全文获取类型
收费全文 | 1073篇 |
免费 | 34篇 |
国内免费 | 13篇 |
专业分类
化学 | 774篇 |
晶体学 | 5篇 |
力学 | 33篇 |
数学 | 110篇 |
物理学 | 198篇 |
出版年
2024年 | 4篇 |
2023年 | 5篇 |
2022年 | 6篇 |
2021年 | 44篇 |
2020年 | 21篇 |
2019年 | 27篇 |
2018年 | 38篇 |
2017年 | 28篇 |
2016年 | 36篇 |
2015年 | 29篇 |
2014年 | 37篇 |
2013年 | 90篇 |
2012年 | 63篇 |
2011年 | 61篇 |
2010年 | 55篇 |
2009年 | 58篇 |
2008年 | 57篇 |
2007年 | 67篇 |
2006年 | 50篇 |
2005年 | 48篇 |
2004年 | 58篇 |
2003年 | 33篇 |
2002年 | 42篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 13篇 |
1997年 | 10篇 |
1996年 | 12篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 10篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1962年 | 2篇 |
排序方式: 共有1120条查询结果,搜索用时 15 毫秒
991.
A series of [W(CO)4]2(-dppa)(-DTA) type complexes [dppa = Ph2P(CH2)
n
PPh2, n = 2(dppe), 4(dppb), 6(dpph), 10(dppd); DTA =
t
BuS(CH2)
m
S
t
Bu, m = 3(DTN), 4(DTD), 5(DTUD), 6(DTDD)] containing doubly bridged bisphosphine and dithiaalkane ligands have been prepared by stepwise replacement of piperidine (pip) from cis-W(CO)4(pip)2 complex. In addition, complexes of general formulae [W(CO)4]2(-dppa)2 and [Mo(CO)4]2(-DTA)2 have been prepared by similar methods. These new complexes have been characterized by i.r. spectroscopy and elemental analysis. 相似文献
992.
Reactions of the intermediate W(CO)5THF, generated photochemically from W(CO)6 in THF, with Ph2P(CH2)
n
PPh2 [ = PP; n = 2 (dppe), 4 (dppb), 6 (dpph), 10 (dppd)] at room temperature in THF solutions gave exclusively bimetallic complexes of the (CO)5WPPW(CO)5 type. In addition, complexes bridged by diphosphine ligands of the (CO)4(pip)MPPM(pip)(CO)4 type (pip = piperidine; M = Mo, W) were prepared by stirring the (CO)4M(pip)2 complexes with bis(diphenylphosphino)alkanes in CH2Cl2 solution at ambient temperatures. These new bis(diphenylphosphino)alkane-bridged complexes were characterized by i.r., 1H- and 31P-n.m.r. spectroscopies, as well as by elemental analysis. 相似文献
993.
A simple kinetic spectrophotometric method was developed for the determination of josamycin in its dosage forms. The method is based on oxidation of the drug with alkaline potassium permanganate at room temperature for a fixed time of 20 min and measuring the produced green color at 611 nm. The absorbance-concentration plot is rectilinear over the range of 2-10 microg/mL (2.4 x 10(6)-1.2 x 10(-5)M) with minimum detectability of 1.0 microg/mL (1.2 x 10(-6)M). The determination of josamycin by fixed concentration and the rate-constant methods is also feasible with the calibration equations obtained, but the fixed-time method proved to be more applicable. The procedure was applied successfully to commercial tablets, and statistical analysis showed that the results compared favorably with those obtained by reference methods. The effect of sensitizers and surfactants on the performance of the proposed method was also studied. A proposal of the reaction pathway was presented. 相似文献
994.
Polysiloxanes with pendant poly(ethylene oxide) side chains (4 were prepared by the dehydrocoupling reaction of poly(methylhydrosiloxane) (PMHS, 3 with 2-(2-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)ethoxy)ethanol (1 and poly(ethylene glycol) methyl ether (2 using a metal catalyst. Catalysts investigated were tin(II) 2-ethylhexanoate, Rh(Ph3P)3Cl, and Pd2(dba)3. The reaction of a cyclic siloxane, D4H, with 1 catalyzed by Pd2(dba)3 was also carried out to synthesize siloxane 6. The polysiloxanes were characterized by 1H NMR, 29Si NMR, FT-IR, and GPC. 29Si NMR study of these comb-like polysiloxanes revealed that there is a significant difference in the structure of the siloxane polymers prepared depending upon the catalyst. M, D, and T units were observed when tin(II) was used as a catalyst, but only M and D units were detected when Rh(Ph3P)3Cl or Pd2(dba)3 was employed. Furthermore, M and T units are negligible for the cyclic siloxane 3 using Pd2(dba)3. A mechanism is proposed to account for these observations. 相似文献
995.
El-Hosiny F. I. Abo-El-Enein S. A. El-Didamony H. Khalil Kh. A. 《Journal of Thermal Analysis and Calorimetry》1996,46(1):105-116
Rice husk ash fired at different temperatures, 450, 700 and 1000°C, was mixed with different concentrations of lime (molar lime/silica ash ratio of 0.2, 0.5 and 1.0). Each dry mixture was first ground and hydrated in the suspension form (water/solid ratio = 10) for various time intervals within the range of 1 to 365 days. The surface properties of the unhydrated and hydrated samples were studied by means of nitrogen adsorption measurements. The results indicated that the surface areas and total pore volumes of unhydrated solid mixtures and hydrated lime-rice husk ash samples, prepared with lime/silica ash ratio of 1.0, decrease with increasing firing temperature of rice husk ash. The effect of varying the lime/silica ash ratio of the solid mixture on the surface area and pore structure was fully discussed. The results of surface area and pore volume measurements could also be related to the crystal structure of silica produced from rice husk ash. 相似文献
996.
The alkali metal ions transfer from water to the nitrobenzene phase containing macrocyclic neutral ionophores (crown polyethers, nonactin) has been investigated, applying the electrolysis at the interface of two immiscible electrolyte solutions (ITIES) in the single-sweep voltammetry mode. The stability constants of complexes formed in the non-aqueous phase have been determined from the peak voltammograms, as well as value of complex formation enthalpy. It has been concluded that the facilitated ion transfer in the temperature range 5–65°C is a diffusion-controlled process. 相似文献
997.
KhalilFaghihi AzizollahMirsamie 《高分子科学》2005,(1):63-69
Eight novel poly(amide-imide)s were synthesized under microwave irradiation by using a domestic microwaveoven from the polycondensation reactions of N,N′-(pyromellitoyl)-bis-L-alanine diacid chloride (1) with eight different derivatives of hydantoin compounds (2a-h) in the presence of a small amount of a polar organic medium such as o-cresol. The polycondensation proceeded rapidly, compared with the conventional solution polycondensation and was completed within 8-10min, producing a series of new poly(amide-imide)s (3a-h) with inherent viscosities about 0.35-0.68dL/g in high yields. The obtained PAIs (3a-h) were fully characterized by means of FT-IR spectroscopy, elemental analyses, inherent viscosity (ηinh), solubility and specific rotation measurements. All of the resulting polymers show optical rotation and are optically active. Thermal properties of the poly(amide-imide)s were investigated by using thermal gravimetric analysis(TGA). 相似文献
998.
A new simple and reliable method for rapid and selective extraction and determination of trace level of Pd(II) ion is developed. Palladium ions are adsorbed quantitatively during passage of aqueous samples through octadecyl silica membrane disks modified with thioridazine·HCl (TRH). The influence of flow rates of eluent and sample solution, amount of ligand, types and least amount of eluent, and pH of samples were studied. Almost all matrix elements were found to pass through the disk to drain. Break through volume and limit of detection of the membrane disks modified by 5 mg of TRH was found to be 1.0 l and 12 μg l−1, respectively. The retained Pd(II) ions are then stripped from the disk with a minimal amount of sulfite solution as eluent and subsequently measured by atomic absorption spectrometry. The proposed method permitted large enrichment factors of about 100 and higher. The method was applied to the recovery of Pd(II) ions from different industrial samples and waters. 相似文献
999.
The molecular complexes of some lanthanides scandium (Sc3+), yttrium (Y3+), lanthanum (La3+), gadolinium (Gd3+), cerium (Ce3+) and ytterbium (Yb3) have been studies in dimethyl formamide (DMF) spectrophtometrically equilibrium constants (K), molar extintion coefficient (epsilon), energy of transition (E) and free energy (delta G*) were calculated. The polymerization of acrylonitrile has been studied and investigated in the presence of Sc3+, Y3+, La3+, Gd3+, Ce3+, and Yb3+ ions. The IR spectra of the formed AN-M (III) Br3 polymer complexes show the absence of the C identical to N band and the presence of two new bands corresponding to NH2 and OH groups. Magnetic moment values and the thermal stabilities of homopolymer and the polymer complexes were studied by means of thermogravimetric analysis and the activation energies for degradation were calculated. 相似文献
1000.
A new prenalterol (Pr) ion-selective PVC membrane electrode based on the ion-pair complex of Pr with sodium tetraphenylborate was prepared and its performance characteristics were studied. The electrode exhibited a linear response with a good Nernstian slope over a relatively wide range of concentration. The electrode whose membrane was made of 8.0% (w/w) of ion pair, 49.5% (w/w) of dioctyl phthalate (DOP) and 43.5% (w/w) of PVC showed characteristics higher than those obtained with the other ones, namely, slope of 56.3 mV per concentration decade, at 25 °C; usable concentration range 1.2×10−5–3.2×10−2 M prenalterol; response time ≤20 s. Up to 24 h continuous soaking, the calibration graph slope was constant at 56.0 mV per concentration decade, at 25 °C, then it decreased gradually as the time of soaking increases reaching 41 mV per decade after 11 days. The changes in pH did not affect the electrode performance within the range 2.2–7.5. The standard electrode potentials were determined at different temperatures and used to calculate the isothermal coefficient of the electrode. The electrode showed very good selectivity for Pr with respect to a large number of inorganic and organic cations. The standard addition method and potentiometric titration were used to determine Pr in pure solutions and in pharmaceutical formulations. The relative standard deviations (R.S.D.) of the three methods used for the determination of prenalterol in pharmaceutical formulations were 0.8, 2.1 and 1.9% of the extrapolation, standard addition and potentiometric methods, respectively. 相似文献