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991.
The nonenzyme browning involves the thermal decomposition of sugars, the caramelization, the decomposition of oxi‐acids, the so called “Maillard reaction” between amino acids and carbohydrates, the reaction between oxidized fats and proteins, and those alterations which take place by the alkaline treatment of proteins. The Maillard reaction is of secondary importance in the case of foodstuffs and fodders with low carbohydrate contents (meats, meat meal, fish meal). By the heat treatment, the sulphur‐containing amino acids of proteins (cystine, methionine) are damaged primarily because of oxidation, but the decrease in the amount of threonine, serine, tryptophan, and lysine is observable too. According to the formation of enzyme resistant cross‐links, the in vitro and in vivo digestibility of protein decreases after the heat treatment and the communication with oxidized fats. Besides the amino acids mentioned, the possibility of enzymatic break‐off of leucine and isoleucine is reduced too. In the course of the heating of proteins the occurance of racemization has to be considered too (formation of alloisoleucine). The basic mechanism of the reaction between sugars and simple amino acids is already essentially explained: amino‐acids break off after the formation glycosilamines and Amadori products but they are linked irreversibly to some, partly unsaturated decomposition products of sugars, types of 6 and 3 carbon atoms. The decrease in the biological usability of amino acids starts already with the Amadori products. The reactivity of the single amino acids depends on the number of carbon atoms, on the basicity, and on the polarity of the amino acid molecule. The especially highly reactive amino acids of proteins are (1) the essential lysine (because of its 6‐NH2 group), (2) other types of basic amino acids, and (3) tryptophan (because of the lability of the indole ring), methionine, cystine and threonine. In the Maillard reaction of tryptophan the —NH— group of the indole ring is involved too. The Maillard reaction is highly influenced by the pH of foodstuffs or other agents. The reduction of pH which may be performed by the increase of fermentation in the baking industry, lessens the decomposition of lysine and tryptophan in proteins. The raise of pH in basic domain enhances the Maillard reaction up to a maximal value but a decrease may be observed when the pH is raised further on. In foodstuffs and in other solid protein‐carbohydrate systems the increase of the moisture content generally enhances the Maillard reaction, the sensibility of the single amino acids to the changes in the moisture content is different. In the case of the alkaline treatment of proteins, we must reckon not only with the decomposition of single amino acids, first of all that of cystine by beta‐elimination, but with the formation of some amino acid derivatives as lysinoalanine, lanthionine, and in ornithinoalanine too. Presently lysinoalanine is of toxicological importance as proved by experiences on rats. The isomerization of essential amino acids may appear too during the alkaline treatment of proteins and this bears a reducing effect on the nutritive’ value. 相似文献
992.
Electrochemical detection techniques in high-performance liquid chromatography are critically surveyed. The general principles of electrochemical detection are outlined and potentiometric, polarographic, voltammetric, coulometric, conductometric and high-frequency detectors are described. The measuring techniques and detector performance are discussed and the operation parameters of electrochemical detectors are briefly compared with the other, most common HPLC detectors. 相似文献
993.
Schlegelová J Nápravníková E Dendis M Horváth R Benedík J Babák V Klímová E Navrátilová P Sustáčková A 《Meat science》2004,66(3):557-565
Meat contaminating bacteria may be the direct cause of foodborne diseases and represent a potential cause for the drug resistance of human pathogenic agents. The prevalence and resistance to 17 antimicrobial drugs of isolates of selected bacterial species were investigated in 70 swabs of beef carcasses and 70 subsequent samples of beef meat. Molecular techniques (coagulase gene typing Staphylococcus aureus and original gene typing Escherichia coli) were used in the differentiation of isolates. Carcasses were already contaminated after evisceration, least frequently with S. aureus strains (7.5% of samples), most frequently with coagulase-negative staphylococci strains (52.2% of samples). During carcass processing, contamination with resistant or polyresistant strains of S. aureus and E. coli significantly increased (P<0.01). Gene typing isolates of S. aureus and E. coli indicated that the strains probably originated in the processing plant. 相似文献
994.
Comparison of HPLC and GLC methodologies for determination of glucosinolates using reference material 总被引:1,自引:0,他引:1
The reference material rapeseed of known glucosinolate composition, was analysed by HPLC and GLC and two commercially available
glucosinolates (glucotropaeolin and sinigrin) were used as internal standards. The HPLC method enabled determination of 11
different glucosinolates (as desulphoglucosinolates) that occur in the rapeseed. Only seven glucosinolates (as trimethylsilylated
desulphoglucosinolates) were separated by GLC. The latter method did not allow the determination of methylsulphinylalkyl glucosinolates
(glucoiberin, glucoraphanin and glucoallysin) and did not separate optical isomers of 2-hydroxy-3-butenyl glucosinolate (progoitrin
and epi-progoitrin). Statistical evaluation of data (t-test, F-test) revealed no significant differences between the tested methods at the 95% confidence level. The advantages and disadvantages
of both widely used chromatographic methods are discussed.
Received: 26 May 1997 / Revised version: 11 July 1997 相似文献
995.
Marta Veningerová Ján Uhnák Viktor Prachar Jana Kovaiová 《Zeitschrift für Lebensmitteluntersuchung und -Forschung A》1996,203(3):309-310
The paper deals with the contamination of human milk with chlorinated phenols. The average and median concentrations of the chlorophenols investigated ranged from 0.75 to 9.74 g.kg–1 and from being not detectable to 5.62 g.kg–1, respectively, for the different compounds. The highest average and median levels were found for pentachlorophenol and 2,4,5-trichlorophenol. The concentrations determined were below the permissible levels. 相似文献
996.
997.
Hájek M Dezortová M Wagnerová D Skoch A Voska L Hejlová I Trunečka P 《Magma (New York, N.Y.)》2011,24(5):297-304
Object
Among several non-invasive methods of liver fat analysis, the most important role is played by MR imaging and spectroscopy (MRS). This study describes the 1H MRS at 3T measurement of liver fat volume fraction ffat{\phi_{{\rm fat}}} in a group of liver transplant patients, an at-risk group for the development of de novo steatosis. 相似文献998.
O. Šolcová L. Matějová P. Topka Z. Musilová P. Schneider 《Journal of Porous Materials》2011,18(5):557-565
Argon(87 K) and nitrogen(77 K) adsorption of three types of porous samples (microporous, microporous-mesoporous and mesoporous)
was evaluated. The shapes of isotherms, specific surface areas, mesopore-, micropore- and net pore volumes, pore-size distributions
(PSD) and positions of micropore and mesopore PSD’s maxima were compared to ascertain the credibility of individual adsorbates
for the texture information estimation. The shapes of adsorption isotherms for Ar and N2 of all samples are similar and the adsorbed amounts at relative pressure x = 0.975 differ slightly. For mesoporous samples some differences are observed between specific mesopore surface areas derived
from nitrogen and argon isotherms. Radii of pore size maxima from Ar(87 K) PSD’s are on average systematically lower than
from N2(77 K) for all samples. The Saito-Foley approach for ZSM-5 samples gives consistently lower mean pore radius from N2(77 K) adsorbate than from Ar(87 K). This difference probably arises from the multiplication factor, Ω, in the Saito-Foley
equation which includes physical properties (magnetic susceptibility, polarizability etc. at 77 and 87 K) and is not easy to obtain with sufficient precision. The use of both adsorbates Ar(87 K) and N2(77 K) possesses some advantages as well some disadvantages and the comparison of textural properties of individual samples
must be evaluated with respect to adsorbate. 相似文献
999.
Poly(ethylene glycol dimethacrylate–1‐vinyl‐1,2,4‐triazole) [poly(EGDMA–VTAZ)] beads with an average diameter of 100–200 μm were obtained by the copolymerization of ethylene glycol dimethacrylate (EGDMA) with 1‐vinyl‐1,2,4‐triazole (VTAZ). The copolymer hydrogel bead composition was determined by elemental analysis and was found to contain 5 EGDMA monomer units for each VTAZ monomer unit. The poly(EGDMA–VTAZ) beads were characterized by swelling studies and scanning electron microscopy (SEM). The specific surface area of the poly(EGDMA–VTAZ) beads was found 65.8 m2/g. Cu2+ ions were chelated on the poly(EGDMA–VTAZ) beads. The Cu2+ loading was 82.6 μmol/g of support. Cu2+‐chelated poly(EGDMA–VTAZ) beads with a swelling ratio of 84% were used in the immobilization of Aspergillus niger glucoamylase in a batch system. The maximum glucoamylase adsorption capacity of the poly(EGDMA–VTAZ)–Cu2+ beads was 104 mg/g at pH 6.5. The adsorption isotherm of the poly(EGDMA–VTAZ)–Cu2+ beads fitted well with the Langmuir model. Adsorption kinetics data were tested with pseudo‐first‐ and second‐order models. The kinetic studies showed that the adsorption followed a pseudo‐second‐order reaction model. The Michaelis constant value for the immobilized glucoamylase (1.15 mg/mL) was higher than that for free glucoamylase (1.00 mg/mL). The maximum initial rate of the reaction values were 42.9 U/mg for the free enzyme and 33.3 U/mg for the immobilized enzyme. The optimum temperature for the immobilized preparation of poly(EGDMA–VTAZ)–Cu2+–glucoamylase was 65°C; this was 5°C higher than that of the free enzyme at 60°C. The glucoamylase adsorption capacity and adsorbed enzyme activity slightly decreased after 10 batch successive reactions; this demonstrated the usefulness of the enzyme‐loaded beads in biocatalytic applications. The storage stability was found to increase with immobilization. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011 相似文献
1000.
Chemical polymorphism may contribute to variation in browsing damage by mammalian herbivores. Earlier, we demonstrated that
essential oil concentration in juniper, Juniperus communis, was negatively associated with herbivore browsing. The aim of the present study was to characterize the volatile chemical
composition of browsed and non-browsed J. communis. By using either gas chromatography with flame ionization detection (GC-FID) or an electronic nose device, we could separate
sheep-browsed or non-browsed juniper shrubs by their essential oil pattern and complex odor matrix. The main components of
the essential oil from J. communis were monoterpenes. We distinguished three chemotypes, dominated either by α-pinene, sabinene, or δ-3-carene. Shrubs belonging
to the α-pinene- or sabinene-dominated groups were browsed, whereas all individuals with the δ-3-carene chemotype were unused
by the local herbivores. The electronic nose also separated the browsed and non-browsed shrubs indicating that their odor
matrix could guide sheep browsing. Responses of sheep could integrate the post-ingestive effects of plant secondary metabolites
with sensory experience that stems from odor–phytotoxin interactions. Chemotype diversity could increase the survival rate
in the present population of J. communis as certain shrubs could benefit from relatively better chemical protection against the herbivores. 相似文献