全文获取类型
收费全文 | 1689篇 |
免费 | 250篇 |
国内免费 | 521篇 |
专业分类
电工技术 | 7篇 |
综合类 | 146篇 |
化学工业 | 218篇 |
金属工艺 | 32篇 |
机械仪表 | 195篇 |
建筑科学 | 102篇 |
矿业工程 | 99篇 |
能源动力 | 65篇 |
轻工业 | 283篇 |
水利工程 | 94篇 |
石油天然气 | 539篇 |
武器工业 | 2篇 |
无线电 | 17篇 |
一般工业技术 | 66篇 |
冶金工业 | 78篇 |
原子能技术 | 505篇 |
自动化技术 | 12篇 |
出版年
2024年 | 6篇 |
2023年 | 45篇 |
2022年 | 81篇 |
2021年 | 106篇 |
2020年 | 93篇 |
2019年 | 92篇 |
2018年 | 92篇 |
2017年 | 75篇 |
2016年 | 109篇 |
2015年 | 85篇 |
2014年 | 106篇 |
2013年 | 282篇 |
2012年 | 110篇 |
2011年 | 113篇 |
2010年 | 85篇 |
2009年 | 74篇 |
2008年 | 68篇 |
2007年 | 83篇 |
2006年 | 88篇 |
2005年 | 99篇 |
2004年 | 68篇 |
2003年 | 70篇 |
2002年 | 48篇 |
2001年 | 58篇 |
2000年 | 40篇 |
1999年 | 32篇 |
1998年 | 30篇 |
1997年 | 22篇 |
1996年 | 25篇 |
1995年 | 29篇 |
1994年 | 24篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 18篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1981年 | 8篇 |
1980年 | 3篇 |
1959年 | 6篇 |
排序方式: 共有2460条查询结果,搜索用时 312 毫秒
161.
食醋中乙酸的碳-13比值(δ13C)与食醋原料有密切关联,通过测定乙酸中δ13C可对食醋原料进行溯源。本工作建立了有机溶剂稀释法与气相色谱-燃烧-稳定同位素比值质谱(GC-C-IRMS)联用法测定食醋中乙酸的δ13C。在乙酸浓度为2%~99.9%的模拟样品测试中获得了稳定的检测结果;同一样品中乙酸的δ13C重复测定16次的标准偏差小于0.15‰;参与欧盟同位素实验室间能力测试时,该方法的测定结果与其中5个国际实验室测定平均值差异为0.17‰。该方法的精密度和稳定性好、准确性高、操作简便快速,适合在食醋真实性技术中应用和推广。 相似文献
162.
应用实时直接分析-高分辨飞行时间质谱(DART-TOF MS)对2,4,6-三硝基甲苯(TNT)、环三亚甲基三硝铵(RDX)、环四亚甲基四硝铵(HMX)、季戊四醇四硝酸酯(PETN)四种常见的有机炸药进行精确质量质谱解析,结果表明,即使在高质量精度的质谱上也不能获得化合物唯一的分子式。借助MassWorks软件对飞行时间质谱数据进行噪音过滤及峰形校正处理,可大大改善高分辨飞行时间质谱对目标化合物分子式识别的准确度,提高定性的准确性,为高分辨质谱解析未知化合物提供参考 相似文献
163.
????????????????????????崿??????????????????λ?????????????????????????????????????????????????????????????????He?????????λ????ɡ??????????????????????????????????????в??????????η?????He???????λ?????????????????He????????????????????????2.9%??5%?????????н?????????????????????????????????????????nobless?????????????????VG5400??????????????????????????????????????Χ??????÷??????????????????????????????????????He???????λ??????????????????Ч?????????????????? 相似文献
164.
165.
采用气相色谱-燃烧-同位素比值质谱(GC-C-IRMS)法对咖啡因化合物和混合体系中特定氨基酸的δ13C和δ15N进行测定,在保证高测定精度和准确度的前提下,探讨GC C IRMS对进样量和信号强度变化的响应特征。分析结果表明,当C质量≥ 1 ng或m/z 44信号≥ 100 mV时,以及N质量≥5 ng或m/z 28信号≥ 100 mV时,咖啡因δ13C和δ15N测定结果的精度(<03‰)和准确度(<02‰)均能够满足实验室测试要求。针对混合体系中氨基酸δ13C和δ15N的测定,GC C IRMS可以在极少的进样量下实现对特定氨基酸色谱峰的有效分离。12种氨基酸δ13C测定结果未表现出对m/z 44信号强度的依赖性(斜率接近0),其δ13C平均测定精度为056‰;而δ15N测定结果表现出良好的时间稳定性(4天),与元素分析 稳定同位素比值质谱(EA IRMS)单独测定结果的平均偏差为077‰。 相似文献
166.
大控蚌温泉位于云南省云县,地处滇藏地热带。温泉区附近断裂发育,主要的热储层岩性为临沧花岗岩。泉水共有五个主要泉眼,水温88.3~96℃。地下热水中主要阳离子为K~+、Na~+、Ca~(2+)和Mg~(2+),主要阴离子为Cl~-、SO_4~(2-)和HCO_3~-,水化学类型为HCO_3~-Na型;热水矿化度0.625~0.702g/L,pH值为6.7~9,偏硅酸含量为161.1~261mg/L,F-含量为5.85~16.1mg/L。根据δ~2 H和δ~(18) O稳定同位素分析得知温泉水源为大气降水;估算出热水补给高程约为1 800m;估算补给区温度约为3.6~7.2℃;热储温度约为143~181℃;循环深度约为2 858~3 496m。综合分析得出大控蚌温泉成因模式为大气降水在周围山区补给区入渗补给,地下水向深大断裂带汇集,在经历深循环过程获得深部热流加热后沿导水断裂带上涌,在上升的过程中与浅部冷水发生混合,最终以高温沸泉和喷泉出露地表。 相似文献
167.
Xiao-Zhong Chu Zhi-Peng Cheng Xin-Xin Xiang Ji-Ming Xu Yi-Jiang Zhao Wei-Guang Zhang Jin-Shun Lv Ya-Ping Zhou Li Zhou Dong-Kyu Moon Chang-Ha Lee 《International Journal of Hydrogen Energy》2014
The separation of a hydrogen isotope mixture on porous materials was studied using equilibrium and breakthrough experiments. The adsorption equilibria of H2 and D2 on SBA-15 with mesopores and molecular sieves 5A, Y, and 10X with micropores were measured at 77 K using the volumetric method. The breakthrough experiments of a H2 and D2 mixture in each adsorbent bed were carried out at various conditions of flow rate and pressure. The equilibrium ratio of D2 to H2 on mesoporous molecular sieves was larger than the ratio on microporous molecular sieves (SBA-15 > 10X > Y > 5A), but the difference among the adsorbents decreased with increases in pressure. On the other hand, the order of breakthrough separation factor showed the opposite result (SBA-15 < 10X < Y < 5A). The breakthrough separation factors for zeolite 10X was approximately equal to the equilibrium ratio of D2 to H2 at the corresponding partial pressures, whereas zeolites 5A and Y showed higher breakthrough separation factors than their equilibrium ratios. In SBA-15, the separation factors from breakthrough results were even smaller than the corresponding equilibrium ratio. In the microporous adsorbent with a limited pore size (zeolite 5A in the study), the diffusion mechanism contributed to the separation of hydrogen isotope gases as one of key factors. 相似文献
168.
Brief review on the development of isotope hydrology in China 总被引:3,自引:0,他引:3
The development of isotope hydrology in China is briefly reviewed. It includes oxygen and hydrogen isotopes of precipitation, application of isotope hydrological methodologies in solving water resources problems, isotope hydrological studies on brines and salt lake waters, as well as isotope hydrological investigations of thermal waters. The review focuses on isotope hydrology in China during the last two decades. 相似文献
169.
Distribution and carbon isotope composition of lipid biomarkers in Lake Erhai and Lake Gahai sediments on the Tibetan Plateau 总被引:1,自引:0,他引:1
Lipid biomarkers and carbon isotopes of n-alkanes, n-fatty acids, and n-alkan-2-ones were used to determine organic carbon sources to surface sediments in lakes Erhai and Gahai on the Tibetan Plateau. All sediment samples contained n-fatty acids with lower concentrations of n-alkanes and n-alkan-2-ones. Long chain n-alkanes in lake sediments were characteristic of a source mixture of epicuticular waxes of higher plants and submerged littoral zone plants while n-fatty acids sources were bacteria and floating and/or submergent macrophytes; the n-alkan-2-ones had a possible source in epicuticular waxes of higher plants and/or grass from catchment which entered the lake and were reworked by the microorganisms. Sediment samples examined from Lake Erhai had heavier δ13C values of bulk organic carbon and specific carbon compounds than samples from Lake Gahai and meadow soil. This heavier isotopic composition can be best interpreted by the enhanced productivity occurring in Lake Erhai due to its lower salinity and possibly to limited CO2 concentrations; for the latter, this may have enriched 13C in the dissolved inorganic carbon pool of the lake water. 相似文献
170.
Invasive sea lamprey (Petromyzon marinus) remain an important source of fish mortality in the Laurentian Great Lakes, yet assessing their impact is hindered by lack of quantitative diet information. We examined nitrogen and carbon stable isotope ratios (δ15N and δ13C) of sea lamprey and host species in six ecoregions of Lake Superior, mainly in 2002–2004. Data implied that most sea lamprey fed primarily on upper trophic level species, including forms of lake trout (Salvelinus namaycush). However, in Ontario waters, particularly semi-enclosed Black Bay, sea lamprey relied heavily on lower trophic levels, such as coregonines (Coregonus spp.) and suckers (Catostomus spp.). Sea lamprey δ15N and δ13C generally increased with sea lamprey size, implying dependence on higher trophic levels later in life. Most parasitic sea lamprey that we captured were attached to either lean lake trout (35% of observed attachments), lake whitefish (Coregonus clupeaformis; 25%), or cisco (C. artedii; 25%); the latter sea lamprey were typically < 15 g. Survey- and fishery-dependent wounding rate data compiled from 1986–2005 suggest that lean and siscowet lake trout were selectively parasitized by sea lamprey, which is consistent with our stable isotope data. Our results largely support the notion that lake trout are the principal host species in Lake Superior. However, stable isotope evidence that sea lamprey feed at lower trophic levels in some regions argues for comprehensive monitoring of sea lamprey impacts throughout the fish community in systems that sea lamprey have invaded. 相似文献