Treść książki

Przejdź do opcji czytnikaPrzejdź do nawigacjiPrzejdź do informacjiPrzejdź do stopki
44
2.GrowthofABO
3andBO
2Crystals
possibletoobservephasetransitions.Thesequestionshavebeenthesub-
jectsofparticularlyintenseexperimentalandtheoreticalstudiesforbarium
titanatecrystalwhichshowsferroelectricpropertiesatroomtemperature
andhasawiderangeofapplications.DSCdatarevealedchangesinphase
transitiontemperaturesinthismaterialinducedbylatticedefects,grainsize
changes[73],theefectofnon-stoichiometry[74],surfacecharges[75]or
agingprocesses[76].Accordingtotheresultsofstudiesonthecriticalsize,
theyindicateaverywiderangeofparticlediametersinwhichphasetransi-
tionshavebeenobserved.ApplyingadditionallyX-raymethods,including
thermodifractionandsynchrotronX-raydifraction,andRamanspectros-
copy,Smithetal.haveshownthatinnanocrystalsthecubic-to-tetragonal
phasetransitionmayoccurinawidetemperaturerange,incontrasttothe
sharptransitionfoundinthebulkmaterial[64].Analyzingthe
137BaNMR
spectra,SedykhandMichelhaveevidencedashiftofthephasetransition
inBaTiO
3by3degreesondecreasingthenanoparticlessizefrom155nm
to15nm[77].Onthebasisofthevariable-temperatureelectrostaticforce
microscopystudies,Spanieretal.haveshownthatforBaTiO
3nanowires
theparticlediameteratwhichphasetransitionfallsbelowroomtempera-
tureis~3nm[75].Studyingultrathinperovskitefilmsofleadtitanate,Fong
etal.haveprovedthestabilityoftheferroelectricphasetillthethickness
of3unitcells,so1.2nm[78].Forthebulkmaterials,preliminaryoptical
characterizationismadebyopticalorpolarizationmicroscopes,butthe
samplessmallerthan300nmarebeyondobservation.Characterization
ofsmallerparticlesneedstheuseofelectronmicroscopyandatransmis-
sionelectronmicroscope(TEM)anditsmodifications[79-81].Theidea
ofTEMisbasedonthedeBrogliewaveconceptionaccordingtowhichthe
lengthofelectronwaveλisrelatedtotheirenergyas:
O
1
122
i
E
where:EistheelectronsenergyineV,λisthewavelengthinnm.For
instance,forelectronsof100keVabeamofwavesofthelengthslightly
below4pmisobtained.Thedevelopmentofelectronmicroscopyhasbeen
triggeredbythestudiesofKnoll&Rusk[82]onfocusinganelectron
beam.ModernTEMproduceverynarrowelectronbeams,typically<5nm
andatthebest<0.1nmindiameter.Theseparameterspermittheobserva-
tionofaverysmallfragmentofthesample.Similarly,asinphotography,
theproblemsarethedepthoffieldandthefieldofview,thereisalsothe
problemofelectronlenses.Onewaytoimprovetheirperformanceisto
insertverysmalllimitingapertures.Theseaperturesobviouslycutdown
(2.15)