全文获取类型
收费全文 | 23480篇 |
免费 | 482篇 |
国内免费 | 736篇 |
学科分类
工业技术 | 24698篇 |
出版年
2023年 | 49篇 |
2022年 | 109篇 |
2021年 | 185篇 |
2020年 | 138篇 |
2019年 | 250篇 |
2018年 | 413篇 |
2017年 | 391篇 |
2016年 | 453篇 |
2015年 | 354篇 |
2014年 | 564篇 |
2013年 | 1532篇 |
2012年 | 911篇 |
2011年 | 1407篇 |
2010年 | 1099篇 |
2009年 | 1280篇 |
2008年 | 1295篇 |
2007年 | 1298篇 |
2006年 | 1133篇 |
2005年 | 1038篇 |
2004年 | 855篇 |
2003年 | 798篇 |
2002年 | 759篇 |
2001年 | 736篇 |
2000年 | 701篇 |
1999年 | 736篇 |
1998年 | 1479篇 |
1997年 | 1050篇 |
1996年 | 909篇 |
1995年 | 565篇 |
1994年 | 457篇 |
1993年 | 373篇 |
1992年 | 239篇 |
1991年 | 245篇 |
1990年 | 156篇 |
1989年 | 170篇 |
1988年 | 120篇 |
1987年 | 86篇 |
1986年 | 59篇 |
1985年 | 59篇 |
1984年 | 37篇 |
1983年 | 31篇 |
1982年 | 26篇 |
1981年 | 29篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 20篇 |
1975年 | 11篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Singular-value-decomposition (SVD)-based moving-average (MA) order determination of non-Gaussian processes using higher-order statistics is addressed. It is shown that the MA order determination of autoregressive moving-average (ARMA) models is equivalent to the rank determination of a certain error matrix, and a SVD approach is proposed. Its simplified form is applied to pure MA models. To improve the robustness of the order selection, a combination of the SVD and the product of diagonal entries (PODE) test is proposed. Some interesting applications of the two SVD approaches are presented, and simulations verify their performance 相似文献
102.
Base excision repair (BER) constitutes a ubiquitous excision repair mechanism, which is responsible for the removal of multiple types of damaged and inappropriate bases in DNA. We have employed a yeast cell-free system to examine the biochemical mechanism of the BER pathway in lower eukaryotes. Using uracil-containing DNA as a model substrate, we demonstrate that yeast BER requires Apn1 protein, an Escherichia coli endonuclease IV homolog. In extracts of an apn1 deletion mutant, the 5'-incision at AP (apurinic/apyrimidinic) sites is not detectable, supporting the notion that yeast contains only one major 5'-AP endonuclease. The processing of the 5'-deoxyribose phosphate moieties was found to be a rate-limiting step. During BER of uracil-containing DNA, repair patch sizes of 1-5 nucleotides were detected, with single nucleotide repair patches predominant. 相似文献
103.
In 1989, 125 African-American and 123 European-American girls aged 12-14 years were enrolled in a 2-year study in which they maintained a menstrual calendar, recording the date and amount of menstrual bleeding. Weight, exercise, and stress during the previous week were recorded at the start of each menstrual cycle. Although only minor ethnic differences were observed in expected cycle length (29.3 vs. 28.8 days for European-American and African-American girls, respectively), more prominent differences were observed in the between-subjects standard deviation of cycle length (2.9 vs. 2.2 days, respectively) and in the odds of having a cycle longer than 45 days (odds ratio=1.86, 95% confidence interval 1.17-2.97) for European-American compared with African-American girls. Low weight for height and high levels of exercise increased the probability of having a cycle longer than 45 days and decreased expected cycle length of 13- to 45-day cycles. Additional investigation of potential ethnic differences in menstrual cycle characteristics is warranted. 相似文献
104.
Large-signal phase retardation with a poled electrooptic fiber 总被引:3,自引:0,他引:3
X.-C. Long S.R.J. Brueck 《Photonics Technology Letters, IEEE》1997,9(6):767-769
A linear electrooptic coefficient of 0.3 pm/V is induced in a germanosilicate fiber by thermal/electric-field poling. Reducing the fiber thickness by simple mechanical polishing following the poling, leads to a half-wave drive voltage of only 75 V for a 12-cm active length with no measurable linear optical loss in the fiber. The induced linear electrooptic coefficient shows no decay at room temperature for over four months, and only a 10% decay after heating to 90/spl deg/C for 1000 h. 相似文献
105.
Myostatin (GDF-8) is a member of the transforming growth factor-beta superfamily and plays a role in muscle growth and development. Mice having targeted disruption of this gene display marked increases in muscle mass, a phenotype similar to the muscular hypertrophy (mh) in several cattle breeds. Physical mapping data developed from YAC clones indicate the bovine myostatin gene lies close to the centromere of bovine Chromosome (Chr) 2 (BTA2) at 2q11, indistinguishable from the cytogenetic location of the mh locus. In addition, a polymorphism in the second intron of the gene was used to show that myostatin maps within the interval previously shown to contain mh. These data suggest myostatin may be the gene causing muscular hypertrophy in cattle. 相似文献
106.
Y Ben-Ari R Khazipov X Leinekugel O Caillard JL Gaiarsa 《Canadian Metallurgical Quarterly》1997,20(11):523-529
The main ionotropic receptors (GABAA, NMDA and AMPA) display a sequential participation in neuronal excitation in the neonatal hippocampus. GABA, the principal inhibitory transmitter in the adult CNS, acts as an excitatory transmitter in early postnatal stage. Glutamatergic synaptic transmission is first purely NMDA-receptor based and lacks functional AMPA receptors. Therefore, initially glutamatergic synapses are 'silent' at resting membrane potential, NMDA channels being blocked by Mg2+. However, when GABA and glutamatergic synapses are coactivated during the physiological patterns of activity, GABAA receptors can facilitate the activation of NMDA receptors, playing the role conferred to AMPA receptors later on in development. Determining the mechanisms underlying the development of this 'ménage à trois' will shed light not only on the wide range of trophic roles of glutamate and GABA in the developing brain, but also on the significance of the transition from neonatal to adult forms of plasticity. 相似文献
107.
DK Bhuyan X Huang G Kuriakose WH Garner KC Bhuyan 《Canadian Metallurgical Quarterly》1997,16(6):519-526
This article takes issue with procedural reductionism, which is the inclination to reduce all matters of judgement and responsibility to the following of some procedure or rule. Two scenarios provide content for a discussion of professional discretion in the context of accountability. The author shows that in professional life there will always be situations that stand beyond the rules of procedures and require the unique judgement of the professional at the time. While this judgement may be determined by the facts available in a situation of uncertainty, it cannot be reduced to the facts (including facts about rules and procedures). The moral judgement will still have an essential indeterminancy about it. 相似文献
108.
109.
P Peretti-Viton D Taieb JM Viton A Flori X Muracciole V Benguigui G Salamon 《Canadian Metallurgical Quarterly》1998,40(12):783-787
Our purpose was to compare prospectively the sensitivity of contrast-enhanced magnetisation transfer (MT) MRI and gradient-echo (GE) T1-weighted images in metastatic disease of the brain. We studied 52 patients with brain metastases, using conventional T1-weighted GE and MT spin-echo (SE) images after the same standard dose of gadolinium. Axial 5-mm reconstructions of GE data were compared with 5-mm MT images in the same plane. Metastases were counted independently by two neuroradiologists. In 12 patients (23%) MT imaging showed more metastases than GE images (P = 0.03). We detected 68 more metastases with the former technique. 相似文献
110.
G. M. Gusev X. Kleber U. Gennser D. K. Maude J. C. Portal D. I. Lubyshev P. Basmaji M. de P. A. Silva J. C. Rossi Yu. V. Nastaushev M. R. Baklanov 《Solid-state electronics》1996,40(1-8):441-446
Electron scattering by a single barrier is predicted to reveal singularities as the magnetic field is changed, because the number of electron collisions with the barrier dramatically increases as chaotic orbits around the barrier are changed into periodic orbits. To test this experimentally we have measured the magnetoresistance of AlGaAs/GaAs heterostructures with a two-dimensional electron gas and a lateral lattice containing a macroscopic number of oval-shaped antidots fabricated using electron lithography. Reproducible fluctuations in the magnetoresistance are observed at low field, which are due to the oscillations of the number of electron collisions with the antidots. The number of collisions N before the electron escapes from the antidot has been calculated as a function of B in an electric field. The position of the maxima in N(B) obtained from calculations and experiment are in reasonable agreement. 相似文献