E-25 Improved ε-poly-L-lysine production of Streptomyces

E-25 Improved ε-poly-L-lysine production of Streptomyces
E-25 Improved ε-poly-L-lysine production of Streptomyces

ORIGINAL ARTICLE

Improvedε-poly-L-lysine production of Streptomyces

sp.FEEL-1by atmospheric and room temperature plasma mutagenesis and streptomycin resistance screening

Liang Wang&Xusheng Chen&Guangyao Wu&

Shuang Li&Xin Zeng&Xidong Ren&Lei Tang&

Zhonggui Mao

Received:23September2014/Accepted:8January2015

#Springer-Verlag Berlin Heidelberg and the University of Milan2015

Abstract To enhance theε-poly-L-lysine(ε-PL)productivity of Streptomyces sp.FEEL-1,a novel breeding strategy of at-mospheric and room temperature plasma(ARTP)mutagenesis with streptomycin resistance screening was developed.After three rounds of recursive breeding,one strain designated AS3-14was selected with aε-PL yield of2.91g/L in shake-flask fermentation,which was elevated by66.3%compared to that of the parent strain FEEL-1.Subsequent batch and fed-batch fermentation was performed in a5L fermenter,and8.2g/L and41.2g/Lε-PL productions were achieved,which were 49.1%and68.1%higher,respectively,than those of the parent strain.Analysis of enzyme activities indicated that key enzymes,e.g.,hexokinase,pyruvate kinase,citrate syn-thase and aspartate kinase,were more active in the mutant AS3-14than in FEEL-1,which might explain the enhanced ε-PL productivity.The results of amplified fragment length polymorphism analysis indicated that the genomic DNA of AS3-14became more variable after ARTP mutagenesis. Therefore,ARTP combined with streptomycin resistance is an efficient breeding approach for the rapid evolution ofε-PL-producing Streptomyces strains.

Keywordsε-Poly-L-lysine.Atmospheric and room temperature plasma.Streptomycin resistance.Batch and fed-batch fermentation.Enzyme activities analysis. Amplified fragment length polymorphism Introduction

ε-Poly-L-lysine(ε-PL)—a natural homopolymer produced mostly by Streptomyces—usually consists of25–35L-lysine residues linked byα-carboxyl andε-amino groups(Shima and Sakai1977,1981a,b).ε-PL is well known for its antimi-crobial activity towards bacteria,fungi,and some viruses,and therefore has been used widely as a biological preservative in Japan,Korea,and the United States(Hiraki2000).Besides,ε-PL is water soluble,biodegradable,edible,and non-toxic to-wards humans and the environment.Thus,the fields of appli-cation ofε-PL are expected to broaden in future(Shih et al. 2006;Ren et al.2014).

However,wildε-PL-producing strains have lowε-PL bio-synthesis capability so it is difficult to meet industrial needs. For example,production ofε-PL by Streptomyces albulus Z-18,Streptomyces noursei NRRL5126,and Streptomyces griseofuscus H1in shake flask fermentation yielded only 0.24,0.42,and0.7g/L,respectively(Zhang et al.2006; Bankar and Singhal2010).Despite fermentation of Streptomyces noursei NRRL5126in optimized medium in shake-flask culture,ε-PL production was still lower than 1g/L(Li et al.2011).Thus,it is necessary to improve theε-PL productivities of wild strains.Until now,several traditional breeding methods have been undertaken forε-PL production enhancement.The most successful example was screening for mutants with S-(2-aminoethyl)-L-cysteine(AEC)plus glycine resistance after treatment of nitrosoguanidine(NTG)and ul-traviolet(UV)mutagenesis.After20years of continuous work for,this breeding strategy had succeeded in isolating a mutant,Streptomyces albulus11011A,with a maximumε-PL production of2.11g/L,nearly10-fold higher than that of the wild strain(Hiraki et al.1998).With this strain,ε-PL produc-tion as high as48.3g/L was obtained in a5-L jar

fermenter L.Wang

:X.Chen(*):G.Wu:S.Li:X.Zeng:X.Ren:

L.Tang

:Z.Mao(*)

The Key Laboratory of Industrial Biotechnology,Ministry of

Education,School of Biotechnology,Jiangnan University,1800Lihu

Road,Wuxi214122,Jiangsu,China

e-mail:chenxs@https://www.360docs.net/doc/993178141.html,

e-mail:maozg@https://www.360docs.net/doc/993178141.html,

Ann Microbiol

DOI10.1007/s13213-015-1039-8

using a two-stage pH control strategy(Kahar et al.2001), and a~1,000t/a industrial scale was accomplished in Chisso Corporation in Japan in2001.Recently,Zong et al.(2012)reported that they had improved theε-PL yield of Streptomyces albulus A-29from an initial0.4g/ L to1.59g/L through atmospheric and room temperature plasma(ARTP)mutagenesis combined with AEC and glycine resistance screening.However,although an effi-cient selective marker,AEC is very expensive at about US$180per gram.Wang et al.(2012)proposed a com-bined resistance screening method with sulfaguanidine+ glycine+L-lysine+DL-3-hydroxynorvaline as selective markers after ARTP mutagenesis.Finally,an improved mutant Streptomyces diastatochromogenes L9yielding ε-PL production of0.77g/L was obtained,which was 15%higher than that of original strain.It can be seen that the primary principle of these latter approaches were generally based on deleting or weakening the feedback inhibition of target end products such as amino acids; however,such strategies are laborious,time-consuming and expensive.

It is well known that drug-resistance can significantly en-hance the production of secondary metabolites in Streptomyces mutants.For example,a streptomycin resistant mutant Streptomyces chattanoogensis can produce fredericamycin in amounts5-fold greater than the parent strain (Hu and Ochi2001).Moreover,the mutants gained not only streptomycin resistance,but also could grow well under higher concentrations of other antibiotics(Hosoya et al. 1998;Ochi et al.2004).ε-PL—a secondary metabolite in Streptomyces—may be enhanced through increasing the tol-erance to antibiotics of the producing strains.Furthermore, antibiotics could also be an effective selective marker for the screening of high-producing mutants.

In our previous studies(Li et al.2012),genome shuf-fling was adopted to enhanceε-PL production of wild-type strains through improving tolerance to high concentrations of carbon sources such as glucose,sulfaguanidine and succinic acid.Although this approach proved effective, theε-PL titer in the shuffled strains remained low.Thus, a further breeding strategy was applied,i.e.,interspecific hybridization involving stochastic protoplast fusion within the shuffled strains.As a result,a hybrid designated Streptomyces sp.FEEL-1was constructed,which had a higherε-PL production of1.12g/L in shake-flask fermen-tation(Li et al.2013).However,genome shuffling and protoplast fusion are time-consuming and laborious due to the lack of an effective selective marker,thus a new breeding method that is brief and effective is urgently needed.In this study,a breeding method combining ARTP mutagenesis with streptomycin resistance has been developed to improve theε-PL production of Streptomyces sp.FEEL-1.The highestε-PL production mutant found was then evaluated in batch and fed-batch fermentations. Finally,differences in key enzymes and genomic DNA polymorphism between parent and mutant strains were investigated.

Materials and methods

Strain and medium

Streptomyces sp.FEEL-1,with aε-PL yield of1.75g/L in shake-flask fermentation,is a hybrid obtained by inter-specific hybridization among Streptomyces in our previ-ous study(Li et al.2013).It was unable to grow on agar plates containing more than3μg/mL streptomycin. BTN medium was used in solid culture,and M3G was employed as seed medium(Li et al.2012;Hiraki et al. 1998).Fermentation medium was composed of glycerol 60g,yeast extract8g,(NH4)2SO45g,MgSO4·7H2O 2g,KH2PO42g,ZnSO40.03g and FeSO40.04g per liter;and its pH was adjusted to 6.8using2mol/L NaOH before sterilization at121°C for20min.

Strain mutagenesis by ARTP

In each ARTP mutagenesis,a loop of starting strain was inoc-ulated in a BTN agar plate and incubated for7days.Spores were washed with10mL sterilized phosphate buffer(2M) and diluted to106–107CFU/mL.Then,10μL spore suspen-sion was poured into a sterilized sample plate and exposed to

the plasma jet downstream of the plasma torch nozzle exit at 2mm distance.The operating parameters were as follows: radio-frequency power input of120W,gas flow at8L/min (Hua et al.2010),pretreatment time of400s with87.3% lethality rate.The detailed ARTP mutagenesis procedure is shown in Fig.1a.

Successive mutagenesis and mutant screening

After ARTP mutagenesis,the treated spores were spread on BTN agar plates containing3μg/mL streptomycin and cultivated for7days at30°C.A library of mutants with rapid growth was constructed,and the corresponding round of mutagenesis was described as AS1.Spores of each colony in AS1were picked out and mixed together, being used as starters for the next round of ARTP muta-genesis(AS2).Four successive rounds of ARTP mutagen-esis were performed by the same method,and in rounds AS2,AS3,and AS4,the concentrations of streptomycin on BTN agar plates were raised to6,12,and24μg/mL strep-tomycin,respectively.The detailed procedures of rounds

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AS1–AS4are illustrated in Fig.1b .No colonies appeared on the plates of AS4,which indicated the end of the mu-tagenesis.After each round of ARTP treatment,100mu-tants were isolated randomly to carry out fermentation tests in shake-flask culture,with the purposes of (1)examining the relationship between ε-PL production and streptomy-cin resistance (Table 2)and (2)calculating the mutation rate and positive mutation rate (Table 3).

Two control assays were carried out,control A (A1-3)and control B (S1-2);the detailed procedure of the two controls is illustrated in Fig.1b .In control A,the spore suspension of FEEL-1was treated by three successive rounds of ARTP mu-tagenesis.After each round,it was spread on BTN agar plates without streptomycin to determine whether the improved ε-PL production could be achieved without the addition of streptomycin.In control B,the spore suspension of FEEL-1was spread on agar plates containing different concentrations of streptomycin,from 3μg/mL to 12μg/mL,without ARTP mutagenesis.No colonies appeared on medium containing streptomycin beyond 6μg/mL,indicating the end of the assay.The ε-PL production of the potential strains in this study allowed 5%random error,i.e.,only those strains with more than a 5%increase in ε-PL yield were considered as positive mutants.

Evaluation efficiency of mutagenesis to Streptomyces sp.FEEL-1by ARTP

The positive mutation rate and the mutation rate were calcu-lated based on the following equations:R M ?M =T eT?100%R P ?P =M eT?100%

Where R M and R P are the mutation rate and the positive mutation rate,respectively.M is the total CFU of the mutants,and P is the CFU of the mutants whose yield of ε-PL increased at least 5%over than that of the starting strains selected from the previous round.The corresponding data are shown in Table 3.

Shake-flask and 5-L fermenter fermentation conditions For the primary screening,three loops of aerial spores were transferred into a 250-mL shake flask containing 40mL fer-mentation medium and incubated at 30°C,200rpm on a rotating shaker (HYL-C,Qiang Le Laboratory Equipment Co.,Taicang,China)for a 96-h cultivation.For

secondary

Fig.1a Successive atmospheric and room temperature plasma (ARTP)treatment.b Mutant screening procedure

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screening,the mutants with the highestε-PL production in each round were selected out through subcultivation and in-cubated in three independent shake flasks;the results were expressed as mean values±standard deviation(Table2).

To evaluate the potential advantage of mutant AS3-14inε-PL production,batch and fed-batch fermentations were per-formed in a5-L jar-fermenter(Baoxing,Shanghai,China)and compared with the parent FEEL-1.The5-L fermenter contain-ing3.2L sterilized fermentation medium was inoculated with 320mL24-h seed culture from a500-mL shake flask cultiva-tion.The initial pH of the culture broth was adjusted to6.8 using NH3·H2O solution(12.5%v/v).When the initial pH was decreased to the optimal pH of3.8,NH3·H2O solution was added automatically to maintain this pH until fermenta-tion finished.The pH decrease was due to(NH4)2SO4con-sumption and some uncertain organic acid production. Dissolved oxygen was maintained at about30%of air satu-ration,and adjusted by varying the agitation speed from200to 800rpm.The residual glycerol during fermentation was main-tained at about10g/L by supplementing the broth with sterile glycerol via automatic pulses using a peristaltic pump. Similarly,the concentration of NH3-N was maintained at about1g/L in the same way as the feeding of glycerol by adding a sterile600g/L(NH4)2SO4solution. Determination of hexokinase,pyruvate kinase,citrate synthase and aspartokinase activities

Spores of strains were inoculated into fermentation medium at 30°C,200rpm on a rotating shaker.After30h incubation, mycelium of each strain in150mL culture broth was prepared for analysis of cell-free crude enzymes(Chen and Mao2013). Enzyme activities of hexokinase(HK),pyruvate kinase(PK), citrate synthase(CS)and aspartokinase(AsK)were deter-mined according to methods previously published by Teichgraber et al.(1972),Morris et al.(1984),Zeng et al. (2014)and Chen and Mao(2013),respectively.All assays of enzyme activities in two strains were performed in triplicate and the results were expressed as mean values±standard de-viation(Table4).Protein concentrations were measured as described by Bradford(1976).All chemicals,including sub-strates,coenzymes and buffers were obtained from Sigma-Aldrich(St.Louis,MO).

Amplified fragment length polymorphism analysis Amplified fragment length polymorphism(AFLP)analysis was conducted as described by Jin et al.(2008)with a little modification.The genomic DNA was obtained by20-mg/mL lysozyme treatment and extraction using a nucleic acid puri-fication kit(Dongsheng Biotech Co.,Guangzhou,China),and then0.5μg genomic DNA was digested with Pst I/Sac II (Takara Biotechnology Co.,Dalian,China).The restriction sites in the digested DNA were ligated to Pst I-and Sac II-adapters(Table1)by adding T4DNA ligase for4h at16°C. Next,the ligated DNA sample was used as a template for preamplification polymerase chain reaction(PCR).The preamplification PCR was conducted by adding Taq polymer-ase,Pst I and Sac II preamplification primers in a Mastercycler pro PCR thermal cycler(Eppendorf,Hamburg, Germany)for20cycles of30s at94°C,1min at56°C and 1min at72°C.After the preamplification PCR,the product obtained was diluted50-fold and used as the template for the following selective amplification PCR.As each of the four selective primers consists of one selective nucleotide (Table1),the restriction enzyme combination of Pst I/Sac II has16unique primer pairs for each strain in the assay.The selective amplification PCR was conducted by adding Taq polymerase,Pst I and Sac II selective primers in a Mastercycler pro PCR thermal cycler with the following pro-gram:(1)1cycle of1min at94°C,(2)12cycles of denatur-ation for30s at94°C,annealing with a decreasing rate at [65?(n?0.5)]°C for30s(here n is the cycle number),and extension for60s at72°C,and(3)22cycles of denaturation for30s at94°C,annealing for30s at56°C,and extension at 72°C for60s.The processes of electrophoresis and silver staining were conducted as described by Jin et al.(2008). Analysis of dry cell weight,ε-PL concentration and glycerol concentration

The fermentation broth was sampled every6h and centri-fuged at4,500g for10min.The resulting precipitate was washed twice with sterile water and dried to a constant weight at105°C to measure the dry cell weight(DCW)of the culture, and the supernatant was used to determine the concentration ofε-PL and glycerol.ε-PL concentration in various solutions was determined according to the method described by Itzhaki (1972).The concentration of glycerol was detected by an Table1Adapter and primer oligonucleotides applied in the process of amplified fragment length polymorphism(AFLP)reaction

Oligonucleotide Sequence(5′-3′)

Adapters

Pst I CTCGTAGACTGCGTACATGCA

TGTACGCAGTCTAC

Sac II TCGTAGACTGCGTACACGC

GTGTACGCAGTCTAC Preamplification primers

Pst I GACTGCGTACATGCAG

Sac II GACTGCGTACACGCGG

Selective primers(Preamplification primers+1bp)

Pst I-A(T,C,G)GACTGCGTACATGCAGA(or T,C,G) Sac II-A(T,C,G)GACTGCGTACACGCGGA(or T,C,G)

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HPLC system (Dionex U-3000,Thermo Fisher Scientific,Sunnyvale,CA)with an ion exchange column (Aminex HPX-87H,Bio-Rad,Hercules,CA)and a refraction index detector (Shodex RI-101,Tokyo,Japan).The elution of the ion exchange column was performed at 60°C with 5mM H 2SO 4,and a flow rate of 0.6mL/min.

Results and discussion

Mutagenesis and selection of mutants with improved ε-PL productivity

After three rounds of streptomycin-aided ARTP mutagen-esis,strain AS3-14was obtained,which has a ε-PL yield of 2.91g/L —66.3%higher than the original strain FEEL-1(Table 2).Statistical analysis on round AS1-3revealed high mutation rates were obtained with 26.1–33.0%,while the rates of UV mutagenesis and chemical mutagenesis in our previous work were only 7.6%and 15.0%,respectively (data not shown).One of the possi-ble reasons for the high mutation rate in Table 3is that the ARTP system produces atmospheric-pressure helium plasma,which consists mainly of different chemically active species upon irradiated of the stainless steel plate during mutagenesis (Zhang et al.2014).The plasma jet temperature was kept below 40°C to make sure that the damage to the spores was caused by active species rather than by high temperature.According to recent studies,variation in the plasma dosage in ARTP mutagenesis may contribute to a diverse pattern of breakage of plas-mid DNA and oligonucleotides,and the mechanism of action differs significantly from UV and chemical muta-genesis as analyzed by the umu test (Oda et al.1985).Besides,another key parameter,positive mutation rate,was 37%–44.4%,which,although expected,was a very large value.Therefore,selection of streptomycin-resistance mutants after ARTP treatment proved to be

an effective approach for breeding of higher ε-PL-producing strains.

The result of control A showed that the highest ε-PL pro-duction of A3-29was only 2.24g/L,which means ARTP mutagenesis without addition of streptomycin can also en-hance the ε-PL production.In recent publications,ARTP mu-tagenesis has been reported to improve the production of avermectin in Streptomyces avermitilis ,and cellulase in Trichoderma viride .Avermectin production in Streptomyces avermitilis increased from 6,100mg/L to 8,540mg/L,and cellulase production in Trichoderma viride showed a 2.61-fold improvement compared to the parent strain (Wang et al.2010;Xu et al.2011).According to the data in Table 2,al-though the average R M of A1-3was a little higher than that of AS1-3,the average R p was much lower than that of AS1-3.Therefore,ARTP seemed to increase the mutation rate a great deal,whereas monitoring streptomycin-resistance resulted in the efficient screening of high-yield strains.In control B,the result showed that no colonies appeared on medium contain-ing streptomycin beyond 6μg/mL,indicating that enhanced streptomycin-resistance could be obtained efficiently only by successive rounds of ARTP mutagenesis.Furthermore,the highest production of strain S2-2was 2.15g/L,which means resistance to streptomycin can also lead to the enhancement of ε-PL production.As stated above,although the sole use of ARTP mutagenesis or streptomycin-resistance can improve the yield to some extent,the difference in ε-PL production between the control and AS3-14was still large.This study demonstrated that the combination of ARTP and streptomycin-resistance contributed to the improvement of

Table 2

Strain screening during AS1–3,control A and control B.ε-PL ε-poly-L -lysine

AS b

Control A c

Control B d FEEL-1a

AS1-24AS2-68AS3-14A1-33A2-57A3-29S1-4S2-2Streptomycin (μg/mL)361236ε-PL production (g/L)

1.75±0.09

2.12±0.05

2.62±0.11

2.91±0.15

1.85±0.13

1.99±0.11

2.24±0.12

1.89±0.06

2.15±0.12

a Initial strain

b Highest ε-PL-producing mutants in group ASn (n =1-3),respectively

c Highest ε-PL-producing mutants in corresponding rounds of control A (ARTP mutagenesis only)d

Highest ε-PL-producing mutants in corresponding rounds of control B

Table 3Comparison of mutation rate (R M )and the positive mutation rate (R P )in AS1–3and control A

AS1

AS2AS3AS a A1A2A3A a R M (%)26.029.033.029.330.034.029.031.0R P (%)

38.4

44.8

42.4

41.9

23.3

20.5

20.6

21.5

a

Average value

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ε-PL production;however,ARTP mutagenesis or streptomy-cin resistance alone has no significant effect.

During three rounds of ARTP mutagenesis,more than 900colonies were observed in culture.Growth rate and morphology was seen to vary among the colonies.The

morphology of mutants with the highest ε-PL produc-tion in each round of mutagenesis is illustrated in Fig.2:the parent strain FEEL-1appeared hoary,flat and glabrate with a few filamentous mycelia.However,mutants obtained from AS1–AS3were rugous and rich in filamentous mycelia.In addition,the color of the spores of some mutants from AS1and AS2changed to green after cultivating for 4days,whereas spores of FEEL-1are gray.Therefore,the various morphologies observed in AS1to AS3proved that ARTP mutagenesis can change mutant morphology.The variety of morphol-ogy in the actinomycetes can reflect gene diversity and alternative products,thus morphological change is usu-ally considered an indicator for gene mutation of a strain.Some explanation of the underlying reasons can be found in a report on Streptomyces avermitilis ,whose entire genome has been reported,which states that many key regulatory steps involved in morphological differen-tiation and secondary metabolism of this organism share common genes (Wang et al.2010).Mutants with differ-ent morphologies produced ε-PL at different levels.The production of stain F,which looks similar to FEEL-1,proved to be the same as FEEL-1.Thus the diversified morphology reduced the difficulties in screening for hyper-yield

strains.

Fig.2Typical images of colonies with the highest ε-PL production at each round of mutagenesis.FEEL-1Parent strain,F a strain morpholog-ically similar to FEEL-1in the assays;S1-4,S2-2mutants obtained in streptomycin mutagenesis (control B);A1-33,A2-57,A3-29mutants de-veloped from treated spores in corresponding rounds of ARTP mutagen-esis (control A);AS1-24,AS2-68,AS3-14mutants selected in correspond-ing rounds of ASn (n =1–3);All pictures were obtained after the plate had been cultivated for 5days at 30

°C

Fig.3Time courses of ε-PL production by mutant strain AS3-14and the parent strain FEEL-1in batch fermentation at pH 3.8.Filled squares Original strain FEEL-1,open squares improved strain AS3-14.Error bars standard deviation

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Performance of mutant AS3-14in batch and fed-batch bioreactor fermentation

After three rounds mutagenesis,a mutant designated AS3-14was selected with the highest ε-PL production of 2.91g/L in shake-flask fermentation.Subsequently,large-scale fermenta-tion was carried out to verify whether advantage could be taken of its high productivity to meet industrial needs.The batch fermentation performance of AS3-14was compared with that of the parent strain FEEL-1in a 5-L fermenter under the same fermentation conditions.As shown in Fig.3,time profiles of those two strains exhibited the same tendency in terms of pH decline,glycerol consumption,cell growth and ε-PL production.With respect to pH,both pHs dropped natu-rally within 16h to pH 3.8–4.0,which is the optimal pH for ε-PL production (Fig.3a ).However,AS3-14exhibited a more rapid glycerol consumption rate and consumed up 60g/L in 50h,i.e.,5h shorter than required by FEEL-1(Fig.3b ).In terms of cell growth,both strains maintained the same growth rate and achieved a maximum biomass of 19±0.5g/L (Fig.3c ).As anticipated,mutant AS3-14accumulated a high ε-PL level of 8.2g/L,which was 49.1%higher than that of the parent FEEL-1,indicating that ε-PL biosynthesis ability of AS3-14has been improved significantly by the mutagenesis strategy used (Fig.3d ).As the fed-batch fermentation ap-proach is widely employed for ε-PL production (Kahar et al.2001;Ren et al.2014;Chen and Mao 2013),AS3-14was further evaluated using this approach by feeding glycerol and ammonium sulfate (Fig.4).After a 192-h fed-batch fer-mentation,ε-PL production reached 41.2g/L,whereas 24.5g/L of ε-PL was realized with the parent strain FEEL-1under the same operational conditions in our previous study (Li et al.2013).The above result showed that the mutant AS3-14has acquired enhanced ε-PL synthetic ability through ARTP mu-tation combined with streptomycin resistance screening.

Analysis of key enzyme activities in mutant AS3-14and the parent FEEL-1

To further explore the improved ε-PL synthetic ability of AS3-14,key enzyme activities (i.e.,HK,PK,CS and AK)were investigated and compared with those of the parent strain FEEL-1.As shown in Table 4,AS3-14possessed clearly higher activities of HK,with a 2.47-fold enhancement com-pared to FEEL-1,which suggested that the Embden-Meyerhof-Parnass (EMP)pathway in AS3-14was more ac-tive than that in FEEL-1.Furthermore,the elevated CS en-zyme activity (1.84-fold enhancement)may direct more met-abolic flux to the tricarboxylic acid (TCA)cycle,which can then provide an abundance of oxaloacetic acid (OAA),the precursor of the diaminopimelate (DAP)pathway.The strengthened TCA cycle may also offer more ATP for ε-PL synthesis.This finding is similar to that of a previous report that adding citrate to the medium could facilitate the ε-PL production of strain USE-51(Hirohara et al.2006).In addition,the activity of AK –the first key enzyme in the DAP pathway —was 160%higher in AS3-14than that in FEEL-1.This improvement resulted in a large metabolic flux through the DAP pathway,which can provide more precursor lysine.The improved activities of these key en-zymes may go some way to explaining the high ε-PL pro-duction of the

mutant.

Fig.4Time course of ε-PL pro-duction by mutant strain AS3-14in fed-batch fermentation at pH 3.8

Table 4Key enzyme activities (U/mg)of AS3-14and FEEL-1after cultivation in shake flask for 30h.HK Hexokinase,PK pyruvate kinase,CS citrate synthase,AsK aspartokinase Strain HK PK CS AsK FEEL-148.6±4.212.9±1.220.5±0.5 2.7±0.1AS3-14

120.2±7.2

16.6±0.7

37.7±1.2

4.1±0.1

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Assessment of genomic variation analysis between parent and mutant strains by AFLP analysis

AFLP analysis combined with restriction enzymes Pst I/Sac II is an efficient method with which to distinguish genomic DNA variation among parent strains and the mutants,the ε-PL productivities of which were 1.75,1.99,2.62,and 2.91g/L,respectively.Sixteen primer pairs were tested to detect the most appropriate pairs of selective primers that yielded the most informative and polymorphic AFLP banding patterns.Results obtained with the six primer pairs selected are illustrated in Fig.5.The number of frag-ments amplified and the polymorphism rate were calculat-ed and are shown in Table 5.The data showed that the average polymorphism rates reached were 1.9%,6.5%,and 13.4%for parent FEEL-1,and mutants AS2-68and AS3-14,respectively,indicating an obvious increasing polymorphism in mutants compared to the parent.Moreover,this result also showed a positive relationship between polymorphism rate and ε-PL yield.The results presented in Fig.5show that the mutant AS2-68had more polymorphic bands than mutant A2-57,suggesting that ge-nomic variation happened more easily and extensively when streptomycin was added in the course of induced mutagenesis.As a result,the strategy of ARTP mutagene-sis combined with streptomycin-resistance resulted in higher genomic variation of mutants and enhanced ε-PL production.

Conclusion

In this study,a mutant Streptomyces sp.AS3-14produc-ing 2.91g/L ε-PL was obtained through ARTP mutagen-esis combined with streptomycin-resistance screening.After a 192-h fed-batch fermentation,ε-PL production of 41.2g/L was realized by mutant AS3-14,suggesting that the strain has potential for industrial application.Further analyses showed that there are significant differ-ences between the mutant AS3-14and parent FEEL-1strain in key enzymes and in genomic DNA polymor-phism.The results demonstrate that the strategy of ARTP mutagenesis combined with

streptomycin-

Fig.5Amplified fragment length polymorphism (AFLP)banding pat-terns of the mutants derived in the course of ARTP mutagenesis assays and the ancestor FEEL-1.The examples demonstrated here were gener-ated with the selective primer combination Pst I-A/Sac II-G.1,2and 4polymorphic bands (arrows )could be distinguished,respectively,from the mutant A2-57obtained from control A,and the mutants AS2-68and AS3-14from rounds AS2and AS3,respectively,which were absent in the ancestor FEEL-1

Table 5

Selected primer pairs and polymorphism for the initial strain FEEL-1and the mutants obtained by mutagenesis

Selective primer pair Total number of bands Number of polymorphic bands Polymorphism rate (%)Ps t I Sac II FEEL-1A2-57AS2-68AS3-14FEEL-1A2-57AS2-68AS3-14FEEL-1A2-57AS2-68AS3-14A T 23232526002300811.5A A 2021212301130 4.8 4.813A G 19192022223410.510.51518.2C C 171818191223 5.911.111.115.8C A 212121220001000 4.5C G

19202023011405517.4Total 1191221251353691816.431.443.980.4Average

19.8

20.3

20.8

22.5

0.5

1

1.5

3

2.7

5.2

7.3

13.4

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resistance screening provides a powerful approach for obtaining enhancedε-PL-producing strains. Acknowledgments This work was supported by the Project of Scien-tific and Technical Supporting Program of Jiangsu(BE2012616),the Cooperation Project of Jiangsu Province among Industries,Universities and Institutes(BY2013015-11),the Open Project Program of the Key Laboratory of Industrial Biotechnology,Ministry of Education,China (KLIBKF201302),the National Natural Science Foundation of China (21376106),and the Jiangsu Province B Collaborative Innovation Center for Advanced Industrial Fermentation^Industry Development Program.

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20072016:黏弹性流体在多孔介质中的新渗流模型

黏弹性流体在多孔介质中的新渗流模型 学号:20072016 姓名:刘超 摘要:对两种类别的常用聚合物:多糖类(黄原胶)和部分水解聚丙烯酰胺(pusher-700)在玻璃珠人造岩心和贝雷砂岩中稳态流动的实验数据进行了分析。用振荡流测量计算聚合物溶解的最长弛豫时间( θ),即本文所涉及的特征弛豫 1f 时间。两种聚合物的稳态流实验数据与所测得的聚合物自身的黏弹性数据一起被换算成在多孔介质中的平均剪切应力-剪切速率数据,因此就得到聚合物流在多孔介质中的平均幂律指数(n)。用 θ、n、岩石渗透率(k)、饱和度(φ)和渗流 1f 速度(μ)计算黏弹性数( N),结果发现黏弹性数V N与多孔介质中的压力梯度密 V 切相关。这种相关性是定义聚合物渗流黏弹性模型的基础,类似于达西定律。新的模型认为渗流速度和压力梯度呈非线性关系,这证实了聚合物的黏弹性变形,并且也证实孔隙的几何尺寸变化是聚合物的分子吸附和机械滞留所致。 关键词:多孔介质;黏弹性流体;人造岩心;贝雷砂岩;渗流模型;特征弛豫时间 一、概述 聚合物在石油工程方面已经得到广泛的应用。在提高采收率方面,将聚合物加到水中是为了增加水的黏度和减小水的相对流度。水相对流度的降低提高了水的体积波及系数和水驱效率。虽然对聚合物在多孔介质中的渗流机理已经研究了几十年,但是至今没有重大突破。 达西定律适用于渗流流体为线性流,且其黏度恒定、孔隙的几何尺寸也恒定的情况。聚合物在多孔介质中的渗流偏离这些假设是因为:①聚合物的黏度是和剪切速率相关的;②聚合物分子的长短是和孔喉尺寸相匹配的,这样才可提高弹性特性;③聚合物分子的吸附和机械滞留改变了孔隙介质的几何尺寸。因此,应用达西定律模拟聚合物在多孔介质中的流动是错误的。模拟聚合物渗流的传统方法是在应用达西定律的同时应用一个有效黏度,即用恒定剪切速率下的黏度代替牛顿黏度。这种方法校正了剪切速率与黏度的相关性,但却没有考虑到非线性流和弹性流的特性。 Van Poollen和Jargon、Willhite和Uhl给出了一个关于非牛顿流体渗流时压降(ΔP)和渗流速率(Q)之间呈非线性关系的简单的经验模型。这种关系可以表示为:

【精品】高等数学习题详解第2章 极限与连续

习题2-1 1.观察下列数列的变化趋势,写出其极限: (1)1n n x n =+; (2)2(1)n n x =--; (3)13(1)n n x n =+-; (4)2 11n x n =-。 解:(1)此数列为12341234,,,,,,23451n n x x x x x n =====+所以lim 1n n x →∞ =。 (2)12343,1,3,1,,2(1),n n x x x x x =====--所以原数列极限不存在。 (3)1234111131,3,3,3,,3(1),234n n x x x x x n =-=+=-=+=+- 所以lim 3n n x →∞ =。 (4)12342111111,1,1,1,,1,4916n x x x x x n =-=-=-=-=-所以lim 1n n x →∞ =- 2.下列说法是否正确: (1)收敛数列一定有界; (2)有界数列一定收敛; (3)无界数列一定发散;

(4)极限大于0的数列的通项也一定大于0. 解:(1)正确. (2)错误例如数列{}(-1)n 有界,但它不收敛。 (3)正确。 (4)错误例如数列21(1)n n x n ??=+-???? 极限为1,极限大于零,但是11x =-小于零。 *3。用数列极限的精确定义证明下列极限: (1)1 (1)lim 1n n n n -→∞+-=; (2)222lim 11 n n n n →∞-=++; (3)3 23125lim -=-+∞→n n n 证:(1)对于任给的正数ε,要使1(1)111n n n x n n ε-+--=-=<,只要1n ε >即可,所以可取正整数1 N ε≥. 因此,0ε?>,1N ε???=???? ,当n N >时,总有1(1)1n n n ε-+--<,所以

第三章粘弹性流体的本构方程

第三章非线性粘弹流体的本构方程 1.本构方程概念 本构方程(constitutive equation),又称状态方程——描述一大类材料所遵循的与材料结构属性相关的力学响应规律的方程。 不同材料以不同本构方程表现其最基本的物性,对高分子材料流变学来讲,寻求能够正确描述高分子液体非线性粘弹响应规律的本构方程无疑为其最重要的中心任务,这也是建立高分子材料流变学理论的基础。 两种。 唯象性方法,一般不追求材料的微观结构,而是强调实验事实,现象性地推广流体力学、弹性力学、高分子物理学中关于线性粘弹性本构方程的研究结果,直接给出描写非线性粘弹流体应力、应变、应变率间的关系。以本构方程中的参数,如粘度、模量、松弛时间等,表征材料的特性。 分子论方法,重在建立能够描述高分子材料大分子链流动的正确模型,研究微观结构对材料流动性的影响。采用热力学和统计力学方法,将宏观流变性质与分子结构参数(如分子量,分子量分布,链段结构参数等)联系起来。为此首先提出能够描述大分子链运动的正确模型是问题关键。 根据研究对象不同, 象性方法和分子论方法虽然出发点不同,逻辑推理的思路不尽相同,而最终的结论却十分接近,表明这是一个正确的科学的研究基础。

目前关于高分子材料,特别浓厚体系本构方程的研究仍十分活跃。 同时,大量的实验积累着越来越多的数据,它们是检验本构方程优劣的最重要标志。 从形式上分, 速率型本构方程,方程中包含应力张量或形变速率张量的时间微商,或同时包含这两个微商。 积分型本构方程,利用迭加原理,把应力表示成应变历史上的积分,或者用一系列松弛时间连续分布的模型的迭加来描述材料的非线性粘弹性。积分又分为单重积分或多重积分。 判断一个本构方程的优劣主要考察: 1)方程的立论是否科学合理,论据是否充分,结论是否简单明了。 2)一个好的理论,不仅能正确描写已知的实验事实,还应能预言至今未知,但可能发生的事实。 3)有承前启后的功能。例如我们提出一个描写非线性粘弹流体的本构方程,当条件简化时,它应能还原为描写线性粘弹流体的本构关系。 4)最后也是最重要的一条,即实验事实(实验数据)是判断一个本构方程优劣的出发点和归宿。实践是检验真理的唯一标准。 本章重点介绍用唯象论方法对一般非线性粘弹流体建立的本构方程。分子论方法在第四章介绍。

常见的金属晶体结构

第二章作业 2-1 常见的金属晶体结构有哪几种它们的原子排列和晶格常数有什么特点 V、Mg、Zn 各属何种结构答:常见晶体结构有 3 种:⑴体心立方:-Fe、Cr、V ⑵面心立方:-Fe、Al、Cu、Ni ⑶密排六方:Mg、Zn -Fe、-Fe、Al、Cu、Ni、Cr、 2---7 为何单晶体具有各向异性,而多晶体在一般情况下不显示出各向异性答:因为单晶体内各个方向上原子排列密度不同,造成原子间结合力不同,因而表现出各向异性;而多晶体是由很多个单晶体所组成,它在各个方向上的力相互抵消平衡,因而表现各向同性。第三章作业3-2 如果其它条件相同,试比较在下列铸造条件下,所得铸件晶粒的大小;⑴金属模浇注与砂模浇注;⑵高温浇注与低温浇注;⑶铸成薄壁件与铸成厚壁件;⑷浇注时采用振动与不采用振动;⑸厚大铸件的表面部分与中心部分。答:晶粒大小:⑴金属模浇注的晶粒小⑵低温浇注的晶粒小⑶铸成薄壁件的晶粒小⑷采用振动的晶粒小⑸厚大铸件表面部分的晶粒小第四章作业 4-4 在常温下为什么细晶粒金属强度高,且塑性、韧性也好试用多晶体塑性变形的特点予以解释。答:晶粒细小而均匀,不仅常温下强度较高,而且塑性和韧性也较好,即强韧性好。原因是:(1)强度高:Hall-Petch 公式。晶界越多,越难滑移。(2)塑性好:晶粒越多,变形均匀而分散,减少应力集中。(3)韧性好:晶粒越细,晶界越曲折,裂纹越不易传播。 4-6 生产中加工长的精密细杠(或轴)时,常在半精加工后,将将丝杠吊挂起来并用木锤沿全长轻击几遍在吊挂 7~15 天,然后再精加工。试解释这样做的目的及其原因答:这叫时效处理一般是在工件热处理之后进行原因用木锤轻击是为了尽快消除工件内部应力减少成品形变应力吊起来,是细长工件的一种存放形式吊个7 天,让工件释放应力的时间,轴越粗放的时间越长。 4-8 钨在1000℃变形加工,锡在室温下变形加工,请说明它们是热加工还是冷加工(钨熔点是3410℃,锡熔点是232℃)答:W、Sn 的最低再结晶温度分别为: TR(W) =(~×(3410+273)-273 =(1200~1568)(℃)>1000℃ TR(Sn) =(~×(232+273)-273 =(-71~-20)(℃) <25℃ 所以 W 在1000℃时为冷加工,Sn 在室温下为热加工 4-9 用下列三种方法制造齿轮,哪一种比较理想为什么(1)用厚钢板切出圆饼,再加工成齿轮;(2)由粗钢棒切下圆饼,再加工成齿轮;(3)由圆棒锻成圆饼,再加工成齿轮。答:齿轮的材料、加工与加工工艺有一定的原则,同时也要根据实际情况具体而定,总的原则是满足使用要求;加工便当;性价比最佳。对齿轮而言,要看是干什么用的齿轮,对于精度要求不高的,使用频率不高,强度也没什么要求的,方法 1、2 都可以,用方法 3 反倒是画蛇添足了。对于精密传动齿轮和高速运转齿轮及对强度和可靠性要求高的齿轮,方法 3 就是合理的。经过锻造的齿坯,金属内部晶粒更加细化,内应力均匀,材料的杂质更少,相对材料的强度也有所提高,经过锻造的毛坯加工的齿轮精度稳定,强度更好。 4-10 用一冷拔钢丝绳吊装一大型工件入炉,并随工件一起加热到1000℃,保温后再次吊装工件时钢丝绳发生断裂,试分析原因答:由于冷拔钢丝在生产过程中受到挤压作用产生了加工硬化使钢丝本身具有一定的强度和硬度,那么再吊重物时才有足够的强度,当将钢丝绳和工件放置在1000℃炉内进行加热和保温后,等于对钢丝绳进行了回复和再结晶处理,所以使钢丝绳的性能大大下降,所以再吊重物时发生断裂。 4-11 在室温下对铅板进行弯折,越弯越硬,而稍隔一段时间再行弯折,铅板又像最初一样柔软这是什么原因答:铅板在室温下的加工属于热加工,加工硬化的同时伴随回复和再结晶过程。越弯越硬是由于位错大量增加而引起的加工硬化造成,而过一段时间又会变软是因为室温对于铅已经是再结晶温度以上,所以伴随着回复和再结晶过程,等轴的没有变形晶粒取代了变形晶粒,硬度和塑性又恢复到了未变形之前。第五章作业 5-3 一次渗碳体、二次渗碳体、三次渗碳体、共晶渗碳体、共析渗碳体异同答:一次渗碳体:由液相中直接析出来的渗碳体称为一次渗碳体。二次渗碳体:从 A 中析出的渗碳体称为二次渗碳体。三次渗碳体:从 F 中析出的渗碳体称为三次渗碳体共晶渗碳体:经共晶反应生成的渗碳体即莱氏体中的渗碳体称为共晶渗碳体共析渗碳体:经共析反应生成的渗碳体即珠光体中的渗

数列极限的概念(经典课件)

第二章 数列极限 引言: 在第一章中我们已经指出,数学分析课程研究的对象是定义在实数集上的函数,那么数学分析用什么方法研究实数集上的函数呢?从本质上来说,这个方法就是极限。极限思想和方法贯穿于数学分析课程的始终,几乎所有的概念都离不开极限,是我们数学分析课程的基础。 §1 数列极限的概念 教学内容:数列极限的概念,应用定义证明简单数列的极限,无穷小数列。 教学要求:使学生逐步建立起数列极限的N ε-定义的清晰概念。深刻理解数列发散、单调、有界和无穷小 数列等有关概念。会应用数列极限的N ε-定义证明数列的有关命题,并能运用N ε-语言正确表述数列不以某实数为极限等相应陈述。 教学重点:数列极限的概念。 教学难点:数列极限的N ε-定义及其应用。 教学方法:讲授为主。 教学学时:2学时。 一、数列概念: 1.数列的定义: 简单的说,数列就是“一列数”,是有一定的规律,有一定次序性的“一列数”。 若函数f 的定义域为全体正整数集合N +,则称:f N R +→或+∈N n n f ),(为数列。 若记()n f n a =,则数列n n n f ,2,1),(=就可写作为:12,,,, n a a a ,简记为{}n a ,其中n a 称为 该数列的通项。 2.数列的例子: (1)(1)111:1,,,, 234n n ??---???? ; (2)11111:2,1,1,1,435 n ? ?+ +++???? (3){}2 :1,4,9,16,25, n ; (4){}1 1(1) :2,0,2,0,2, n ++- 二、数列极限的概念: 1.引言: 对于这个问题,先看一个例子:古代哲学家庄周所著的《庄子. 天下篇》引用过一句话:“一尺之棰,日取其半,万世不竭”。把每天截下的部分的长度列出如下(单位为尺): 第1天截下 12,第2天截下2111222?=,第3天截下23111222?=,…,第n 天截下1111 222 n n -?=,… 得到一个数列:? ?? ?? ?n 21: 231111 ,,,,,2222n 不难看出,数列12n ?? ? ??? 的通项12n 随着n 的无限增大而无限地接近于零。 一般地说,对于数列{}n a ,若当n 无限增大时,n a 能无限地接近某一个常数a ,则称此数列为收敛数列,常数a 称为它的极限。不具有这种特性的数列就不是收敛的数列,或称为发散数列。

广义Oldroyd-B粘弹性流体Stokes第一问题

Stokes' first problem for a viscoelastic fluid with the generalized Oldroyd-B model1 Haitao Qi Department of Applied Mathematics and Statistics, Shandong University at Weihai Weihai, P. R. China 264209 htqi@https://www.360docs.net/doc/993178141.html, Mingyu Xu School of Mathematics and Systematical Science, Shandong University Jinan, P.R. China, 250100 Abstract The flow near a wall suddenly set in motion for a viscoelastic fluid with the generalized Oldroyd-B model is studied. The fractional calculus approach has been taken into account in the constitutive relationship of fluid model. Exact analytical solutions of velocity and stress are obtained by using the discrete Laplace transform of the sequential fractional derivative and the Fox-H function. The obtained results indicate that some well known solutions for the Newtonian fluid, the generalized second grade fluid as well as for the ordinary Oldroyd-B fluid, as limiting cases, are included in our solutions. Keywords: Generalized Oldroyd-B fluid, Stokes' first problem, Fractional calculus, Exact solution, Fox- H function. 1Introduction Navier-Stokes equations are the most fundamental motion equations in fluid dynamics. However, there are few cases in which their exact analytical solutions can be obtained. Exact solutions are very important not only because they are solutions of some fundamental flows, but also because they serve as accuracy checks for experimental, numerical, and asymptotic methods. The inadequacy of the classical Navier-Stokes theory to describe rheologically complex fluids such as polymer solutions, blood and heavy oils, has led to the development of several theories of non-Newtonian fluids. In order to describe the non-linear relationship between the stress and the rate of strain, numerous models or constitutive equations have been proposed. The model of differential type and those of rate type have received much attention [1]. In recent years, the Oldroyd-B fluid has acquired a special status amongst the many fluids of the rate type, as it includes as special cases the 1 Supported by the National Natural Science Foundation of China (10272067), the Doctoral Program Foundation of the Education Ministry of China (20030422046) and the Natural Science Foundation of Shandong University at Weihai.

高等数学习题详解-第2章-极限与连续

习题2-1 1. 观察下列数列的变化趋势,写出其极限: (1) 1 n n x n = + ; (2) 2(1)n n x =--; (3) 13(1)n n x n =+-; (4) 211n x n =-. 解:(1) 此数列为12341234,,,,,,23451 n n x x x x x n =====+L L 所以lim 1n n x →∞=。 (2) 12343,1,3,1,,2(1),n n x x x x x =====--L L 所以原数列极限不存在。 (3) 1234111131,3,3,3,,3(1),234n n x x x x x n =-=+=-=+=+-L L 所以lim 3n n x →∞ =。 (4) 123421111 11,1,1,1,,1,4916n x x x x x n =-= -=-=-=-L L 所以lim 1n n x →∞=- 2.下列说法是否正确: (1)收敛数列一定有界 ; (2)有界数列一定收敛; (3)无界数列一定发散; (4)极限大于0的数列的通项也一定大于0. 解:(1) 正确。 (2) 错误 例如数列{} (-1)n 有界,但它不收敛。 (3) 正确。 (4) 错误 例如数列21(1) n n x n ?? =+-??? ? 极限为1,极限大于零,但是11x =-小于零。 *3.用数列极限的精确定义证明下列极限: (1) 1 (1)lim 1n n n n -→∞+-=; (2) 22 2 lim 11 n n n n →∞-=++; (3) 3 2 3125lim -=-+∞→n n n 证:(1) 对于任给的正数ε,要使1(1)111n n n x n n ε-+--= -=<,只要1 n ε >即可,所以可取正整数1 N ε ≥ . 因此,0ε?>,1N ε?? ?=???? ,当n N >时,总有 1(1)1n n n ε-+--<,所以

铁素体马氏体和奥氏体的区别

铁素体和奥氏体的区别 钢的组织和特性?铁是钢的基本组成元素。铁在固态有两种晶体结构,一是体心立方结构(存在于两个温度范围内,?912?℃?以上称?α? 铁,?1394?℃?以上称?δ?铁);另一是面心立方结构(存在 于?912?~?1394?℃?之间,称?γ?铁)。碳是钢中另一主要元素,对钢的组织和性能起重要作用,通常随着含碳量的增加,钢的强度增加、塑性下降。碳在钢中主要有两种存在形式,一是溶入铁中与铁形成固溶体(两种以上化学组分互相溶解而形成的均匀固相);另一是与铁形成铁碳化合物,称渗碳体(?Fe?3C?),其硬度高、脆性大。碳溶于?α?铁中形成的固溶体称铁素体;溶于?γ?铁中形成的固溶体称奥氏体,其最大溶解度为??%。钢在冷却过程中,过饱和的奥氏体将发生分解,形成铁素体和渗碳体。铁素体和渗碳体组成的呈片状相间排列的混合物称珠光体。一般碳素钢在室温下的金相组织由铁素体、珠光体和渗碳体组成? 铁素体是碳溶解在a-Fe中的间隙固溶体,常用符号F表示。 不锈钢中的“铁素体”,指的是碳溶解在a-Fe中的间隙固溶体,其溶碳能力很小,常温下仅能溶解为%的碳,在727℃时最大的溶碳能力为%, 它仍保持的体心立方晶格.常用符号F表示。

由于铁素体含碳量很低,其 c:\iknow\docshare\data\cur_work\&aid=6148&sid=&click=1&url=http:的是在使用状态下以铁素体组织为主的不锈钢。它的含铬量在11%~30%,具有体心立方晶体结构,至于不锈钢含铁量与它是否是铁素体不锈钢并无关系.铁素体不锈钢只取决于在使用状态下,它是否以铁素体组织为主. 铁素体有磁性. 在使用状态下以铁素体组织为主的不锈钢。含铬量在11%~30%,具有体心立方晶体结构。这类钢一般不含镍,有时还含有少量的Mo、Ti、Nb等到元素,这类钢具导热系数大,膨胀系数小、抗氧化性好、抗应力腐蚀优良等c:\iknow\docshare\data\cur_work\&aid=6025&sid=&click=1&url=http:727℃1148℃727℃是奥氏体不锈钢的三大元素之一(碳、铬、镍)。镍在奥氏体不锈钢中的作用是与碳紧密结合(不锈钢含碳量越大越容易生锈,为了使奥氏体不锈钢既具有强度又不容易生锈,就需要控制碳的含量,而镍正好弥补这一缺陷),增加其强度及硬度。因为镍抗磁性元素,所以奥氏体不锈钢是没有磁性的。因为铁素体不锈钢主要用于加工装饰方面,需具有良好的塑性与韧性,所以它只含极少量的镍元素,因而它是有磁性的。B. 因为马氏体和铁素体的内部电子都有规则的排列;决定磁性的关键因素是排列规则的电子有规律的运动.而镍正好破坏了电子间这种有规则的排列。 为什么不锈钢不生锈铬具有耐腐蚀性。奥氏体不锈钢、马氏体和铁素体不锈钢都含有12%——30%的铬元素,所以它们不生锈。

极限概念

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第一章流体流动答案

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