化学环境结合起来,系统探讨了饱和指数和反应条件指数与第三系砂岩水Eh值之间的关系,初步划分了氧化带、过渡带和还原带的位置,并对受盐池、黄花沟排泄源控制的铀矿化地段进行了初步预测。研究结果表明铀矿化地段位于北;隹顶山地区0号线Z14-Z16之间距Z14孔0.9km至4.6km处的3.7km范围内。  相似文献   
543.
The Reaction of Unsaturated Aliphatic Oxygenates with Ozone   总被引:1,自引:0,他引:1  
The reaction of ozone with unsaturated aliphatic oxygenates has been studied at ambient T (287–297 K) and p = 1 atm. of air (RH = 55 ± 10%) with sufficient cyclohexane added to scavenge the hydroxyl radical. Reaction rate constants, in units of 10-18 cm3 molecule-1 s-1, are 10.7 ± 1.4 for methyl trans-3-methoxy acrylate, 63.7 ± 9.9 for 4-hexen-3-one (predominantly the trans isomer), 125 ± 17 for trans-4-methoxy-3-buten-2-one, 148 ± 13 for cis-4-heptenal, 439 ± 37 for 3- methyl-2-buten-1-ol and 585 ± 132 for (cis + trans)-ethyl 1-propenyl ether. The influence of the oxygen-containing substituents on reactivity toward ozone is examined. Unsaturated ethers react with ozone faster than their alkene structural homologues; the reverse is observed for unsaturated esters and unsaturated carbonyls. Major reaction products have been identified by liquid chromatography with ultraviolet detection (LC-UV), particle beam-mass spectrometry (PB- MS) and gas chromatography-mass spectrometry (GC-MS) and are methyl formate and methyl glyoxylate from methyl trans-3-methoxy acrylate, acetaldehyde and 2-oxobutanal from 4-hexen-3-one, propanal and succinic dialdehyde from cis-4-heptenal, hydroxyacetaldehyde and acetone from 3-methyl-2-buten-1-ol, and ethyl formate and acetaldehyde from (cis + trans)-ethyl 1-propenyl ether. PB-MS and GC- MS were also employed to identify new reaction products and to confirm the structure of products tentatively identified in a previous study of the reaction of ozone with five unsaturated oxygenates (Grosjean and Grosjean, 1997a): formic acid and methyl glyoxylate from methyl acrylate, formic acid and formic acetic anhydride from vinyl acetate, 2-oxoethyl acetate and 3-oxopropyl acetate from cis-3-hexenyl acetate, ethyl formate and formic acid from ethyl vinyl ether, and methyl formate from trans-4-methoxy-3- buten-2-one. The nature and formation yields of the reaction products are consistent with (and supportive of) the reaction mechanism: O3 + R1R2C=CR3X (R1COR2 + R3C(X)OO) + (1 - )(R3COX + R1C(R2)OO), where R1, R2 and R3 = H or alkyl, X is the oxygen-containing substituent, R1COR2 and R3COX are the primary products and R1C(R2)OO and R3C(X)OO are the carbonyl oxide biradicals. The variations of the coefficient , which ranges from 0.25 to 0.61, are discussed in terms of the number and nature of alkyl and oxygen-containing substituents. Subsequent reactions of the alkyl-substituted biradicals R1C(R2)OO and of the biradicals R3C(X)OO that bear the oxygen-containing substituent are discussed. For the biradical CH3CHOO, the ratio ka/kb for the competing pathways of rearrangement to acetic acid (CH3CHOO CH3C(O)OH, reaction (a) and formation of an unsaturated hydroperoxide (CH3CHOO CH2=CH(OOH), reaction (b) is <0.25 for ethyl 1-propenyl ether and <0.27 for 4-hexen-3-one. Concentrations measured in co- located samples, one downstream of a water impinger and the other without water impinger, show the uptake in water impingers to be high (from 83.2 to >99.9%) and comparable to that for formaldehyde (98.4%) for formic acetic anhydride and for difunctional oxygenated compounds. Uptake in water impingers was lower (19–78%) for monofunctional aldehydes and ketones.  相似文献   
544.
A simple kinetic mechanism of nitrate radicals uptake on dry sea-salt NaCl, NaBr surfaces is proposed. The mechanism includes adsorption/desorption equilibrium and unimolecular decomposition of the adsorbed complex: NO3(g) + NaX(s) (NO3-NaX)(s); (NO3-NaX)(s) NaNO3 + X(s) Two techniques were used: the matrix isolation ESR and mass spectrometry. The uptake coefficient () is found to be dependent on exposure time of salt to NO3 for raw coating. The initial (t0) is higher than the observable steady-state obs. At room temperature obs is independent of [NO3] at low [NO3] = 3 × 109 - 1011 cm-3, but it is inversely proportional to [NO3] at concentrations higher than 1012 cm-3. At temperatures above 100 °C, obs becomes independent of [NO3] in a wider range of [NO3]. An increased number of dislocations is supposed to exist in the case of raw coating. Due to a wide spread of the surface sites binding energy with the ionic lattice near dislocations, the part of surface complexes has lower binding energy and "burns" more rapidly. That burning determines the transition from (t0) down to obs.The kinetic parameters and elementary rate coefficients are obtained. The recommended for low atmospheric NO3 concentration are in the range of 0.002 ± 0.04 for NaCl and 0.1-0.3 for NaBr depending on a mechanism of the (t) relaxation.  相似文献   
545.
作者根据北京小汤山一带热、矿泉水文地质特征,把90%的白云岩和10%的花岗岩作为试料,把二次蒸馏水作为反应溶液,将它们一起放在纯钛高压釜内,在50—200℃的温度和(200—500)×10 ̄5Pa的压力下进行水—岩反应动力学的实验。实验结果表明:水从岩石中萃取出的K ̄(+)、Na ̄(+)、Ca ̄(2+)、Mg ̄(2+)、Al ̄(3+)、HCO、NO、F ̄(-)、Cl ̄(-),SO、∑Fe和SiO_2等离子和氧化物的含量随着岩石的成分、温度、压力、水的pH值、岩石粒度、氧化—还原环境、时间和水的循环次数呈规律性变化。这些规律揭示了小汤山一带热、矿泉形成的机理。为热田的开发和人造温泉提供了理论上的依据。  相似文献   
546.
姜继圣  陈永清 《岩石学报》1993,9(4):400-410
黑龙江省东部麻山群中堇青片麻岩的主要矿物组合有石榴石+夕线石+堇青石+石英+钾长石+黑云母;石榴石+夕线石+堇青石+钾长石+尖晶石;堇青石+夕线石+斜长石;柱晶石+堇青石+斜长石+夕线石+石榴石+电气石;堇青石+紫苏辉石+石榴石+斜长石+黑云母+石英。这些含堇青石组合是在麻粒岩相条件下已存石榴石+夕线石+石英矿物组合分解反应的产物,其形成压力(0.45-0.5GPa_比麻粒岩相“高峰”压力明显低,  相似文献   
547.
何锡文  史长虹 《岩矿测试》1993,12(3):209-216
收集了1986~1991年间参考文献,评述了物种的溶液状态及其对分析反应性能的影响,并论述其进展及代表性工作。内容包括概述、研究溶液状态的方法(其中含测定缔合数、缔合度的新方法)及其在分析化学中的应用(包括新型掩蔽剂、液相化学发光、氧化态和还原态的转换、溶剂效应、吸附与解吸、物种缔合的应用)。引用文献79篇。  相似文献   
548.
A simplified method is introduced to obtain the fundamental frequency of offshore wind turbines supported by monopile foundations. Soil-pile interaction is modeled based on Winkler approach and concept of beam on elastic foundation. The soil is considered to have linearly varying modulus of subgrade reaction along depth which is a typical assumption for cohesionless soils. Rayleigh method which is based on conservation of total energy of the system is utilized. Firstly the natural frequency of the system with rigid pile is derived and then an innovative procedure is introduced to take pile flexural stiffness into consideration. Comparison between results of the present method with those of a numerical FE model for a typical 2 MW wind turbine structure shows excellent agreement for rigid pile and flexible pile with small value of slenderness ratio. The agreement is also good for flexible pile with higher slenderness ratios. A parametric study is carried out to investigate the effect of important parameters of foundation including pile slenderness ratio, pile aspect ratio and pile mass on the system natural frequency.  相似文献   
549.
The infalling movement of the matter accreted onto a magnetized neutron star is discussed. A one-dimensional accretion column model is presented to describe the variations of the infalling velocity, density and temperature of the infalling plasma. The column can be divided from top down into four zones, impact, deceleration of ideal gas, deceleration of degenerate gas and outflow. As an example, the accretion column for an accretion rate of 1017 g/s and a polar magnetic field of ≈ 108 T was calculated. We discuss thermonuclear reaction inside the column, and consider that it may be related to the quasi-periodic oscillation (QPO) of the X-ray flux in low-mass close binaries.  相似文献   
550.
热质输运-反应体系对于热液成矿、污染物迁移等均十分重要.该体系包括热质输运、流体渗流、地球化学平衡计算、多组分化学反应-输运过程耦会等动力学过程.介绍了求解热质输运-反应动力学方程的指数拟合有限体积方法,并编制了相应的计算机软件.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号