全文获取类型
收费全文 | 5235篇 |
免费 | 229篇 |
国内免费 | 300篇 |
专业分类
系统科学 | 244篇 |
丛书文集 | 140篇 |
教育与普及 | 88篇 |
理论与方法论 | 17篇 |
现状及发展 | 47篇 |
研究方法 | 24篇 |
综合类 | 5202篇 |
自然研究 | 2篇 |
出版年
2024年 | 29篇 |
2023年 | 60篇 |
2022年 | 95篇 |
2021年 | 86篇 |
2020年 | 78篇 |
2019年 | 18篇 |
2018年 | 55篇 |
2017年 | 64篇 |
2016年 | 64篇 |
2015年 | 113篇 |
2014年 | 168篇 |
2013年 | 156篇 |
2012年 | 161篇 |
2011年 | 175篇 |
2010年 | 155篇 |
2009年 | 206篇 |
2008年 | 236篇 |
2007年 | 244篇 |
2006年 | 192篇 |
2005年 | 183篇 |
2004年 | 133篇 |
2003年 | 129篇 |
2002年 | 124篇 |
2001年 | 159篇 |
2000年 | 172篇 |
1999年 | 328篇 |
1998年 | 284篇 |
1997年 | 283篇 |
1996年 | 272篇 |
1995年 | 228篇 |
1994年 | 183篇 |
1993年 | 174篇 |
1992年 | 151篇 |
1991年 | 146篇 |
1990年 | 128篇 |
1989年 | 101篇 |
1988年 | 92篇 |
1987年 | 68篇 |
1986年 | 44篇 |
1985年 | 13篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1978年 | 1篇 |
1974年 | 2篇 |
1971年 | 1篇 |
1967年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有5764条查询结果,搜索用时 31 毫秒
971.
硼系表面活性剂复配性能的研究 总被引:6,自引:0,他引:6
以二乙醇胺与硼酸酯化合成硼酸酯(DEAB),再与1-溴十二烷烷基化合成了一类新型含硼两性表面活性剂(RDEAB)。用二元表面活性剂溶液的热力学,研究了RDEAB-AES、RDEAB-AEO9 2种水溶液的混合体系胶束中表面性剂分子间相互作用参数β^m和胶束的组成,2种混合体系的β^m值分别为:-6.47、-3.46。 相似文献
972.
A defined range of guard cell calcium oscillation parameters encodes stomatal movements. 总被引:59,自引:0,他引:59
G J Allen S P Chu C L Harrington K Schumacher T Hoffmann Y Y Tang E Grill J I Schroeder 《Nature》2001,411(6841):1053-1057
Oscillations in cytosolic calcium concentration ([Ca2+]cyt) are central regulators of signal transduction cascades, although the roles of individual [Ca2+]cyt oscillation parameters in regulating downstream physiological responses remain largely unknown. In plants, guard cells integrate environmental and endogenous signals to regulate the aperture of stomatal pores and [Ca2+]cyt oscillations are a fundamental component of stomatal closure. Here we systematically vary [Ca2+]cyt oscillation parameters in Arabidopsis guard cells using a 'calcium clamp' and show that [Ca2+]cyt controls stomatal closure by two mechanisms. Short-term 'calcium-reactive' closure occurred rapidly when [Ca2+]cyt was elevated, whereas the degree of long-term steady-state closure was 'calcium programmed' by [Ca2+]cyt oscillations within a defined range of frequency, transient number, duration and amplitude. Furthermore, in guard cells of the gca2 mutant, [Ca2+]cyt oscillations induced by abscisic acid and extracellular calcium had increased frequencies and reduced transient duration, and steady-state stomatal closure was abolished. Experimentally imposing [Ca2+]cyt oscillations with parameters that elicited closure in the wild type restored long-term closure in gca2 stomata. These data show that a defined window of guard cell [Ca2+]cyt oscillation parameters programs changes in steady-state stomatal aperture. 相似文献
973.
由于褐煤矿质组成的特点和提质工艺的复杂性,决定了褐煤提质废水在水质组成及含量上具有自身独特的环境学特征。为了建立高效、稳定以及出水水质安全的褐煤提质废水处理技术体系,采用常规分析、色谱、质谱等技术,分析研究了洗选蒸脱法褐煤提质工艺废水的污染特性和特征污染物。研究结果表明,褐煤提质加工的废水的CODCr、BOD5、NH4+-N、挥发酚、氰化物等常规污染指标较高,可生化性差,有机污染组分复杂且不同组分含量差异较大,需经过严格的水处理工艺才能满足污水排放标准的要求。研究为褐煤提质废水处理技术的开发提供了科学依据。 相似文献
974.
采用Petri网作为建模工具,把活动分到每个角色,基于然色建立过程模型,该模型很容易获得参与动态联盟的“企业”在实现企业目标的过程中行为的重要信息。一个建立在Web/Intranet平台上的原型系统提供图形化的建模工具,当业务活动按模型执行时,触发相应的处理单元PSA,可动态地显示企业过程,整个企业的经营管理活动按一定的规则和次序有控地进行。 相似文献
975.
本文总结了胭珠蚧属 Porphyrophora Brandt et Ratz.(1933)国内外研究成果 ,发现已记录之 P.ningxiana Yang (1979)和 P.xinjiangana Yang (1979)为 P.sophorae (Arch.,1935)之新异名 ;另新记录 2种 :即 P.polonica(L.,1758)和 P.ussuriensis Borchs.(194 9)及评定 P.yamenica Yang (1979)为 P.hirsutissima(Hall,192 4 )之新异名 相似文献
976.
N-acetamide C60 nitride (N-acetamide-1,2-dihydro-1,2-aza-fullerene [60]) has been synthesized by the reaction of C60 and azidoacetamide and initially characterized. Its mechanism was that the objective product is formed through intermediate
triazoline. The computational study shows that N-nitride structure is thermodynamically more stable by using the semi-empirical
AM1 SCF-MO method. The properties of optical limiting were tested by using double frequency Nd: YAG pulse laser system and
showed that the optical limiting mechanism is reverse saturation absorption. At wavelength of 532 nm, compared with optical
limiting for ns pulse, this new material has good optical limiting character for ps pulse. At wavelength of 707 nm, the capacity
of optical limiting of the derivative is prior to C60. 相似文献
977.
Cr(VI)-bearing wastewater can be treated by natural pyrrhotite which is used for reductant to reduce Cr(VI) and precipitant
to precipitate Cr(III) simultaneously. The disposal products can be divided into three parts in the beakers, namely supernatant
in the upper part, the yellowish colloidal precipitates in the middle part and the pyrrhotite in the lower part. The content
of total Cr=Cr(VI)+Cr(III) in the supernatant liquid is 0.06 mg/L, which is lower than 1.5 mg/L of the discharge standard
of China and near to 0.05 mg/L of the standard of potable water. This one-step disposal composing of both reduction and precipitation
which is traditionally divided into two independent steps called reducing technology and precipitating technology respectively.
The new method is of obvious economic advantage and favourable to decreasing surplus mud derived from adding Ca(OH)2 to precipitate Cr(III) traditionally so as to avoid recontamination. In fact, sodium sulfite (Na2SO3) used in disposal of Cr(VI) was traditionally produced from natural mineral of pyrrhotite (FeS). One molecule of FeS is 4
times more than that of Na2SO3 from a view point of rational use of mineral resources. Therefore the prospective of application of the one-step disposal
of Cr(VI) method is full of promise. 相似文献
978.
Inheritance of resistance toHelicoverpa armigera of 3 kinds of transgenicBt strains available in upland cotton in China 总被引:1,自引:0,他引:1
Canming Tang Jing Sun Xiefei Zhu Wangzhen Guo Tianzhen Zhang Jinliang Shen Congfen Gao Weijun Zhou Zhixian Chen Sandui Guo 《科学通报(英文版)》2000,45(4):363-367
There are 3 kinds of transgenicBt strains, Shanxi 94-24, Zhongxin 94, and R19, in upland cotton in China. Their transgenicBt insect-resistance cultivars or hybrids have been developed and grown by farmers. Genetic studies indicate that the resistance
of the 3 transgenicBt cotton strains toHelicoverpa armigera is controlled by one pair of non-allelic dominant genes. Linkage relationship between the resistant genes of R19 and Shanxi
94-24 transgenicBt strains shows that they may be inserted in the same chromosome. F1 hybrids crossed among the 3 strains show that high levels of protection from feeding damage are the same as that of their
parents. Therefore, there is no co-suppression phenomenon in many transgenic plants. The results presented here afford a fundamental
reliance in developing transgenicBt insect-resistant cultivars and exploiting the heterosis of hybrids in upland cotton. 相似文献
979.
Al2O3颗粒增强不锈钢基表面复合材料腐蚀性能的研究 总被引:2,自引:1,他引:1
针对湿法磷酸工况,设计了不锈钢基体的化学成分;在Al2O3颗粒表面,通过化学气相沉积Ni涂层,解决了颗粒与基体的润湿性问题;采用负压铸渗工艺制备了氧化铝-锈钢基表面复合材料,并研究了该复合材料在此工况下的静态耐蚀性能,发现了耐蚀性能超过了高铬钢。 相似文献
980.
双金属MEL结构杂原子分子筛催化性能的研究 总被引:1,自引:0,他引:1
研究了H2O存在下双金属CrCo-、VCr-、VCo-、VNi-、CrNiMEL结构杂原子分子筛对环己醇、2-丁醇、2-丙醇、环己烯的选择氧化催化性能,并考察了反应温度及H2O2浓度对环己烯催化氧化反应的影响。结果表明,反应物不同,活性亦不同,其顺序为环为已烯〉环己醇〉2-丁醇〉2-丙醇;CrNi、CrCoMEL分子筛活性较高,而VCo-、VCr-、VNiMEL分子筛活性较低,反应温度和H2O2浓 相似文献