全文获取类型
收费全文 | 1876篇 |
免费 | 76篇 |
国内免费 | 19篇 |
专业分类
化学 | 1351篇 |
晶体学 | 19篇 |
力学 | 108篇 |
数学 | 205篇 |
物理学 | 288篇 |
出版年
2023年 | 25篇 |
2022年 | 82篇 |
2021年 | 88篇 |
2020年 | 61篇 |
2019年 | 93篇 |
2018年 | 85篇 |
2017年 | 63篇 |
2016年 | 108篇 |
2015年 | 71篇 |
2014年 | 104篇 |
2013年 | 182篇 |
2012年 | 129篇 |
2011年 | 121篇 |
2010年 | 96篇 |
2009年 | 77篇 |
2008年 | 91篇 |
2007年 | 67篇 |
2006年 | 55篇 |
2005年 | 36篇 |
2004年 | 30篇 |
2003年 | 29篇 |
2002年 | 31篇 |
2001年 | 13篇 |
2000年 | 15篇 |
1999年 | 7篇 |
1998年 | 17篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1968年 | 2篇 |
1967年 | 3篇 |
排序方式: 共有1971条查询结果,搜索用时 10 毫秒
991.
In 2019, an exceptional Ptolemaic‐era tomb was discovered in Al‐Diabat archaeological site, Sohag, Upper Egypt. Numerous fallen painted fragments in the floor of tomb were collected and studied. The microstructural and chemical makeup of samples was discerned by virtue of a field‐emission scanning electron microscopy (FE‐SEM) with an energy‐dispersive x‐ray (EDX) spectrometry. Further, the molecular‐vibrational property was acknowledged by Fourier‐transform infrared (FT‐IR) and micro‐Raman (μ‐Raman) spectrometers. Also, the crystalline structure of stone and plaster samples was defined by x‐ray diffraction (XRD) analysis. The results have demonstrated that the murals were implemented on a thick mud plaster followed by a thin preparation layer, which, unusually, was made of calcite. Gypsum was also detected in variable quantity throughout the render samples. Colouring substances of Egyptian blue, green earth, red ochre, yellow ochre and carbon black were recognized in the studied murals. Besides, maghemite (γFe2O3) was disclosed in the red pigment sample. Unexpectedly, gold was detected in the yellow paint areas, possibly associated with a religious tradition. As well, the recognition of an organic binder (probably egg yolk) nominates the application of tempera technique. Noteworthy, this research contributes in presenting significant findings on painting materials used in unusual rock‐cut tomb from the Ptolemaic era. 相似文献
992.
This paper proposes a robust method to detect and extract silhouettes of foreground objects from a video sequence of a static
camera based on the improved background subtraction technique. The proposed method analyses statistically the pixel history
as time series observations. The proposed method presents a robust technique to detect motions based on kernel density estimation.
Two consecutive stages of the k-means clustering algorithm are utilized to identify the most reliable background regions and decrease the detection of false
positives. Pixel and object based updating mechanism for the background model is presented to cope with challenges like gradual
and sudden illumination changes, ghost appearance, non-stationary background objects, and moving objects that remain stable
for more than the half of the training period. Experimental results show the efficiency and the robustness of the proposed
method to detect and extract the silhouettes of moving objects in outdoor and indoor environments compared with conventional
methods. 相似文献
993.
Three lanthanide complexes (La(III), Er(III), and Yb(III)) derived from ferrocene-based Schiff base ligand (HL) were synthesized from condensation of 2-aminophenol with 2-acetylferrocene. The ligand and metal complexes were characterized based on elemental analyses, IR, 1H NMR, molar conductance, SEM and thermal analyses (TG, DTG). The molar conductance revealed that all the metal chelates were electrolytes having the general composition [M(L)(Cl)(H2O)3]Cl·4H2O. HL and its complexes were screened for their antibacterial and antifungal activity by agar diffusion method. The results of these studies showed that the metal complexes are more effective antibacterial and antifungal agents as compared with the free ligand. The anticancer activity was screened against human breast cancer cell line (MCF-7). Results indicated that metal complexes showed an increased cytotoxicity in proliferation to cell lines as compared to free ligand. Molecular docking studies were performed to identify the binding orientation or conformation of a complex in the active site of the protein. HL and its complexes were docked with crystal structure of DDB1 of breast cancer, crystal structure of HCV, RNA-dependent RNA polymerase, receptors of HBV core protein, crystal structure of the Fab fragment of anti-HAV. 相似文献
994.
Heba A. El Gizawy Sylvia A. Boshra Ahmed Mostafa Sara H. Mahmoud Muhammad I. Ismail Aisha A. Alsfouk Azza T. Taher Ahmed A. Al-Karmalawy 《Molecules (Basel, Switzerland)》2021,26(19)
In response to the urgent need to control Coronavirus disease 19 (COVID-19), this study aims to explore potential anti-SARS-CoV-2 agents from natural sources. Moreover, cytokine immunological responses to the viral infection could lead to acute respiratory distress which is considered a critical and life-threatening complication associated with the infection. Therefore, the anti-viral and anti-inflammatory agents can be key to the management of patients with COVID-19. Four bioactive compounds, namely ferulic acid 1, rutin 2, gallic acid 3, and chlorogenic acid 4 were isolated from the leaves of Pimenta dioica (L.) Merr (ethyl acetate extract) and identified using spectroscopic evidence. Furthermore, molecular docking and dynamics simulations were performed for the isolated and identified compounds (1–4) against SARS-CoV-2 main protease (Mpro) as a proposed mechanism of action. Furthermore, all compounds were tested for their half-maximal cytotoxicity (CC50) and SARS-CoV-2 inhibitory concentrations (IC50). Additionally, lung toxicity was induced in rats by mercuric chloride and the effects of treatment with P. dioca aqueous extract, ferulic acid 1, rutin 2, gallic acid 3, and chlorogenic acid 4 were recorded through measuring TNF-α, IL-1β, IL-2, IL-10, G-CSF, and genetic expression of miRNA 21-3P and miRNA-155 levels to assess their anti-inflammatory effects essential for COVID-19 patients. Interestingly, rutin 2, gallic acid 3, and chlorogenic acid 4 showed remarkable anti-SARS-CoV-2 activities with IC50 values of 31 µg/mL, 108 μg/mL, and 360 µg/mL, respectively. Moreover, the anti-inflammatory effects were found to be better in ferulic acid 1 and rutin 2 treatments. Our results could be promising for more advanced preclinical and clinical studies especially on rutin 2 either alone or in combination with other isolates for COVID-19 management. 相似文献
995.
Adel Mahmoud Gomaa 《Czechoslovak Mathematical Journal》2017,67(2):339-365
We give existence theorems for weak and strong solutions with trichotomy of the nonlinear differential equation where {?(t): t ∈ R} is a family of linear operators from a Banach space E into itself and f: R × E → E. By L(E) we denote the space of linear operators from E into itself. Furthermore, for a < b and d > 0, we let C([?d, 0],E) be the Banach space of continuous functions from [?d, 0] into E and f d : [a, b] × C([?d, 0],E) → E. Let \(\hat {\mathcal{L}}:[a,b] \to L(E)\) be a strongly measurable and Bochner integrable operator on [a, b] and for t ∈ [a, b] define τ t x(s) = x(t + s) for each s ∈ [?d, 0]. We prove that, under certain conditions, the differential equation with delay has at least one weak solution and, under suitable assumptions, the differential equation (Q) has a solution. Next, under a generalization of the compactness assumptions, we show that the problem (Q) has a solution too.
相似文献
$$\dot x\left( t \right) = \mathcal{L}\left( t \right)x\left( t \right) + f\left( {t,x\left( t \right)} \right),t \in \mathbb{R}$$
(P)
$$\dot x\left( t \right) = \hat {\mathcal{L}}\left( t \right)x\left( t \right) + {f^d}\left( {t,{\tau _t}x} \right),ift \in \left[ {a,b} \right],$$
(Q)
996.
Russul M. Adnan Malak Mezher Alaa M. Abdallah Ramadan Awad Mahmoud I. Khalil 《Molecules (Basel, Switzerland)》2023,28(1)
This study aims to investigate the effect of magnesium (Mg) doping on the characteristics and antibacterial properties of copper oxide (CuO) nanoparticles (NPs). The Mg-doped CuO NPs were fabricated by the co-precipitation method. NPs were characterized by X-ray Powder Diffraction (XRD), Transmission Electron Microscope (TEM), Energy Dispersive X-ray (EDX) analysis, Fourier Transform Infrared Spectroscopy (FTIR), and Photoluminescence (PL). Broth microdilution, agar-well diffusion, and time-kill assays were employed to assess the antibacterial activity of the NPs. XRD revealed the monoclinic structure of CuO NPs and the successful incorporation of Mg dopant to the Cu1−xMgxO NPs. TEM revealed the spherical shape of the CuO NPs. Mg doping affected the morphology of NPs and decreased their agglomeration. EDX patterns confirmed the high purity of the undoped and Mg-doped CuO NPs. FTIR analysis revealed the shifts in the Cu–O bond induced by the Mg dopant. The position, width, and intensity of the PL bands were affected as a result of Mg doping, which is an indication of vacancies. Both undoped and doped CuO NPs exhibited significant antibacterial capacities. NPs inhibited the growth of Gram-positive and Gram-negative bacteria. These results highlight the potential use of Mg-doped CuO NPs as an antibacterial agent. 相似文献
997.
Mahmoud Filali 《Journal of Functional Analysis》2007,250(1):144-166
The paper proposes a unified approach to many key theorems proved in the last twenty years in different areas of abstract harmonic analysis. This approach is based on the so-called slowly oscillating functions which were introduced in coarse geometry. In addition to this method being the most natural and simple, it also leads to the generalisation of some of the results and to the achievement of some new results. Several of these results concern the topological centres of convolution algebras and semigroup compactifications. 相似文献
998.
Three‐component condensation of ninhydrin, malononitrile, and some nucleophilic reagents in the presence of piperidine under microwave irradiation without solvent afforded the corresponding spiro‐fused pyran derivatives. The structures of the products were proved by elemental analyses, IR, 1H NMR and MS spectroscopy. 相似文献
999.
M Hosein Arbab-Zavar G Hosein Rounaghi Mahmoud Chamsaz Mahboube Masrournia 《Analytical sciences》2003,19(5):743-746
A technique for determination of mercury is described; it is based on electrolytic reduction of Hg(II) ion on a graphite cathode, the trapping of mercury vapor and its volatilization into a quartz tube aligned in the optical path of an atomic absorption spectrometer. The electrochemical cell consisted of a graphite cathode and an anode operating with constant direct current for the production of mercury atoms. A pre-activated graphite rod was used as the cathode material. The optimum conditions for electrochemical generation of mercury cold vapor (the electrolysis time and current, the flow rate, the type of electrode and electrolyte) were investigated. The characteristic electrochemical data with chemical cold vapor using NaBH4-acid were compared. The presence of cadmium(II), arsenic(III), antimony(III), selenium(IV), bismuth(III), silver(I), lead(II), lithium(I), sodium(I) and potassium(I) showed interference effects which were eliminated by suitable separation techniques. The calibration curve is linear over the range of 5-90 ng ml(-1) mercury(II). The detection limit is 2 ng ml(-1) of Hg(II) and the RSD is 2.5% (n = 10) for 40 ng ml(-1). The accuracy and recovery of the method were investigated by analyzing spiked tap water and river water. 相似文献
1000.
Ahmed S. Abu Zaid Ahmed E. Aleissawy Ibrahim S. Yahia Mahmoud A. Yassien Nadia A. Hassouna Khaled M. Aboshanab 《Molecules (Basel, Switzerland)》2021,26(13)
Background: This study aimed to produce, purify, structurally elucidate, and explore the biological activities of metabolites produced by Streptomyces (S.) griseus isolate , a recovered soil bacterium previously screened in our lab that showed promising cytotoxic activities against various cancer cell lines. Methods: Production of cytotoxic metabolites from S. griseus isolate KJ623766 was carried out in a 14L laboratory fermenter under specified optimum conditions. Using a 3-(4,5-dimethylthazol-2-yl)-2,5-diphenyl tetrazolium-bromide assay, the cytotoxic activity of the ethyl acetate extract against Caco2 and Hela cancer cell lines was determined. Bioassay-guided fractionation of the ethyl acetate extract using different chromatographic techniques was used for cytotoxic metabolite purification. Chemical structures of the purified metabolites were identified using mass, 1D, and 2D NMR spectroscopic analysis. Results: Bioassay-guided fractionation of the ethyl acetate extract led to the purification of two cytotoxic metabolites, R1 and R2, of reproducible amounts of 5 and 1.5 mg/L, respectively. The structures of R1 and R2 metabolites were identified as β- and γ-rhodomycinone with CD50 of 6.3, 9.45, 64.8 and 9.11, 9.35, 67.3 µg/mL against Caco2, Hela and Vero cell lines, respectively. Values were comparable to those of the positive control doxorubicin. Conclusions: This is the first report about the production of β- and γ-rhodomycinone, two important scaffolds for synthesis of anticancer drugs, from S. griseus. KJ623766相似文献