当前位置：文档库 › Preparation and properties of cellulose nanocrystals - Rods, spheres, and network

Preparation and properties of cellulose nanocrystals - Rods, spheres, and network

Carbohydrate Polymers 82 (2010) 329–336

Contents lists available at ScienceDirect

Carbohydrate

Polymers

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c a r b p o

Preparation and properties of cellulose nanocrystals:Rods,spheres,and network

Ping Lu,You-Lo Hsieh ?

Fiber and Polymer Science,University of California,One Shields Avenue,Davis,CA 95616,USA

a r t i c l e i n f o Article history:

Received 6March 2010

Received in revised form 21April 2010Accepted 22April 2010

Keywords:

Cellulose nanocrystals Acid hydrolysis Freeze-drying

Mesoporous structure

a b s t r a c t

Cellulose nanocrystals with rod,sphere,and network morphologies were prepared by acid hydrolysis of cotton cellulose,followed by freeze-drying.Hydrolysis with sulfuric acid introduced sulfate groups to these nanocrystal surfaces permitting their dispersion in aqueous as well as organic media,includ-ing ethanol and N,N-dimethylformamide,in a matter of seconds.Freeze-drying,on the other hand,induced mesoporosity (91.99±2.57?average pore width)and signi?cantly improved speci?c surface (13.362m 2/g)that is about 9times of the original cellulose (1.547m 2/g).Moreover,the nanocrystals exhibited improved thermal conductivity and considerably higher (nearly 30%)carbonaceous residue,possibly due to direct solid-to-gas decomposition.These results demonstrated that a combination of surface charge introduction and ?xation of mesoporosity in cellulose nanocrystals is an ef?cient route to prepare large quantity of high quality cellulose nanocrystals with quick re-dispersion capability for practical applications.

1.Introduction

Cellulose,the most abundant biomass in the world,is a linear syndiotactic homopolymer of ?-(1→4)-glycosidic bonds linked d -anhydroglucopyranose (Kim,Yun,&Ounaies,2006).Native cel-lulose is generally known to be ?brillar and crystalline (Saxena &Brown,2005)and the cellulose ?brils play a signi?cant role in con-tributing to the high strength of plant cell walls (Zuluaga et al.,2009).Crystalline nano?bers with crystallinities from 65%to 95%have been extracted from a broad range of natural sources including cotton (Favier,Chanzy,&Cavaille,1995),tunicate (Terech,Chazeau,&Cavaille,1999),algae (Hanley,Giasson,Revol,&Gray,1992),bac-teria (Grunert &Winter,2002)and wood (Beck-Candanedo,Roman,&Gray,2005).These cellulose ?brils were reported to be 2–20nm wide.Their aspect ratios varied from 40(～200nm long and 5nm wide)for cotton to around 66(～1?m long and 15nm wide)for tunicin whiskers (Samir,Alloin,&Dufresne,2005).

The bending strength and modulus of the cellulose nano?brils estimated (Helbert,Cavaille,&Dufresne,1996;Sakurada,Nukushina,&Ito,1962;Sturcova,Davies,&Eichhorn,2005)and measured by Raman spectroscopy (Sturcova et al.,2005)were impressively high at ～10and ～150GPa,respectively.Cellulose nano?bers thus have a bending strength that is nearly one-sixth of the 63GPa for the carbon nanotubes whose tensile strength is predicted to be as high as

?Corresponding author.Tel.:+15307520843;fax:+15307527584.E-mail address:ylhsieh@https://www.wendangku.net/doc/1f8832167.html, (Y.-L.Hsieh).～300GPa at E of ～1TPa (Wong,Sheehan,&Lieber,1997;Yu et al.,2000),but can be prepared far more economically from read-ily available renewable resources.Various cellulose nano?brils,nanocrystals and whiskers have been incorporated into polymer matrices to produce reinforced composites with several tens to hundreds folds higher mechanical strength (Beecher,2007;Lu &Hsieh,2009;Svagan,Samir,&Berglund,2008)as well as enhanced optical transparency (Ifuku et al.,2007).Cellulose nano?brils have been used as substrates to determine cellulase activity (Helbert,Chanzy,Husum,Schuelein,&Ernst,2003)and as carriers for targeted delivery of therapeutics (Dong &Roman,2007).With the layer-by-layer (LbL)technique,cellulose nanowires have been assembled into antire?ective ?lms (Podsiadlo et al.,2007)and high performance nanocomposites (Podsiadlo et al.,2005).

The major challenge of developing the cellulose nano?bers as advanced materials and for further applications is their tendency to form bundles or aggregates.During drying,the abundant hydro-gen bonds of cellulose draw the cellulose nanocrystals together to pose signi?cant problems in their re-dispersion for effec-tive processing (Tingaut,Zimmermann,&Lopez-Suevos,2010).To enable better utilization,it is crucial to develop methods to isolate the nano?brils after the solvent evaporation in their preparation.

This study was to investigate the hydrolysis and drying pro-cesses with the intent to minimize hydrogen bonding,thus reduce and even eliminate aggregation of the cellulose nanocrystals.Homogenous and stable cellulose nanocrystals suspensions were generated by hydrolyzing native cellulose with sulfuric acid to introduce negative charges to the nanocrystal surfaces.Esteri?-

330P.Lu,Y.-L.Hsieh/Carbohydrate Polymers82 (2010) 329–336

cation of surface hydroxyl groups of cellulose nanocrystals has shown to introduce sulfate groups to form stable suspensions (Beck-Candanedo et al.,2005).The focus was then to prevent hydro-gen bond formation by sustaining repulsion among the cellulose nanocrystals with fast freezing of water among the well-dispersed cellulose nanocrystals with liquid nitrogen to keep them separated and?xed in the solidi?ed ice.The high vacuum in freeze-drying then sublimates the ice in-between the cellulose nanocrystals to substantially reduce or prevent hydrogen bonding,the driving force for the cellulose nanocrystals to aggregate.The induced morpholo-gies and properties of the cellulose nanocrystals were studied to relate to the hydrolysis and drying processes.

2.Experimental

2.1.Materials

Cotton cellulose from?lter paper(Q2,Whatman)was purchased from Fisher Scienti?c(Pittsburgh,PA).Sulfuric acid(95–98%)for hydrolysis was provided by EMD(Gibbstown,NJ).Water used in all experiments was puri?ed by a Millipore Milli-Q Plus(Billerica, MA)water puri?cation system.

2.2.Sulfuric acid hydrolysis

The cellulose?lter was milled(Thomas-Wiley Laboratory Mill model4,Thomas Scienti?c,USA)to pass through a60-mesh screen.Hydrolysis was performed using64–65%(w/w)sulfuric acid (10mL/g cellulose)at45?C for60min and stopped by diluting with 10-fold cold(4?C)water(Beck-Candanedo et al.,2005).The suspen-sion was washed once by centrifugation at4500rpm for10min and then was dialyzed using Fisherbrand(Pittsburgh,PA)regenerated cellulose dialysis membranes with12–14kDa molecular weight cut off and against ultra-pure water(Millipore Milli-Q UF Plus)until neutral pH was reached.The solid aggregates in the suspension were disrupted by sonication(Branson ultrasonic processor model 2510,Danbury,CT)for2h in an ice bath.The suspension was kept over ion-exchange resin(Rexyn I-300H–OH from Fisher Scienti?c, Inc.)for7days and then?ltered through Whatman541?lter paper (Maidstone,Kent,England).The?nal concentration of the suspen-sion was about0.75%(w/w).

2.3.Freeze-drying

The cellulose nanocrystals suspension was quickly frozen by liq-uid nitrogen(N2)in an ice-tray and the formed ice cubes were put into a freeze-dryer(FreeZone1.0L Benchtop Freeze Dry System, Labconco,Kansas City,MO)overnight to remove the solvent water. The ice cubes kept their shape during the entire freeze-drying process without any observable shrinkage.The dried product was stored in vacuum for the following characterizations.

2.4.Characterization

The concentration of produced cellulose nanocrystals in the suspension was determined by drying5mL suspension in air and the mass after water evaporation was weighed.Optical light micro-scope observations were performed by using a Leica DM2500.The magni?cations used were100×and400×under crossed-polars. The microstructures and the surface morphologies of samples were examined by a scanning electron microscope(SEM)(XL30-SFEG, FEI/Philips,Hillsboro,OR,USA)after gold coating(Bio-Rad SEM coating system).For transmission electron microscopy(TEM),the freeze-dried cellulose nanocrystals were re-dispersed into ethanol.

A drop of10?l about0.005%(w/v)cellulose nanocrystals suspen-sion was added onto the carbon-coated electron microscopy grid (Ted Pella Inc.,Redding,CA)and the excess liquid was absorbed by a?lter paper.The specimens were then negatively stained with 2%uranyl acetate.Excess solution was blotted out with a?lter paper and allowed to dry by evaporation at ambient condition.The sample grids were observed at100kV using a Philip transmission electron microscope(Philip CM12).The?ber diameters and their distribution were evaluated by an image analyzer(analySIS FIVE,Soft Imaging System GmbH,Munster,Germany).The FTIR spectra were measured from4000to400cm?1at a resolution of 4cm?1by a Nicolet6700spectrometer(Thermo Fisher Scienti?c, Pittsburgh,PA,USA).The thermal behavior of cellulose powder and nanocrystals was studied by differential scanning calorimetry (DSC)(DSC-60,Shimadzu,Japan)and thermogravimetric analyzer (TGA-50,Shimadzu,Japan)in N2at a heating rate of10?C/min from30to500?C(or600?C for TGA).The crystalline phases present in the samples were measured by X-ray diffraction(XRD) collected on a Scintag XDS2000powder diffractometer(Cupertino, CA)at45kV and40mA from5?to40?with a Ni-?ltered Cu K?1 radiation( =1.542?).The components of cellulose powder and cellulose nanocrystals were measured by the energy-dispersive X-ray spectroscopy(EDS).The surface areas,pore sizes and pore size distribution were measured by N2adsorption–desorption at 77K or the Brunauer–Emmett–Teller(BET)method using a surface area and porosity analyzer(ASAP2020,Micromeritics,Norcross, GA,USA).

3.Results and discussion

3.1.Preparation of cellulose nanocrystals and homogenous suspension

Acid hydrolysis of cellulose in sulfuric acid involves rapid pro-tonation of glucosidic oxygen(path1)or cyclic oxygen(path2)by protons from the acid,followed by a slow splitting of glucosidic bonds induced by the addition of water(Fig.1a).This hydrolysis process yields two fragments with shorter chains while preserving the basic backbone structure.In native cellulose,the amorphous regions are more accessible to acid molecules and susceptible to the hydrolytic actions than the crystalline region(Hon&Shiraishi, 1991).With sulfuric acid,the amorphous regions are digested very fast.Under the conditions employed,i.e.,60min at45?C,most of the amorphous regions were removed leaving cellulose nanocrys-tals in the reaction solution.

Besides chain scission,hydrolyzing cellulose with sulfuric acid also involves esteri?cation of the hydroxyl groups(Fig.1b).This esteri?cation reaction generally proceeds to yield acid half-ester or the so-called‘cellulose sulfate’.The presence of sulfate groups on the cellulose nanocrystal surfaces results in negatively charged surfaces above acidic pH.This anionic stabilization via the repul-sion forces of electrical double layers was shown to be very ef?cient in preventing the aggregation of cellulose nanocrystals driven by hydrogen bonding(Marchessault,Morehead,&Koch,1961). Observations during dialysis also con?rmed this.The colloidal gel was observed at the beginning of dialysis when the cellulose nanocrystals suspension was still acidic,then disappeared as the pH became neutral with the removal of the residual acid.In order to avoid the possible desulfation of the sulfate groups on the surface of the cellulose nanocrystals,ultrasonic treatment was carried out in an ice bath.

3.2.Cellulose nanocrystals prepared by freeze-drying

Under crossed-polars of the optical microscope,the cellulose nanocrystals prepared by freeze-drying showed brighter color and more intensive contrast than the original cellulose powder,a clear

P.Lu,Y.-L.Hsieh/Carbohydrate Polymers82 (2010) 329–336

331

Fig.1.(a)Acid hydrolysis mechanism and(b)esteri?cation of cellulose nanocrystals surfaces.

indication of higher anisotropy(Fig.2).This is expected from pref-erential acid hydrolysis of the less ordered regions in the cellulose, leaving the more highly crystalline cellulose aligned along the?b-ril’s axis.

The SEM images of cellulose powder before acid hydrolysis clearly show the cellulose micro?brils(Fig.3a and b).Following acid hydrolysis at45?C,these micro?brils in the original cellulose were swollen and separated into much smaller crystalline cellulose products in three forms,i.e.,rods(Fig.3c and d),spheres(Fig.3e and f)and porous network(Fig.3g and h).The most abundant form was the spherical nanocrystals while the least was porous network. These three different forms of nanocrystals could not be separated by the common methods of?ltration and centrifugation.The TEM image of cellulose nanocrystal rods showed widths less than10nm and lengths between200and400nm(Fig.3d).The rodlike cellu-lose nanocrystals appeared longer under the SEM,with lengths up to several microns and width in the range of30–50nm.It seems that some aggregations of cellulose nanocrystal rods did occur

to Fig.2.Optical microscope images of(a)cellulose powder and(b)cellulose nanocrystals under crossed-polars.

332P.Lu,Y.-L.Hsieh /Carbohydrate Polymers

82 (2010) 329–336

Fig.3.SEM images of cellulose (a and b)powder and SEM (left,except g which is TEM)and TEM (right)images of nanocrystals in the forms of (c and d)rods,(e and f)spheres,(g and h)porous network.

form larger bundles during freeze-drying.However,these bundles of cellulose nanocrystals rods were loosely packed and easily sepa-rated by ethanol in preparing the TEM sample for observation.The majority of the products from acid hydrolysis and freeze-drying were spherical cellulose nanocrystals.The sources of these cellu-lose spheres (Fig.3e)could be from self-assembled short cellulose rods via interfacial hydrogen bonds and/or the process involved in preparing cotton ?bers into the ?lter paper.The spherical cellu-lose nanocrystals were loosely packed as shown by SEM (Fig.3e)while the isolated spherical cellulose nanocrystals in 10–100nm diameters were observed by TEM (Fig.3f).

While rodlike and spherical cellulose nanocrystals by acid hydrolysis have been reported previously (Beck-Candanedo et al.,2005;Zhang,Elder,Pu,&Ragauskas,2007),some network-

P.Lu,Y.-L.Hsieh/Carbohydrate Polymers82 (2010) 329–336333

structured cellulose nanocrystals were also observed by TEM (Fig.3g and h).The extended network in several microns in both the length and width directions was thought to result from the less dispersed cellulose nanocrystals in ethanol than in aqueous media. The strong H-bonding among cellulose nanocrystals overcomes the repulsion of surface negative charges,leading to the forma-tion of self-assembled porous networks(van den Berg,Capadona, &Weder,2007).These network-structured cellulose nanocrystals were not observed under SEM.It is possible that the network-structured cellulose nanocrystals were imbedded in the abundant spherical cellulose nanocrystals membranes in the freeze-drying process.Another possibility is that the network-structured cel-lulose was actually formed by the over-irradiation of electron beams during TEM observation.However,these network struc-tures were even clearly seen in large quantities under low energy electron beam(80kV).Therefore,further inquiry is needed to fully understand the source and formation of these cellulose networks observed by TEM.Most signi?cantly,the cellulose nanocrystals prepared by freeze-drying could be directly dispersed into water, ethanol,DMF and other solvents in a matter of seconds without son-ication.No aggregate except the networked cellulose was observed by TEM,indicating that the?brils in the as-prepared cellulose nanocrystals samples were isolated.This easily dispersive behavior without aggregation in a wide range of common solvents is highly desirable for versatile processing and making many potential appli-cations possible.

3.3.Chemical and crystalline structures of as-prepared cellulose nanocrystals

The original cellulose powders and nanocrystals showed simi-lar FTIR spectra(Fig.4A).The broad bands in the3650–3000cm?1 region are O–H stretching vibrations and the peaks at2900cm?1 correspond to C–H stretching vibrations.The1430cm?1band can be assigned to bending of the–C-6CH2-while the deformation, wagging and twisting modes of the anhydroglucopyranose unit are observed from1800to600cm?1and are consistent with oth-ers’reports(Mo,Zhao,Chen,Niu,&Shi,2009).Both the cellulose powder and nanocrystals are cellulose I?type.Due to the dif-ferent hydrogen-bonding strength of native cellulose I?and I?, their O–H stretching and out-of-plane bending bands are different, i.e.,stretching at3240cm?1for I?and3270cm?1for I?and out-of-plane bending at750cm?1for I?and710cm?1for I?(Wada, Kondo,&Okano,2003).The crystalline I?characteristic peaks are present in the original powder as well as the nanocrystals:OH stretching at3270cm?1and OH out-of-plane bending at710cm?1.

The main differences in the FTIR are the stronger peak of the adsorbed water(H2O)in the cellulose powders at1641cm?1and the1205cm?1sulfate peak of the cellulose nanocrystals from the esteri?cation reaction.The presence of the sulfate groups was also con?rmed by the EDS(Fig.4B)where the cellulose nanocrystals had a weak sulfur peak which accounted for only0.85at%as com-pared to64.38at%for the carbon peak.This atomic ratio indicates that about1out12of the anhydroglucopyranose units reacted with sulfuric acid to form esters.This is consistent with the notion that the resultant sulfate groups were present mainly on the crystal sur-faces and greatly improved the separation of nanocrystals by charge repulsion.

Both X-ray diffraction diagrams of cellulose powder(Fig.5a) and nanocrystals(Fig.5b)showed three cellulose I characteris-tic peaks at2?=14.7?,16.4?,and22.6?(Wada,Heux,&Sugiyama, 2004).The22.6?peak of the(200)plane of cellulose nanocrystals becomes sharper,indicating higher perfection of the crystal lattice in the(200)plane than original cellulose.The peak for the(1ˉ10) plane(2?=14.7?)also became more intense and separated from the 110(2?=16.4?)peak for cellulose nanocrystals.The XRD

results Fig.4.(A)FTIR spectra of cellulose(a)powder and(b)nanocrystals.(B)EDS of cellulose(a)powder and(b)nanocrystals.

together with the FTIR data con?rm that the cellulose nanocrys-tals retained the cellulose I?crystalline structure following the acid hydrolysis and freeze-drying process while becoming more crystalline than the original cellulose.

3.4.Thermal properties of cellulose nanocrystals

The cellulose nanocrystals also exhibited signi?cantly different thermal behaviors than the original cellulose powders(Fig.6).The original cellulose showed the typical decomposition with onset temperature just above300?C(Fig.6A)and this coincided with a massive mass loss leaving only2.87%ash at600?C(Fig.6B).The cellulose nanocrystals,on the other hand,showed more gradual thermal transitions that started at a lower temperature around 150?C in both DSC and TGA.The cellulose nanocrystals lost nearly 40%of its mass in the150–300?C region followed by another30% mass loss between300and600?C,leaving signi?cantly higher residue,nearly30%,at600?C.These major thermal behavioral differences between the cellulose nanocrystals and the original cel-

334P.Lu,Y.-L.Hsieh /Carbohydrate Polymers

82 (2010) 329–336

Fig.5.XRD of (a)cellulose powder and (b)cellulose nanocrystals.

lulose powders may involve different decomposition–gasi?cation processes.Cellulose is known to decompose to levoglucosan (1,6-anhydro-?-d-glucopyranose)at around 180?C which then gasi?es ef?ciently at 300?C.This gradual mass loss of the cellulose nanocrystals in the 150–300?C temperature range and the appear-ance of small endotherms around 160and 210?C suggest a

different

Fig.6.DSC (A)and TGA (B)of cellulose (a)powder and (b)nanocrystals.

decomposition mechanism,possibly direct solid-to-gas phase tran-sitions catalyzed by surface sulfate groups.It has been reported that the activation energies of the degradation of cellulose nanocrys-tals were signi?cantly lowered by introducing sulfate groups via sulfuric acid hydrolysis (Roman &Winter,2004).Therefore,the thermostability of cellulose nanocrystals was compromised by sulfate groups,agreed well with the above DSC and TGA results.Fur-thermore,the high surface area of cellulose nanocrystals might also play an important role in diminishing their thermostability due to the increased exposure surface area to heat.Moreover,the decom-position of cellulose nanocrystals occurring at lower temperatures from 150to 300?C might also indicate faster heat transfer in cellu-lose nanocrystals.The cellulose nanocrystals have been reported to function as ef?cient pathways for phonons,leading to their higher thermal conductivity (Shimazaki et al.,2007).The better thermal conductivity of cellulose nanocrystals might be ascribed to smaller phonon scattering in the bundle of crystallized cellulose chains in the cellulose nanocrystals than the amorphous random chains in cellulose powder.The lower weight loss of 70.25%(w/w)for cel-lulose nanocrystals indicated nearly one-third mass remained as the residue which was con?rmed to be carbon by FTIR.This is also consistent with the proposed solid-state decomposition of cellu-lose nanocrystals.Since oxygen cannot be entirely eliminated in the circulating nitrogen in the DSC and TGA measurements,further improvement in the yield of amorphous carbon is highly possible.3.5.Speci?c surface areas and pore structures of the freeze-dried cellulose nanocrystals

The N 2adsorption–desorption isotherms at 77K of both the original cellulose (Fig.7a)and the cellulose nanocrystals (Fig.7c)are type IV accompanied by type H3hysteresis,consistent with the report on cellulose (Sing et al.,1985).The Barrett–Joyner–Halenda (BJH)pore size distributions show that most pore diameters of freeze-dried cellulose nanocrystals are below 200?(Fig.7d)while those of cellulose powders are above 200?(Fig.7b),showing mesoporous structure in the cellulose nanocrystals.The mesopores which had a measured mean pore width of 91.99±2.57?,are much smaller than the amorphous pores in the original cellulose with a mean pore width of 214.64±7.23?(Table 1).Such meso-pores could be present in the spheres and network structures,but not in the rods as con?rmed by the SEM and TEM observations (Fig.3).It is very possible that the mesopores also existed as the inter-crystal voids resulting from the evaporation of water and the corresponding driving force to move isolated cellulose nanocrystals closer.

The cumulative pore volume for cellulose nanocrystals (0.03396±0.00059cm 3/g)is 4times larger than that for the orig-inal cellulose (0.00839±0.00026cm 3/g).The smaller cumulative pore volume of the original cellulose was expected with the rigid and highly hydrogen bonded cellulose structures.The pore volume of cellulose nanocrystals increased after loosely packed structures took shape to create a huge amount of mesopores among the nanocrystals.These mesopores can be clearly observed in the SEM images of the freeze-dried nanocrystals samples (Fig.3).The same trend was observed with cumulative surface area of pores for cel-lulose powder and cellulose nanocrystals,but with much larger differences.The cumulative pores surface area of the cellulose nanocrystals (14.771±0.158m 2/g)was 10times of that of the original cellulose powder (1.563±0.019m 2/g).While the original cellulose possessed 0.0910m 2/g micropores in its structure,the negative Y -intercept calculated in t -plot (not shown here)for cel-lulose nanocrystals indicated absence of micropores.This result suggests that aggregation by strong hydrogen-bonding effect in air-drying could be signi?cantly reduced in freeze-drying.This con-clusion was further evidenced by the 8times higher BET surface

P.Lu,Y.-L.Hsieh/Carbohydrate Polymers82 (2010) 329–336

335

Fig.7.N2adsorption and desorption isotherms at77K of cellulose(a)powder and(c)nanocrystals.BJH pore size distribution derived from adsorption isotherm of cellulose (b)powder and(d)nanocrystals.

Table1

BET surface areas and pore characteristics of cellulose powder and nanocrystals.

BET surface area (m2/g)t-Plot external

surface area(m2/g)

BJH adsorption cumulative

surface area of pores(m2/g)a

BJH adsorption cumulative

volume of pores(cm3/g)a

BJH adsorption average

pore width(4V/A)(?)

Cellulose powders 1.547±0.012 1.480±0.032 1.563±0.0190.00839±0.00026214.64±7.23 Cellulose nanocrystals13.362±0.03415.093±0.21514.771±0.1580.03396±0.0005991.99±2.57

a Between17and3000?.

area of cellulose nanocrystals(13.362m2/g)compared to that of the original cellulose(1.547m2/g).

4.Conclusions

Cellulose nanocrystals with rod,sphere,and network-structured morphologies were prepared by acid hydrolysis and freeze-drying of cotton cellulose.Hydrolysis with sulfuric acid removed amorphous cellulose to produce isolated cellu-lose nanocrystals with newly introduced sulfate groups on the nanocrystal surfaces.Repulsion among the negatively charged cellulose nanocrystals and quick freezing with liquid nitrogen were very effective in preventing aggregate formation driven by the strong hydrogen bonding.The cellulose nanocrystals retained their chemical and I?crystalline structures,while exhibited signi?cantly different thermal and decomposition behaviors, suggesting higher thermal conductivity and possibly direct solid-to-gas decomposition mechanism.These cellulose nanocrystals could be readily and directly dispersed into water,ethanol,DMF and other solvents without any additional preparation or aids. Furthermore,the cellulose nanocrystals formed a mesoporous structure with a mean pore width of91.99±2.57?and possessed a high surface area of13.362m2/g.The results demonstrated that the acid hydrolysis not only produced nanocrystalline structures but also introduced surface charges for their effective separation. Coupling with freeze-drying,the cellulose nanocrystals exhibit the desirable quick re-dispersion capability which is essential for their practical applications.

Acknowledgement

This research was made possible by funding from the National Textile Center(project M02-CD05),the Jastro-Shields Graduate Research Award,and Summer Graduate Researcher Award from the University of California,Davis.

References

Beck-Candanedo,S.,Roman,M.,&Gray,D.G.(2005).Effect of reaction conditions on the properties and behavior of wood cellulose nanocrystal suspensions.

Biomacromolecules,6(2),1048–1054.

Beecher,J.F.(2007).Organic materials:Wood,trees and nanotechnology.Nature Nanotechnology,2(8),466–467.

Dong,S.,&Roman,M.(2007).Fluorescently labeled cellulose nanocrystals for bioimaging applications.Journal of the American Chemical Society,129(45), 13810–13811.

336P.Lu,Y.-L.Hsieh/Carbohydrate Polymers82 (2010) 329–336

Favier,V.,Chanzy,H.,&Cavaille,J.Y.(1995).Polymer nanocomposites reinforced by cellulose whiskers.Macromolecules,28(18),6365–6367.

Grunert,M.,&Winter,W.T.(2002).Nanocomposites of cellulose acetate butyrate reinforced with cellulose nanocrystals.Journal of Polymers and the Environment, 10(1/2),27–30.

Hanley,S.J.,Giasson,J.,Revol,J.F.,&Gray,D.G.(1992).Atomic force microscopy of cellulose micro?brils:Comparison with transmission electron microscopy.

Polymer,33(21),4639–4642.

Helbert,W.,Cavaille,J.Y.,&Dufresne,A.(1996).Thermoplastic nanocomposites ?lled with wheat straw cellulose whiskers.Part I:Processing and mechanical behavior.Polymer Composites,17(4),604–611.

Helbert,W.,Chanzy,H.,Husum,T.L.,Schuelein,M.,&Ernst,S.(2003).Fluores-cent cellulose micro?brils as substrate for the detection of cellulase activity.

Biomacromolecules,4(3),481–487.

Hon,D.N.S.,&Shiraishi,N.(1991).Wood and cellulosic chemistry.New York:M.

Dekker.

Ifuku,S.,Nogi,M.,Abe,K.,Handa,K.,Nakatsubo,F.,&Yano,H.(2007).Surface mod-i?cation of bacterial cellulose nano?bers for property enhancement of optically transparent composites:Dependence on acetyl-group DS.Biomacromolecules, 8(6),1973–1978.

Kim,J.,Yun,S.,&Ounaies,Z.(2006).Discovery of cellulose as a smart material.

Macromolecules,39(12),4202–4206.

Lu,P.,&Hsieh,Y.-L.(2009).Cellulose nanocrystal-?lled poly(acrylic acid)nanocomposite?brous membranes.Nanotechnology,20(41), 415604/415601–415604/415609.

Marchessault,R.H.,Morehead,F.F.,&Koch,M.J.(1961).Hydrodynamic properties of neutral suspensions of cellulose crystallites as related to size and shape.Journal of Colloid Science,16,327–344.

Mo,Z.-l.,Zhao,Z.-l.,Chen,H.,Niu,G.-p.,&Shi,H.-f.(2009).Heterogeneous prepara-tion of cellulose-polyaniline conductive composites with cellulose activated by acids and its electrical properties.Carbohydrate Polymers,75(4),660–664. Podsiadlo,P.,Choi,S.-Y.,Shim,B.,Lee,J.,Cuddihy,M.,&Kotov,N.A.(2005).

Molecularly engineered nanocomposites:Layer-by-layer assembly of cellulose nanocrystals.Biomacromolecules,6(6),2914–2918.

Podsiadlo,P.,Sui,L.,Elkasabi,Y.,Burgardt,P.,Lee,J.,Miryala,A.,et al.(2007).Layer-by-layer assembled?lms of cellulose nanowires with antire?ective properties.

Langmuir,23(15),7901–7906.

Roman,M.,&Winter,W.T.(2004).Effect of sulfate groups from sulfuric acid hydrolysis on the thermal degradation behavior of bacterial cellulose.Biomacro-molecules,5(5),1671–1677.

Sakurada,I.,Nukushina,Y.,&Ito,T.(1962).Experimental determination of the elastic modulus of crystalline regions in oriented polymers.Journal of Polymer Science, 57,651–660.Samir,M.A.S.A.,Alloin,F.,&Dufresne,A.(2005).Review of recent research into cellulosic whiskers.Their properties and their application in nanocomposite ?eld.Biomacromolecules,6(2),612–626.

Saxena,I.M.,&Brown,R.M.,Jr.(2005).Cellulose biosynthesis:Current views and evolving concepts.Annals of Botany(Oxford,United Kingdom),96(1),9–21. Shimazaki,Y.,Miyazaki,Y.,Takezawa,Y.,Nogi,M.,Abe,K.,Ifuku,S.,et al.(2007).

Excellent thermal conductivity of transparent cellulose nano?ber/epoxy resin nanocomposites.Biomacromolecules,8(9),2976–2978.

Sing,K.S.W.,Everett,D.H.,Haul,R.A.W.,Moscou,L.,Pierotti,R.A.,Rouquerol,J., et al.(1985).Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity(Recommendations 1984).Pure and Applied Chemistry,57(4),603–619.

Sturcova,A.,Davies,G.R.,&Eichhorn,S.J.(2005).Elastic modulus and stress-transfer properties of tunicate cellulose whiskers.Biomacromolecules,6(2),1055–1061.

Svagan,A.J.,Samir,M.A.S.A.,&Berglund,L.A.(2008).Biomimetic foams of high mechanical performance based on nanostructured cell walls reinforced by native cellulose nano?brils.Advanced Materials,20(7),1263.

Terech,P.,Chazeau,L.,&Cavaille,J.Y.(1999).A small-angle scattering study of cellulose whiskers in aqueous suspensions.Macromolecules,32(6),1872–1875.

Tingaut,P.,Zimmermann,T.,&Lopez-Suevos,F.(2010).Synthesis and characteri-zation of bionanocomposites with tunable properties from poly(lactic acid)and acetylated micro?brillated cellulose.Biomacromolecules,11(2),454–464.

van den Berg,O.,Capadona,J.R.,&Weder,C.(2007).Preparation of homogeneous dispersions of tunicate cellulose whiskers in organic solvents.Biomacro-molecules,8(4),1353–1357.

Wada,M.,Heux,L.,&Sugiyama,J.(2004).Polymorphism of cellulose I family:Rein-vestigation of cellulose IVI.Biomacromolecules,5(4),1385–1391.

Wada,M.,Kondo,T.,&Okano,T.(2003).Thermally induced crystal transforma-tion from cellulose I.alpha.to I.beta.Polymer Journal(Tokyo Japan),35(2),155–159.

Wong,E.W.,Sheehan,P.E.,&Lieber,C.M.(1997).Nanobeam mechanics:Elasticity, strength,and toughness of nanorods and nanotubes.Science(Washington,DC), 277(5334),1971–1975.

Yu,M.-F.,Lourie,O.,Dyer,M.J.,Moloni,K.,Kelly,T.F.,&Ruoff,R.S.(2000).Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load.

Science,287(5453),637–640.

Zhang,J.G.,Elder,T.J.,Pu,Y.Q.,&Ragauskas,A.J.(2007).Facile synthesis of spherical cellulose nanoparticles.Carbohydrate Polymers,69(3),607–611.

Zuluaga,R.,Putaux,J.L.,Cruz,J.,Velez,J.,Mondragon,I.,&Ganan,P.(2009).Cellulose micro?brils from banana rachis:Effect of alkaline treatments on structural and morphological features.Carbohydrate Polymers,76(1),51–59.

雅思口语素材汇总之端午节

雅思口语素材汇总之端午节雅思口语素材:Dragon Boat Festival(端午节) Qu Yuan The Dragon Boat Festival, also called the Duanwu Festival, is celebrated on the fifth day of the fifth month according to the Chinese calendar. For thousands of years, the festival has been marked by eating zong zi (glutinous rice(糯米)wrapped to form a pyramid using bamboo or reed leaves) and racing dragon boats. The festival is best known for its dragon-boat races, especially in the southern provinces where there are many rivers and lakes. This regatta(赛舟会)commemorates the death of Qu Yuan , an honest minister who is said to have committed suicide by drowning himself in a river. Qu was a minister of the State of Chu situated in present-day Hunan and Hubei provinces, during the Warring States Period (475-221BC)(战国时期). He was upright, loyal and highly esteemed for his wise counsel that brought peace and prosperity to the state. However, when a dishonest and corrupt prince vilified Qu, he was disgraced and dismissed from office. Realizing that the country was now in the hands of evil and corrupt officials, Qu grabbed a large stone and leapt into the Miluo River on the fifth day of the fifth month. Nearby fishermen rushed over to try and save him but were unable to even recover his body. Thereafter, the state declined and was eventually conquered by the State of Qin. The people of Chu who mourned the death of Qu threw rice into the river to feed his ghost every year on the fifth day of the fifth month. But one year, the spirit of Qu appeared and told the mourners that a huge reptile(爬行动物)in the river had stolen the rice. The spirit then advised them to wrap the rice in silk and bind it with five different-colored threads before tossing it into the river. During the Duanwu Festival, a glutinous rice pudding called zong zi is eaten to symbolize the rice offerings to Qu. Ingredients such as beans, lotus seeds(莲子), chestnuts(栗子), pork fat and the golden yolk of a salted duck egg are often

操作系统文件管理_答案

第六部分文件管理 1、文件系统的主要目的就是( )。 A、实现对文件的按名存取 B、实现虚拟存储 C、提供外存的读写速度 D、用于存储系统文件 2、文件系统就是指( )。 A、文件的集合 B、文件的目录集合 C、实现文件管理的一组软件 D、文件、管理文件的软件及数据结构的总体 3、文件管理实际上就是管理( )。 A、主存空间 B、辅助存储空间 C、逻辑地址空间 D、物理地址空间 4、下列文件的物理结构中,不利于文件长度动态增长的文件物理结构就是( )。 A、顺序文件 B、链接文件 C、索引文件 D、系统文件 5、下列描述不就是文件系统功能的就是( )。 A、建立文件目录 B、提供一组文件操作 C、实现对磁盘的驱动调度 D、实现从逻辑文件到物理文件间的转换 6、文件系统在创建一个文件时,为它建立一个( )。 A、文件目录 B、目录文件 C、逻辑结构 D、逻辑空间 7、索引式(随机)文件组织的一个主要优点就是( )。 A、不需要链接指针 B、能实现物理块的动态分配 C、回收实现比较简单 D、用户存取方便 8、面向用户的文件组织机构属于( )。 A、虚拟结构 B、实际结构 C、逻辑结构 D、物理结构 9、按文件用途来分,编译程序就是( )。 A、用户文件 B、档案文件 C、系统文件 D、库文件 10、将信息加工形成具有保留价值的文件就是( )。 A、库文件 B、档案文件 C、系统文件 D、临时文件 11、文件目录的主要作用就是( )。 A、按名存取 B、提高速度 C、节省空间 D、提高外存利用率 12、如果文件系统中有两个文件重名,不应采用( )。 A、一级目录结构 B、树型目录结构 C、二级目录结构 D、A与C 13、文件系统采用树型目录结构后,对于不同用户的文件,其文件名( )。 A、应该相同 B、应该不同 C、可以不同,也可以相同 D、受系统约束 14、文件系统采用二级文件目录可以( )。 A、缩短访问存储器的时间 B、实现文件共享 C、节省内存空间 D、解决不同用户间的文件命名冲突

雅思口语常用背诵素材

口语常用背诵素材一，喜好和厌恶： 1.（释放压力）I guess most young people enjoy listening to music. It’s a perfect way to reduce stress/relieve stress. (*It’s a perfect stress-buster.) 2.（休闲）I think the best way to relax and kill time (kick back and relax) is to be with friends. Chatting with them helps me unwind/wind down (* Chatting with them is very therapeutic). 3.（放松）I found it pretty relaxing to flick through a magazine before going to bed (*before hitting the sack/hitting the hay). 4.（运动）Jogging on sunny days and simply basking in the sun(bathing in the sun) is super enjoyable(pleasurable). /Jogging is my favorite workout. It can always cheer me up after I’ve been down and depressed. And it can help me shed some unwanted weight. 5.（喜欢，提升心情）I’m quite into watching sitcom (I’m fascinated by, I’m crazy about, I adore, I’m very fond of...). Spending 20 minutes in front of the screen and having a good laugh after dinner really puts me in a better mood/ switch my mood/ lift my spirits. 6.（天气）I fancy spring most compared to the other three. Summer is simply too hot and stuffy (*muggy), and you can’t stay out for two minutes without getting sweaty and smelly. Autumn is too dry and winter too freezing cold. 7.（电影）I’m a movie buff (Movies are my biggest pastime), and I would either catch one on my couch or go to the movie theatre when they have the one I like on the big screen./ It's a great form of relaxation for me to while away the hours of a long flight, to keep my family together on the sofa, and to give me a bit of a thrill. 8.（无聊困难）I’m terrible at what I study now./(History is never my forte/ my strong suit). The classes are kind of a drag (It’s super dull/ *It’s mind-numbing), and there’s always loads of (tons of) homework to do. Memorizing those facts and years is a lot of effort (is tricky; is no picnic; is challenging for me). 9.（不能忍）I can’t stand (I hate, I totally dislike) the busy traffic in my city. I’m sick and tired of always getting stuck in traffic for a long time during rush hours. It really bothers me (It drives me crazy, *It drives me up the wall). 10. （受欢迎）Hip-hop is really trendy (is catching on, is all the rage) among young people these days. 二，时间和地点 11. （优点）What’s great about my home town is that there are plenty of shopping malls, coz I’m a shopaholic and I can shop for hours. 12. （远）The stadium is quite far away from where I live, and every time I want to do there, it takes me roughly 40 minutes by bus to arrive. (It’s one-hour drive from the city center.) 13. （方便）My apartment is in the center/heart of the city, a handy location where I can find supermarkets and clubs right on my doorstep. It’s a bit noisy though. 14. （近）I’m a movie-goer and and there’s a movie theater pretty close to my home, so I go there quite often (frequently). (close to my home= *within walking distance of my home , *in close proximity to my home) 15. （经常）I have a heavy workload at school, so I have to go to the library to study pretty much

智能化系统建设方案

精品文档一.背景描述：江南海岸总体规划和设计均体现了传统中国居家理想和现代生活方式的有机融合，是依照21世纪人居标准精心打造的高级住宅小区。整个小区无不营造一个舒适休闲的生活空间，是一所环境优雅，闹中取静的花园式住宅小区，满足住户对高品质生活的追求。二.工程说明：江南海岸位于三明市列东区，由14栋高层住宅小区组成，总建筑面积29.7627万平方米，其中包括4栋27层，6栋25层，4栋29 层，会所1间，负一层，一层。住户总数为1182户。项目要求: 江南海岸，是集住宅、花园、会所于一体的高级住宅小区。小区智能化系统的工程建设具有投资大、工程复杂、专业性强等特点。小区要求建设成具有国内先进水平的，既具有自身特点，又具有时代潮流特色的高尚住宅楼宇。整个工程规划、设计、实施上要求充分体现技术的先进性、系统的复杂性、严密的安防集控管理。注重整体功能强大，中心设备完善，系统配置科学合理，真正体现高技术、高标准、高水平的现代化智能小区。四.需求分析: 4.1分析与评估：

本方案以江南海岸小区住宅智能化管理及安全防范为设计目标为将力求建设成为高水平、高质量、高标准的信息化智能小区。我方提出以下见解，请发包方领导参考。 ①小区建设要求基于系统可靠、稳定、先进的基础上，既能满足用户住宅的实际需求，同时又力求经济、实用、合理。 ②整个系统的结构要求清晰合理，小区实现全封闭管理，各个子系统既相互关联又相对独立，形成一个全方位智能安防管理系统。 ③要求考虑未来系统扩展的需求，为小区以后系统功能的增加、升级，提供良好的环境空间。因此，考虑江南海岸属于大型的综合住宅小区，建筑规模庞大、结构复杂，小区各项功能模块齐备，因此在智能化建设方面，产品的集成度、统一化、高效管理方面尤为重要，同时，还必须考虑小区规模的不断扩大，智能化产品必须具备高度的扩展及冗余，顺应小区的发展。我方进行多项分析与评估，结合小区建筑结构的分布特点、规模发展，以及对小区各功能模块的深层了解，建议江南海岸智能化系统工

linux下各目录作用和功能

/bin:是binary的缩写,这个目录是对Unix系统习惯的沿袭,存放着使用者最经常使用的命令。如:ls,cp,cat等。 /boot:这里存放的是启动Linux时使用的一些核心文档。 /dev:是device的缩写.这个目录下是任何Linux的外部设备,其功能类似Dos下的.sys 和Win下的.vxd。在Linux中设备和文档是用同种方法访问的。例如:/dev/hda代表第一个物理IDE硬盘。 /etc:这个目录用来存放任何的系统管理所需要的配置文档和子目录。 /home:用户主目录,比如说有个用户叫sina,那他的主目录就是/home/sina，说到这里打个岔.您现在应该明白，在我们访问一些个人网页。如:https://www.wendangku.net/doc/1f8832167.html,/sina的时候,sina就是表示访问 https://www.wendangku.net/doc/1f8832167.html, 站点中的用户sina的用户主目录.假如这个网站的操作系统是Linux,那就是表示/home/sina。 /lib:这个目录里存放着系统最基本的动态链接共享库,其作用类似于Windows里的.dll文档。几乎任何的应用程式都需要用到这些共享库。 /lost+found:这个目录平时是空的,当系统不正常关机后,这里就成了一些无家可归的文档的避难所。对了,有点类似于Dos下的.chk文档。 /mnt:这个目录是空的,系统提供这个目录是让用户临时挂载别的文档系统。 /proc:这个目录是个虚拟的目录,他是系统内存的映射,我们能够通过直接访问这个目录来获取系统信息。也就是说，这个目录的内容不在硬盘上而是在内存里啊。 /root:系统管理员,也叫终极权限者的用户主目录。当然系统的拥有者,总要有些特权啊。/sbin:s就是Super User的意思,也就是说这里存放的是一些系统管理员使用的系统管理程式。 /tmp:这个目录不用说,一定是用来存放一些临时文档的地方了。 /usr:这是个最庞大的目录,我们要用到的很多应用程式和文档几乎都存放在这个目录了。具体来说: /usr/X11R6:存放X-Windows的目录。 /usr/bin:存放着许多应用程式. /usr/sbin:给终极用户使用的一些管理程式就放在这. /usr/doc:这就是Linux文档的大本营. /usr/include:Linux下研发和编译应用程式需要的头文档在这里找. /usr/lib:存放一些常用的动态链接共享库和静态档案库. /usr/local:这是提供给一般用户的/usr目录,在这安装软件最适合. /usr/man:是帮助文档目录. /usr/src:Linux开放的源代码,就存在这个目录,爱好者们别放过哦! /var:这个目录中存放着那些不断在扩充着的东西,为了保持/usr的相对稳定,那些经常被修改的目录能够放在这个目录下,实际上许多系统管理员都是这样干的.顺便说一下,系统的日志文档就在/var/log目录中. /usr/local/bin 本地增加的命令 /usr/local/lib 本地增加的库根文件系统通常情况下，根文件系统所占空间一般应该比较小，因为其中的绝大部分文件都不需要, 经常改动，而且包括严格的文件和一个小的不经常改变的文件系统不容易损坏。除了可能的一个叫/ v m l i n u z标准的系统引导映像之外，根目录一般不含任何文件。所有其他文件在根文件系统的子目录中。

雅思口语素材

雅思口语素材 Document serial number【LGGKGB-LGG98YT-LGGT8CB-LGUT-

Useful Expressions: Words and phrases Friends and communication: solidify/ strengthen/ enhance/ promote communication / connection with mutual understanding relationship network/circle of f r i e n d s cultivate/develop friendship with s b . keep steady relationship with sb. establish interpersonal networksac build up the social circle spur message transmission Knowledge and experience widen one’s outlook broaden one’s vision/horizon acquire knowledge and skills comprehensive/overall quality

expand/enlarge one’s scope of knowledge knowledge reserve/base/storage theoretical knowledge practical skills social experience broaden one’s knowledge base promote one’s overall/ comprehensive competence accumulate experiences learn lessons from past experiences Work and experience the scarcity of employment o p p o r t u n i t i lay the foundations for career p r o s p e r i t y

告诉你C盘里面每个文件夹是什么作用

Documents and Settings是什么文件？答案：是系统用户设置文件夹，包括各个用户的文档、收藏夹、上网浏览信息、配置文件等。补：这里面的东西不要随便删除，这保存着所有用户的文档和账户设置，如果删除就会重新启动不能登陆的情况，尤其是里面的default user、all users、administrator和以你当前登陆用户名的文件夹。 Favorites是什么文件？答案：是收藏夹，存放你喜欢的网址。可以在其中放网址快捷方式和文件夹快捷方式，可以新建类别（文件夹）。 Program Files是什么文件？答案：应用软件文件夹装软件的默认路径一般是这里！当然里面也有些系统自身的一些应用程序Common Files是什么文件？答案： Common Files. 这个文件夹中包含了应用程序用来共享的文件,很重要，不能乱删除Co mmon Files这个文件是操作系统包扩系统程序和应用程序Common Files是应用程序运行库文件数据库覆盖了大约1000多个最流行的应用程序的插件,补丁等等文件夹com mon files里很多都是系统文件，不能随意删除，除非确定知道是干什么用的，没用的可以删掉。不过就算删掉了有用的东西，也没大的关系，顶多是某些软件用不了，不会造成系统崩溃。 ComPlus Applications是什么文件？答案： ComPlus Applications：微软COM+ 组件使用的文件夹，删除后可能引起COM+ 组件不能运行 DIFX是什么文件？答案：不可以删除，已有的XML数据索引方法从实现思想上可分为两类:结构归纳法和节点定位法.这两种方法都存在一定的问题,结构归纳法的缺点是索引规模较大而且难以有效支持较复杂的查询,而节点定位法的主要缺点是容易形成过多的连接操作.针对这些问题,提出了一种新的动态的XML索引体系DifX,它扩展了已有的动态索引方法,采用一种动态的Bisimil arity的概念,可以根据实际查询需求以及最优化的要求动态决定索引中保存的结构信息,以实现对各种形式的查询最有效的支持.实验结果证明DifX是一种有效而且高效的XML索引方法,其可以获得比已有的XML索引方法更高的查询执行效率. Internet Explorer是什么文件？答案：不用说了，肯定不能删除，IE，浏览网页的！ Kaspersky Lab是什么文件？答案：卡巴斯基的文件包，这个是卡巴的报告，在C:\Documents and Settings\All Users\Application Data\Kaspersky Lab\AVP6\Report 的更新文件中有很多repor t文件很占地

雅思口语素材

U s e f u l E x p r e s s i o n s: Words and phrases Friends and communication: mutual understanding solidify/ strengthen/ enhance/ promote communication / connection with relationship network/circle of friends cultivate/develop friendship with sb. keep steady relationship with sb. establish interpersonal networksac build up the social circle spur message transmission Knowledge and experience widen one’s outlook broaden one’s vision/horizon acquire knowledge and skills comprehensive/overall quality expand/enlarge one’s scope of knowledge knowledge reserve/base/storage theoretical knowledge practical skills social experience broaden one’s knowledge base promote one’s overall/ comprehensive accumulate experiences competence learn lessons from past experiences Work and experience the scarcity of employment opportunities lay the foundations for career prosperity immerse oneself in endless job tasks boost/augment/enhance efficiency be adept in boost one’s c ompetitiveness Health and pressure diminish individuals' leisure time drive away lassitude lighten one’s burden homework/workforce overload

智能化系统配置项目及要求

智能化系统初步配置方案一、设计原则 1、智能化系统设计，应综合考虑项目投资额度的可控性、设备选型的灵活性、工程施工的可行性、系统功能的可扩展性、系统运行维护的便利性和物业管理的规范性等要求。 2、智能化系统的设计应参照本要求，其配置标准不得低于”初步配置方案” 的要求，同时应有一定的升级和扩展能力，并预留相应的接口。 3、智能化系统设计除了满足国家标准与规范的相关规定，以及本标准基本配置要求外，还应满足建筑、结构以及与智能化系统存在设计相关联的其他专业的设计规范和要求，特别需要注意符合项目当地现行有关标准和规范的特殊要求。 4、智能化系统设计与工程项目建设地点的实际情况相适应。 5、智能化系统所采用的设备及线路材料等均应符合国家现行的规定，并具有产品合格证、质量检验证书和产品须通过国家的CCC认证。二、智能化系统基本配置要求 1、安全防范系统 a、安全防范系统包括闭路电视监控、防盗报警系统、门禁系统、电子巡更系统及无线对讲系统。 b、闭路电视监控系统主要设置重要出入口，如公共场所、重要房间、楼梯通道、楼层电梯通道、电梯轿厢、室外主干道及交叉路口等处，电梯轿厢安装电梯专用监控镜头并加装抗干扰器，户外监控镜头须带有红外夜视功能。所有的监控镜头全部采用网络彩色摄像机，采用矩阵主机切换至电视墙，录像以全天实时高清图像质量保存至少30天（具体保存时间可按照当地派出所要求确定）。 c、防盗报警系统主要设置在财务室、档案室等重要房间，并设置手动报警开关或脚挑报警开关，要求闭路电视监控系统同时显示出报警地点画面。 d、重要设备机房、财务室、档案室等重要房间设置门禁系统。 e、电子巡更系统要求采用离线式，设置于重要机房、楼层楼梯口、电梯厅。 f、无线对讲系统要求具有可进行信道改写和信道加密功能，满足内部通讯需要。 g、各系统设备品牌须选用技术成熟，性能稳定可靠的产品。 2、通讯网络系统

文件夹的作用

All Users文件夹：『Win9x/ME』所有用户文件夹，里面里面包括系统缺省登录时的桌面文件和开始菜单的内容。『Win2000』在Win2000的系统目录中没有这个文件夹，Win2000将用户的信息放在根目录下的Documents and Settings文件夹中，每个用户对应一个目录，包括开始菜单、桌面、收藏夹、我的文档等等。 Application Data文件夹：『Win9x/ME』应用程序数据任务栏中的快捷方式，输入法的一些文件等等。根据你系统中使用不同的软件，该目录中的内容也有所不同。『Win2000』在Documents and Settings文件夹中，每个用户都对应一个Application Data 文件夹，同样每个用户由于使用的软件不同，目录内容也相同。 Applog文件夹：『Win9x/ME』应用程序逻辑文件目录。逻辑文件是用来记录应用软件在运行时，需要调用的文件、使用的地址等信息的文件。要查看这些文件，用记事本打开即可。 Catroot文件夹：『Win9x』计算机启动测试信息目录，目录中包括的文件大多是关于计算机启动时检测的硬软件信息。『WinME』该文件夹位于系统目录的system目录中。『Win2000』该文件夹位于系统目录的system32目录中。 Command文件夹：『Win9x/ME』DOS命令目录。包括很多DOS下的外部命令，虽说都是些小工具，但真的很好用，特别是对于系统崩溃时。『Win2000』这些DOS命令位于系统目录的system32目录中。 Config文件夹：『Win9x/ME/2000』配置文件夹，目录中包括一些MIDI乐器的定义文件。 Cookies文件夹：『Win9x/ME』Cookies又叫小甜饼，是你在浏览某些网站时，留在你硬盘上的一些资料，包括用户名、用户资料、网址等等。『Win2000』每个用户都有一个Cookies文件夹，位于Documents and Settings文件夹的每个用户目录中。 Cursors文件夹：『Win9x/ME/2000』鼠标动画文件夹。目录中包括鼠标在不同状态下的动画文件。 Desktop文件夹：『Win9x/ME』桌面文件夹。包括桌面上的一些图标。『Win2000』这个文件夹在系统目录中也存在，同时在Documents and Settings文件夹的每个用户目录中还有“桌面”文件夹。

近期雅思口语卡片新题素材汇总

近期雅思口语卡片新题素材汇总人物Describe a classmate of yours Describe a good friend Describe a happy person Describe a colleague Describe a neighbor Describe your own personality Describe a family member Describe a child you know Describe an old person Describe an old person who has influenced you the most Describe a successful person Describe a singer Describe a sportsman Describe a movie star Describe a character in TV or movie Describe a teacher of yours Describe a famous person that you want to spend a day with.地点Describe a building at schools Describe a historical place

Describe a monument Describe an interesting building Describe a lake, river or sea. Describe a peaceful place Describe a leisure place Describe a park Describe a place of interest Describe a natural beauty Describe a city you want to live in Describe a place you have visited Describe a place you always go for shopping

linux下各文件夹的结构说明及用途详细介绍解析

linux下各文件夹的结构说明及用途介绍: /bin:二进制可执行命令。 /dev:设备特殊文件。 /etc:系统管理和配置文件。 /etc/rc.d:启动的配置文件和脚本。 /home:用户主目录的基点,比如用户user的主目录就是/home/user,可以用~user 表示。 /lib:标准程序设计库,又叫动态链接共享库,作用类似windows里的.dll文件。 /sbin:系统管理命令,这里存放的是系统管理员使用的管理程序。 /tmp:公用的临时文件存储点。 /root:系统管理员的主目录。 /mnt:系统提供这个目录是让用户临时挂载其他的文件系统。 /lost+found:这个目录平时是空的,系统非正常关机而留下“无家可归”的文件就在这里。 /proc:虚拟的目录,是系统内存的映射。可直接访问这个目录来获取系统信息。 /var:某些大文件的溢出区,比方说各种服务的日志文件。 /usr:最庞大的目录,要用到的应用程序和文件几乎都在这个目录。其中包含: /usr/x11r6:存放x window的目录。 /usr/bin:众多的应用程序。

/usr/sbin:超级用户的一些管理程序。 /usr/doc:linux文档。 /usr/include:linux下开发和编译应用程序所需要的头文件。 /usr/lib:常用的动态链接库和软件包的配置文件。 /usr/man:帮助文档。 /usr/src:源代码,linux内核的源代码就放在/usr/src/linux 里。 /usr/local/bin:本地增加的命令。 /usr/local/lib:本地增加的库根文件系统。通常情况下,根文件系统所占空间一般应该比较小,因为其中的绝大部分文件都不需要经常改动,而且包括严格的文件和一个小的不经常改变的文件系统不容易损坏。除了可能的一个叫/vmlinuz标准的系统引导映像之外,根目录一般不含任何文件。所有其他文件在根文件系统的子目录中。 1. /bin目录 /bin目录包含了引导启动所需的命令或普通用户可能用的命令(可能在引导启动后。这些命令都是二进制文件的可执行程序(bin是binary的简称,多是系统中重要的系统文件。 2. /sbin目录 /sbin目录类似/bin,也用于存储二进制文件。因为其中的大部分文件多是系统管理员使用的基本的系统程序,所以虽然普通用户必要且允许时可以使用,但一般不给普通用户使用。 3. /etc目录

Simon考官范文-雅思口语素材2(Cook整理)

一.雅思P2相关话题一日假期和外国朋友一起吃饺子的建议 1.The best way to eat a dumpling is in one bite. “Dumplings are designed to be consumed in one mouthful, as it’s the best way to enjoy the combination of the meat filling and the very thin and springy flour wrapper,” If you can’t use chopsticks, eat your dumplings with your fingers. Avoid using a fork at all costs, as piercing the dumpling will compromise the flavour. 3.Mix two parts vinegar with one part soy sauce for the perfect dumpling sauce. Add young ginger slices too， Chili oil is also a great addition when available. 4.Dumplings are just one element of dim sum. “Dim sum doesn’t just include dumplings. It’s also braised dishes like pork ribs, chicken feet, and beef balls. It’s actually small tapas-style dishes that are eaten in Cantonese restaurants at lunchtime,” 5.Xiao long bao dumplings are different from others as they contain broth. They originated in the Jiangnan region of China and are prepared in bamboo steaming baskets called xiao long, hence the name. 6.When eating xiao long bao or a dumpling with a ~soupy~ interior, opt for chopsticks and a spoon. “As soon as the dumplings arrive at your table, lift one from the steamer basket onto a soup spoon. Next, tear the skin of the dumpling by pressing the chopsticks from the side of the dumpling onto the spoon. The broth will ooze out onto the spoon. Sip the soup then enjoy the dumpling in one mouthful.” 7.You can tell whether your dumpling was cooked fresh or frozen by looking at the skin. “The skin of a freshly made dumpling is springy and light. “Frozen ones tend to be soggy.” 8.When makingdumplingsat home, try to keep your packages small.

让你知道C盘的每个文件夹代表什么,其作用是什么

让你知道C盘的每个文件夹代表什么,其作用是什么 C:\Program Files文件夹介绍列出常见的几个文件夹： 1、C:\Program Files\common files Common Files （存放软件会用到的公用库文件）安装一些软件会在里面产生文件夹比如visual studio symentec antivirus gtk lib 等他是一些共享资源，这里的共享是指，一个公司所出的一系列软件都需要用这里的文件比如：vb vc 要用里面visual studio 文件夹下的文件 norton fire wall norton antivirus 等要用里面symentec shared文件夹下的文件acrobat reader photoshop 要用里面adobe 文件夹下的文件公有文件不能删除正常的话会有： Microsoft Shared MSSoap ODBC SpeechEngines System Direct X Common Files这个文件是操作系统包扩系统程序和应用程序 Common Files是应用程序运行库文件数据库覆盖了大约1000多个最流行的应用程序的插件,补丁等等文件夹common files里很多都是系统文件，不能随意删除，除非确定知道是干什么用的，没用的可以删掉。不过就算删掉了有用的东西，也没大的关系，顶多是某些软件用不了，不会造成系统崩溃。另外也有说各种软件的注册信息也在里面。 2、C:\Program Files\ComPlus Applications ComPlus Applications：微软COM+ 组件使用的文件夹，删除后可能引起COM+ 组件不能运行显示名称：COM+ System Application ◎微软描述：管理基于COM+ 组件的配置和跟踪。如果服务停止，大多数基于 COM+ 组件将不能正常工作。如果本服务被禁用，任何明确依赖它的服务都将不能启动。 ◎补充描述：如果 COM+ Event System 是一台车，那么 COM+ System Application 就是司机，如事件检视器内显示的 DCOM 没有启用。一些 COM+软件需要，检查你的C:\Program Files\ComPlus 3、C:\Program Files\InstallShield Installation Information 发现这个网上一些解释的很含糊，说是用来存放部分软件安装信息的文件夹。比较准确的说

(完整word版)雅思口语素材整理汇总

雅思口语素材训练 by Tina Li Do you like music??A—肯定:Definitely yes, everyone enjoys music, and I am no exception! I love... 否定 :Well, honestly speaking, music is really not my cup of tea, simply because... ( 给出直接原因) What—pop, techno ( 电音音乐), hip-hop, rock, meditation ( 冥想乐) and especially light music.( 罗列名词) Where—Normally speaking, I would like to listen to music with my earphones when I take a ride on public transportation. ( 给出一个具体的场景) When—As long as I couldn’t go to sleep, I’d like to listen to some light music to calm myself down. ( 给出一个条件 :As long as I..., I would...)

Who—My most favourite singers include Adele, James Blunt, Avril Lavigne, and so forth. ( 喜欢的歌手) Why—I am fond of music mainly because it can cheer me up greatly when I feel down/low/ blue/bored/tired/depressed. ( 心情不好的时候让我高兴起来)?Besides, I also believe that music is an indispensable part of culture and tradition, through which I could have a better understanding of different cultures around the world, including cowboy culture, African-American street culture, the three main reli- gions and so on. ( 有助于理解不同的文化) Do you like watching movies?? A—Speaking of movies, yes, I am a big fan of all types of movies, such as...?What—comedy, action, romance, sci-fi, manga, vampire, zombie, animation...