简介:从理论上推导了声学超表面对平面声波的作用模型,该理论模型计及声波高阶衍射模态,从而能够计及超表面微结构之间的声学干扰.通过与数值结果对比,该模型预测的反射频率精度得到了一定程度的提高,并能够分辨出相邻孔声场之间的耦合模态.讨论了声学超表面吸声特性与阻抗特性对高超声速边界层内Mack第2模态的抑制机理,研究发现通过设计超表面阻抗特性,使得入射声波与反射声波在壁面处相位相反,同样可以抑制Mack第2模态.基于理论模型,分别优化设计得到最优的微结构几何尺寸,并通过对Mach6平板边界层流动进行稳定性分析,验证了超表面不同声学特性的抑制效果.
简介:Duetotheirsize-dependentpropertiesandflexibleprocessingchemistry(Alivisatos,1996),monodispersednanocrystalshaveattractedconsiderableattentionfortheirdistinguishedroleinfundamentalstudiesandtechnicalapplications.AlthoughmuchhasbeenreportedonfabricationofmonodispersedinorganicnanocrystalssuchasFe2O3,CdSe,PbSeandZrO2(Peng&Peng,2002;Zhuetal.,2002;Niederbergeretal.,2002;Pangetal.,2001),littlepublishedonthepreparationofBaSO4nanocrystalsandtheircrystalsizeandmorphologywhichaffecttheirperformanceinapplication.PolymeradditiveswithcomplexfunctionshavebeenusedastemplatestodirectorientednucleationandgrowthofBaSO4(Xiaoetal.,2001;Yuetal.,2002;Summersetal.,2002),asanewclassoffunctionalpolymers,theso-calleddouble-hydrophilicblockcopolymers,designedtocontrolthemorphologyofBaSO4(Colfenetal.,2002),includingtheirnanofiberssynthesizedinAOTmicroemulsion(Li&Mann,2000).ThoughsomuchresearchhasbeendevotedtocontrollingtheBaSO4morphologies,thepresentnotewillreportonthesynthesisofBaSO4nanocrystalswithsizecontrolling.
简介:Well-dispersedBaSO4nanoparticlesweresynthesizedinthepresenceofsodiumpolyacrylate(PAAS)byasimpleprecipitationmethod,withBaCl2and(NH4)2SO4asreactants.ThedifferentrolesperformedbyPAASinthesynthesisofBaSO4nanoparticleswereinvestigatedusingX-raydiffractometry,Fouriertransforminfraredspectroscopy,andtransmissionelectronmicroscopy.TheresultsindicatethattheassynthesizedBaSO4nanoparticlesweresphereswithanaveragediameterof30nmandthattheirsurfaceswereaffectedbythePAAS.Underatypicalprocedureemployed,PAASreactedwithBaCl2toyieldanintermediate,servingasacontrolreleasingagentandseparatingthenucleationandcrystalgrowthprocessesoftheBaSO4nuclei.DuringformationoftheBaSO4nanospheres,theintermediateslowlydissolvedandreleasedbariumandpolyacrylateions,inhibitingthegrowthandaggregationofnewlyformedBaSO4seedsandresultinginparticlesofnarrowdiameterdistributionandimproveddispersibility.Moreover,thesepolyacrylateionsfurthermodifiedthesurfacesoftheBaSO4nanoparticles.
简介:Fatiguepropertiesofage-hardenedAlalloy2017-T4underultrasonicloadingfrequency(20kHz)wereinvestigatedandcomparedwiththeresultsunderconventionalloadingofrotatingbending(50Hz).Thegrowthofacrackretardedatabout500μminsurfacelengthunderultrasonicloading,whileatabout20μmunderrotatingbending.Althoughstriationsbeingatypicalfracturemechanismwereobservedunderconventionalloading,mostoffracturesurfacewascoveredwithmanyfacetsunderultrasonicloading.Thesefacetswerealsoobservedunderrotatingbendinginnitrogengas.Thedifferenceingrowthmechanismdependingontheloadingfrequencyandtheretardationofacrackgrowthunderultrasonicloadingmaybecausedbytheenvironmentatthecracktipduetohighcrackgrowthrateunderultrasonicloading.
简介:TheinfluenceofNa2SO4ontheformationofZnOwhiskerswasinvestigatedinthispaper.ZnOwhiskerswithaspectratiosofupto50weresynthesizedbydissolvingε-Zn(OH)2precursorinNaOH/Na2SO4solutionatroomtemperature,followedbyagingoftheresultingsolutionat140℃for6h.Fouriertransforminfraredspectroscopy,Ramanspectroscopy,andX-rayphotoelectronspectroscopyanalysesrevealedthatSO42-ionswereprimarilyadsorbedonthe(100)planeoftheZnOwhiskersviaanouter-spherecomplexconfiguration(O-H…O),therebypromotingtheone-dimensionalgrowthofZnOwhiskersalongthec-axis.
简介:有从30~80nm的尺寸的Fe3O4nanoparticles被湿milling铁粉末在一家行星的球工厂综合。阶段作文和同样综合的产品的形态学被X光检查衍射(XRD)测量,扫描电子显微镜学(SEM)和传播电子显微镜学(TEM)。NanosizedFe3O4粒子被湿milling准备金属性的铁粉末(?200网孔,99%)在与不锈钢装备的一家行星的球工厂,使用的小瓶与50:1并且以300rpm的旋转速度的ball-to-powder团比率在提取的水下面熨球。在这个方法的铁球的使用在Fe3O4formation起了一个关键作用。现在的技术简单,这个过程是容易的执行。
简介:Octahedron-likeBaMoO4microcrystalsweresynthesizedbytheco-precipitationmethodatroomtemperatureandprocessedinmicrowave-hydrothermalat413Kfordifferenttimes(from30minto5h).ThesemicrocrystalswereanalyzedbyX-raydiffraction(XRD),field-emissiongunscanningelectronmicroscopy(FEG-SEM)andtransmissionelectronmicroscopy(TEM).XRDpatternsshowedthatthismaterialpresentsatetragonalstructurewithoutthepresenceofdeleteriousphases.FEG-SEMandTEMmicrographsrevealedthattheBaMoO4microcrystalspresentanoctahedron-likemorphologywithagglomeratenatureandpoly-disperseparticlesizedistribution.Thesemicrographsalsoindicatedthatthemicrocrystalsgrowalongthe[001]direction.Theobservedcrystallographicplanesinthesestructuresweremodeledcomputationallyandacrystalgrowthmodelwasproposedinordertoexplainthemorphologicalchangesasafunctionofprocessingtime.
简介:TiO2nanoparticlesweresynthesizedbyahomogeneouscontrolledprecipitationmethodusingindustrialtitaniumsulfate(Ti(SO4)2).TheobtainedpowderswerecharacterizedbyX-raydiffraction(XRD),transmissionelectronmicroscope(TEM),Fouriertransforminfraredspectroscopy(FTIR)andICPplasmaspectrometer.EDTAwasusedascomplexingagenttoimprovethepurityandtheformationofTiO2nanoparticles.Experimentalresultsindicatedthatthehigh-purityTiO2nanoparticleswere20nminmeansizeandnearlymonodispersed.