全文获取类型
收费全文 | 33789篇 |
免费 | 4775篇 |
国内免费 | 2649篇 |
专业分类
化学 | 7313篇 |
晶体学 | 112篇 |
力学 | 3777篇 |
综合类 | 996篇 |
数学 | 16538篇 |
物理学 | 12477篇 |
出版年
2024年 | 49篇 |
2023年 | 300篇 |
2022年 | 569篇 |
2021年 | 788篇 |
2020年 | 778篇 |
2019年 | 719篇 |
2018年 | 748篇 |
2017年 | 1107篇 |
2016年 | 1251篇 |
2015年 | 977篇 |
2014年 | 1706篇 |
2013年 | 2196篇 |
2012年 | 1759篇 |
2011年 | 2121篇 |
2010年 | 1960篇 |
2009年 | 2277篇 |
2008年 | 2407篇 |
2007年 | 2453篇 |
2006年 | 2130篇 |
2005年 | 1826篇 |
2004年 | 1663篇 |
2003年 | 1446篇 |
2002年 | 1368篇 |
2001年 | 1140篇 |
2000年 | 964篇 |
1999年 | 933篇 |
1998年 | 812篇 |
1997年 | 695篇 |
1996年 | 611篇 |
1995年 | 566篇 |
1994年 | 395篇 |
1993年 | 359篇 |
1992年 | 296篇 |
1991年 | 292篇 |
1990年 | 208篇 |
1989年 | 163篇 |
1988年 | 177篇 |
1987年 | 131篇 |
1986年 | 92篇 |
1985年 | 119篇 |
1984年 | 93篇 |
1983年 | 59篇 |
1982年 | 91篇 |
1981年 | 80篇 |
1980年 | 60篇 |
1979年 | 83篇 |
1978年 | 49篇 |
1977年 | 38篇 |
1976年 | 25篇 |
1974年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Brian J. Brisdon 《Journal of organometallic chemistry》1977,125(2):225-230
Compounds of the type [XM(CO)2(ν-allyl)L2] (where X = Cl and Br; M = Mo and W; L2 = Ph2PCH2PPh2 and Ph2 PCH2CH2PPh2) have been prepard from the corersponding MeCN complexes. The spectral properties of these compounds and the effects of chelate rign size on 31P coordination shifts and J(183W—31P) have been investigated. 相似文献
982.
For lattices with two kinds of points (black and white), distributed according to a translation-invariant joint probability distribution, we study statistical properties of the sequence of consecutive colors encountered by a random walker moving through the lattice. The probability distribution for the single steps of the walk is considered to be independent of the colors of the points. Several exact results are presented which are valid in any number of dimensions and for arbitrary probability distributions for the coloring of the points and the steps of the walk. They are used to derive a few general properties of random walks on lattices containing traps.Presented at the Symposium on Random Walks, Gaithersburg, MD, June 1982. 相似文献
983.
低浓度HPAM/AlCit交联体系的27Al NMR研究 总被引:2,自引:0,他引:2
用27Al NMR谱研究了高分子量低浓度的部分水解聚丙烯酰胺(HPAM)与柠檬酸铝(AlCit)体系交联反应过程中Al的化学位移和Al的自旋-晶格弛豫时间的变化. 结果表明, HPAM与AlCit反应后, 与HPAM分子链上的羧基发生配位交联的Al的化学位移向低场移动, 而不参与交联反应的AlCit分子结构中Al的化学位移基本不变. HPAM/AlCit交联体系中存在三种形态的Al, 分别对应三种不同的自旋-晶格弛豫时间. 当HPAM的质量浓度≤200 mg/L时, HPAM与AlCit反应过程中交联态Al的自旋-晶格弛豫时间τ13随反应进行变小, HPAM与AlCit主要发生分子内交联反应. 当HPAM的质量浓度≥250 mg/L时, HPAM与AlCit反应过程中交联态Al的自旋-晶格弛豫时间τ13随反应进行变大, HPAM与AlCit主要发生分子间交联反应. 相似文献
984.
Luisa Fernanda Rojas Paola Zapata Laura Ruiz-Tirado 《Current Opinion in Green and Sustainable Chemistry》2022
According to the Food and Agriculture Organization of the United Nations, approximately 1.3 billion tons of food is wasted each year, equivalent to approximately one-third of world production. Agri-food wastes are the source of proteins, carbohydrates, lipids, and other essential minerals that have been exploited for value-added products by the development of biorefineries and sustainable business as important elements of circular economies. The innovation and materialization of these types of processes, including the use of disruptive technologies on microbial bioconversion and enzyme technology, such as nanotechnology, metabolic engineering, and multi-omics platforms, increase the perspectives on the waste valorization process. Lignocellulolytic enzymes, pectinases, and proteases are mainly used as catalyzers on agri-food waste treatment, and their production in house might be the trend in near future for agro-industrial countries. Another way to transform the agri-food wastes is via aerobic or anaerobic microbial process from fungal or bacterial cultures; these processes are the key to produce waste enzymes. 相似文献
985.
Zusammenfassung An unterschiedlich verstreckten und hydrothermisch nachbehandelten, danach gefriergetrockneten Polyacrylnitril-Gelfasern (PAN) wurden Messungen der Röntgenkleinwinkelstreuung (RKWS), Röntgenweitwinkelstreuung (RWWS) und Tieftemperatur-Gassorption (TTGA) durchgeführt. Die RKWS-Messungen zeigen, daß sowohl in den isotropen als auch in den orientierten Proben eine exponentialstatistische Verteilung der Form und Größe der Inhomogenitäten (Hohlräume, Cluster) vorliegt, welche durch die Kohärenzlängel
c bestimmt wird. Eine erhöhte Röntgenabsorption wurde bei den verstreckten Proben festgestellt. Die Analyse der aus der Kohärenzlänge und den Volumsanteilen der Phasen berechneten charakteristischen RK WS-Parameter (z.B. innere Oberfläche, Durchschußlänge) ergibt, daß jede Behandlung der Gelfasern eine Zu- oder Abnahme der etwa gleich großen Hohlräume verursacht (nucleotic change). Das relative Ausmaß dieser Veränderungen wird besprochen.Der Vergleich mit den Ergebnissen der TTGA an den gleichen Fasern zeigt, daß die Tendenzen bei der spezifischen Oberfläche und dem Porenvolumen (Zu- bzw. Abnahme) bei beiden Methoden annähernd gleich sind. Die Absolutwerte des Hohlraumvolumens unterscheiden sich aber z.T. um den Faktor 40. Dies führt zur Schlußfolgerung, daß mit der RK WS Faserbereiche als Hohlräume erfaßt werden, die bei der TTGA nicht angezeigt werden, weil sie für das Meßgas unzugänglich sind.
Structural studies on wet-spun freeze-dried polyacrylonitrile fibers
Summary Polyacrylonitrile fibers (PAN) were drawn to different extents, hydrothermally aftertreated, and at last freeze-dried. The products were studied by small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), and low temperature gassorption (LTGS). The SAXS data show that the shape and the size of the inhomogeneities (voids, clusters) have an exponential statistical distribution determined by the correlation lengthl c, both in the isotropic fibers and in the fiber-oriented ones. It has also been observed that stretching always causes a higher X-ray absorption. An analysis of the characteristic SAXS parameters (e.g., internal surface, mean chord length), which were calculated from the correlation length and the volume fractions of the phases, evidences that in all samples an augmentation or diminution of the number of the voids with nearly the same shape and size takes place (nucleotic change). Its relative number has been pointed out.Comparison with the results of the LTGS investigations at the same fibers shows that the tendencies of the specific surface and porosity (increase, decrease) are approximately the same with both methods. However, the absolute values of void volume differ in some cases by the factor 40. This leads to the conclusion that the SAXS method records domains of these fibers as voids which are not registered by LTGS (voids not accessible for the gas).
Herrn em.o.Univ. Prof. Dr.Josef Schurz zum 70. Geburtstag gewidmet 相似文献
986.
G.B. Song C.Q. Han J.K. Liang Q.L. Liu F.S. Liu G.H. Rao 《Journal of solid state chemistry》2004,177(7):2394-2403
The subsolidus phase relations of the PrOx-CaO-CuO pseudo-ternary system sintered at 950-1000°C have been investigated by X-ray powder diffraction. In this system, there exist one compound Ca10Pr4Cu24O41, one Ca2Pr2Cu5O10-based solid solution, seven three-phase regions and two two-phase regions. The crystal structures of Ca10Pr4Cu24O41 and Ca2Pr2Cu5O10-based solid solution have been determined. Compound Ca10Pr4Cu24O41 crystallizes in an orthorhombic cell with space group D2h20−Cccm, Z=4. Its lattice parameters are a=11.278(2) Å, b=12.448(3) Å and c=27.486(8) Å. The crystal structure of Ca2Pr2Cu5O10-based solid solution is an incommensurate phase based on the orthorhombic NaCuO2 type subcell. The lattice parameters of the subcell of the Ca2.4Pr1.6Cu5O10 are a0=2.8246(7) Å, b0=6.3693(5) Å, c0=10.679(1) Å, and those of the orthorhombic superstructure are with a=5a0, b=b0, c=5c0. The Ca2.4Pr1.6Cu5O10 structure can also be determined by using a monoclinic supercell with space group C2h5−P21/c, Z=4, a=5a0, b=b0, and β=104.79(1)° or 136.60(1)°, V=5a0b0c0. 相似文献
987.
A search system is presented that utilizes Fourier domain data. This system removes dominating features from the infrared absorbance data, transforms the data into the Fourier domain and performs a complete library search. The actual matching algorithm is very simple and excellent search results were attained using 16 cm–1 resolution infrared absorbance spectra before transformation. This method can be used to distinguish very similar spectra that cannot be distinguished by many other search methods. 相似文献
988.
Summary Retention indices and their energy equivalents were determined for ten alkylthiazoles analysed on the stationary phases Apiezon
L, Triton X-305 and Carbowax 1000. It was found that the energy contributions of identical alkyl groups to the partial molar
free energy of sorption of a molecule are not equivalent. Smaller energy contribututions are due to the α-position of alkyl
groups with respect to the heteroatoms in the thiazole ring (α-effect). This effect was most pronounced for 2-alkylthiazoles.
Higher sorption energies were found for dimethylthiazoles with two methylgroups in a vicinal position. The retention indices
were predicted for ten other di- and tri-substituted methyl, ethyl and propyl thiazoles on the basis of the so-called α- and
ortho-effects. The predicted values are in good agreement with the experimentally determined values.
Dedicated to Prof. Dr. E. sz. Kováts (Ecole Polytechnique Fédérale de Lausanne) on the occasion of his sixtieth birthday. 相似文献
989.
Óscar López 《Tetrahedron》2005,61(38):9058-9069
An efficient method for the preparation of urea-bridged cyclodextrins using triphosgene in the isocyanation step in an aqueous two-phase system is reported. Per-O-acetylated glycopyranosylamines and 2-amino-2-deoxy-α and β-d-glucose were also transformed into the corresponding isocyanates using either an aqueous two-phase or an anhydrous dichloromethane medium, and converted in situ into ureas. An alternative method for the preparation of sugar-derived ureas consisting of desulfurization of sugar thioureas with mercury oxide is also presented. 相似文献
990.
阐述了ICP-AES专家系统中AES电离和激发模型的基本原理,研究了模拟中应用non-LTE理论的正确性以及所需的支持数据库。利用Boltzmann曲线,讨论了Boltzmann温度的测量值随能级激发能变化的现象,并与类似条件下的经验Boltzmann曲线进行对比,证明了专家系统中AES电脑和激发模型可靠性。 相似文献