全文获取类型
收费全文 | 1012篇 |
免费 | 110篇 |
国内免费 | 150篇 |
专业分类
化学 | 867篇 |
晶体学 | 13篇 |
力学 | 65篇 |
综合类 | 5篇 |
数学 | 19篇 |
物理学 | 303篇 |
出版年
2023年 | 12篇 |
2022年 | 25篇 |
2021年 | 34篇 |
2020年 | 40篇 |
2019年 | 26篇 |
2018年 | 28篇 |
2017年 | 29篇 |
2016年 | 34篇 |
2015年 | 35篇 |
2014年 | 48篇 |
2013年 | 78篇 |
2012年 | 62篇 |
2011年 | 52篇 |
2010年 | 39篇 |
2009年 | 54篇 |
2008年 | 73篇 |
2007年 | 66篇 |
2006年 | 69篇 |
2005年 | 49篇 |
2004年 | 40篇 |
2003年 | 42篇 |
2002年 | 31篇 |
2001年 | 35篇 |
2000年 | 43篇 |
1999年 | 31篇 |
1998年 | 24篇 |
1997年 | 27篇 |
1996年 | 14篇 |
1995年 | 21篇 |
1994年 | 13篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 6篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1977年 | 1篇 |
排序方式: 共有1272条查询结果,搜索用时 234 毫秒
21.
The effect of the biomass presence on the overall circulation velocity, the linear velocities both in the riser and the downcomer
and the overall gas hold-up was studied in a three-phase internal loop airlift reactor (ILALR). The measured data were compared
with those obtained using a two-phase system (air—water). All experiments were carried out in a 40 dm3 ILALR at six different biomass concentrations (ranging from 0 g dm−3 to 7.5 g dm−3), at a temperature of 30°C, under atmospheric pressure. Air and water were used as the gas and liquid model media, respectively.
Pellets of Aspergillus niger produced during the fermentation of glucose to gluconic acid in the ILALR were considered solid phase. In addition, liquid
velocities were measured during the fermentation of glucose to gluconic acid using Aspergillus niger. All measurements were performed in a bubble circulation regime. At given experimental conditions the effect of the biomass
on the circulation velocities in the ILALR was negligible. However, increasing of the biomass concentration led to lower values
of the total gas hold-up.
Presented at the 33rd International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 22–26 May
2006. 相似文献
22.
99Mo1、31I和89Sr等医用同位素对人类健康和医学的发展具有非常重要的作用。与靶辐照反应堆相比,用水溶液堆生产99Mo1、31I和89Sr具有安全性好,结构简单,经济价值高,无靶件制备、溶解工艺,产生废物少等优点,用水溶液堆生产医用同位素具有很好的发展前景。由于多堆芯水溶液堆、高功率水溶液堆均能显著提高产率,低富集度235U水溶液堆符合核不扩散条约中对235U浓缩度的要求,因此这三种堆是水溶液堆未来的发展方向。 相似文献
23.
The effect of the shear rate on pullulan production from beet molasses by Aureobasidium pullulans P56 in an airlift reactor was investigated. A maximum polysaccharide concentration (18.5 g/L), biomass dry weight (14.0 g/L),
polysaccharide yield (38.5%), and sugar utilization (96%) was achieved at a shear rate of 42 s−1. A. pullulans grown on beet molasses produced a mixture of pullulan and other polysaccharides. The highest value of pullulan proportion
(30% of total polysaccharide) was obtained at a low shear rate (42 s−1). The apparent viscosity of the fermentation broth increased as the shear rate increased up to 42 s−1 and then decreased. On the other hand, the dissolved oxygen concentration and the volumetric mass transfer coefficient increased
with the increase of the shear rate from 21 to 84 s−1. The external addition of L-glutamic acid, olive oil, and Tween-80 improved significantly the production of crude polysaccharide (27.0 g/L), but the
pullulan content of the polysaccharide was low (20%). 相似文献
24.
甲烷直接氧化制甲醇Ⅲ.反应气吹扫催化膜反应器(RSCMR) 总被引:2,自引:0,他引:2
用溶胶-凝胶法制备了“SiO_2/陶瓷”非对称无机膜,并用该膜制备了反应气吹扫催化膜反应器(RSCMR)装置。在RSCMR上考察了甲烷一步催化氧化制甲醇反应。结果表明,在研究的范围内,增加氧气或甲烷的浓度和吹扫气的流速(即总的反应气流量)有利于提高甲醇的收率;甲醇在反应条件下的热不稳定性是影响目标反应选择性的重要原因。RSCMR较膜反应器(CMR)系统能更有效地抑制甲醇的热分解,因而可得到较CMR更高的甲醇收率。当反应温度为700℃时,甲醇的收率在CMR中为0.5g/m ̄2h,在RSCMR中可达0.9g/m ̄2h。 相似文献
25.
26.
脉冲电晕环境中13X分子筛对NO分解作用的研究 总被引:2,自引:0,他引:2
在常温、常压下,利用脉冲电晕放电产生冷等离子体,使一氧化氮发生分解,直接生成氮气和氧气,用四极杆质谱仪在线测量反应过程中反应物和生成物的变化。在自行研制的实验台上,考察了13X分子筛在不同的脉冲电晕放电条件下,表现出来的不同特性及其对反应转化率的影响。在30 ℃~430 ℃、流量375 mL/min~1 333 mL/min,分析了该反应过程中13X分子筛对转化率的促进作用。在同一脉冲放电条件下,控制13X分子筛的温度为200 ℃,转化率从30 ℃时的1.2%上升到19.7%;转化率最高可以达到35.9%。并对反应过程中13X分子筛的吸附特性做了初步探讨。 相似文献
27.
The Total Isomerization Process developed by Union Carbide in 1970 (Gary, 1987) for the conversion of normal paraffin's to their isomers consists of a reactor followed by a PSA unit each operating at similar pressures and temperatures. The combination of these two operations in one unit in a Pressure Swing Adsorption Reactor (PSAR) process may provide an increased throughput and a significant cost saving in ancillary equipment.The simulation of a mathematical model linking the catalyst packed-bed and the adsorbent packed-bed is reported. The catalyst is a Pd/Y-zeolite and the adsorbent is 5A zeolite. The simulated feed consists of 17% each of n- and isopentane with the remainder being hydrogen. The mathematical model assumes dispersed plug-flow in both sections, constant velocity in the reactor section but varying in the adsorber, with mass transfer in the adsorber section due to external fluid film resistance and macropore diffusion in series. The fraction of the total column length occupied by the catalyst (denoted by ) is accounted for in the model by solving numerically using orthogonal collocation on finite elements. Parameters varied are the ratio of catalyst/column length (), temperature range (506–533 K), high pressure (15–20 bars), with the low pressure held constant at 2 bars. The catalyst/column ratio has a strong effect at low temperatures. The optimum catalyst/column length ratio appears to be controlled by the low pressure step and occurs at = 0.7 for the assumptions used in this work. 相似文献
28.
Enhancing the enzymatic hydrolysis of cellulosic materials using simultaneous ball milling 总被引:1,自引:0,他引:1
Mais Ursula Esteghlalian Ali R. Saddler John N. Mansfield Shawn D. 《Applied biochemistry and biotechnology》2002,98(1-9):815-832
One of the limiting factors restricting the effective and efficient bioconversion of softwood-derived lignocellulosic residues
is the recalcitrance of the substrate following pretreatment. Consequently, the ensuing enzymatic process requires relatively
high enzyme loadings to produce monomeric carbohydrates that are readily fermentable by ethanologenic microorganisms. In an
attempt to circumvent the need for larger enzyme loadings, a simultaneous physical and enzymatic hydrolysis treatment was
evaluated. A ball-mill reactor was used as the digestion vessel, and the extent and rate of hydrolysis were monitored. Concurrently,
enzyme adsorption profiles and the rate of conversion during the course of hydrolysis were monitored. α-Cellulose, employed
as a model substrate, and SO2-impregnated steam-exploded Douglas-fir wood chips were assessed as the cellulosic substrates. The softwood-derived substrate
was further posttreated with water and hot alkaline hydrogen peroxide to remove >90% of the original lignin. Experiments at
different reaction conditions were evaluated, including substrate concentration, enzyme loading, reaction volumes, and number
of ball beads employed during mechanical milling. It was apparent that the best conditions for the enzymatic hydrolysis of
α-cellulose were attained using a higher number of beads, while the presence of air-liquid interface did not seem to affect
the rate of saccharification. Similarly, when employing the lignocellulosic substrate, up to 100% hydrolysis could be achieved
with a minimum enzyme loading (10 filter paper units/g of cellulose), at lower substrate concentrations and with a greater
number of reaction beads during milling. It was apparent that the combined strategy of simultaneous ball milling and enzymatic
hydrolysis could improve the rate of saccharification and/or reduce the enzyme loading required to attain total hydrolysis
of the carbohydrate moieties. 相似文献
29.
Studies of the Methane Steam Reforming Reaction at High Pressure in a Ceramic Membrane Reactor 下载免费PDF全文
The effects of temperature and pressure on the steam reforming of methane 3H2+CO) were investigated in a membrane reactor (MR) with a hydrogen permeable membrane. The studies used a novel silica-based membrane prepared by using the chemical vapor deposition (CVD) technique with a permeance for H2 of 6.0×l0-8 mol·m-2·s-1·Pa-1 at 923 K. The results in a packed-bed reactor (PBR) were compared to those of the membrane reactor at various temperatures (773-923 K) and pressures (1-20 atm, 101.3-2026.5 kPa) using a commercial Ni/MgAl2O4 catalyst. The conversion of methane was improved significantly in the MR by the countercurrent removal of hydrogen at all temperatures and allowed product yields higher than the equilibrium to be obtained. Pressure had a positive effect on the hydrogen yield because of the increase in driving force for the permeance of hydrogen. The yield of hydrogen increased with pressure and reached a value of 73×10-6 mol·g-1·s-1 at 2026.5 kPa and 923 K which was higher by 108% than the value of 35×10-6 mol·g-1·s-1 obtained for the equilibrium yield. The results obtained with the silica-based membrane were similar to those obtained with various other membranes as reported in the literature. 相似文献
30.