Environmental Engineering Reference
In-Depth Information
is.not.related.to.the.nanoscale.dimensions.or.properties..Prior.to.embedment.or.in.
the. case. of. release. at. end. of. life. disposal,. the. unique. properties. of. nanomaterials.
do.have.a.very.different.effect..The.most.obvious.case.is.that.of.worker.exposure..
With.particles.roughly.1/1000th.the.diameter.of.chopped.glass.ibers,.the.concern.is.
that.iltration.and.ventilation.regulations.are.not.effective..The.behavior.also.is.not.
monotonic.with.size..Some.properties.may.actually.make.it.easier.to.ilter.or.collect.
any.stray.nanomaterials..For.example,.the.Brownian.motion.of.nanoparticles.results.
in.a.more.tortuous.travel.path.that.may.make.capture.easier..Similarly,.the.high.reac-
tivity.of.the.surface-dominated.particles.can.lead.to.a.greater.ease.of.collection;.for.
example,.nanoparticles.tend.to.agglomerate.into.much.larger.clusters,.making.them.
easier.to.detect.and.ilter.
3.3.2 t
he
i
mpaCt
OF
“e
ngineered
” n
anOmaterials
More. than. 10. years. ago,. as. capabilities. of. measuring. particles. below. 100. nano-
meters. (nm). were. developed,. signiicant. research. focused. on. “ultraine”. particles.
resulting. from. vehicle. emissions. and. combustion-related. manufacturing. processes.
such.as.welding..Since.that.initial.research.into.nanoparticles.as.byproducts,.inter-
est. in. engineered. nanoparticles. has. grown.. The. breadth. of. processes. creating. and.
utilizing.nanoscale.materials.raises.more.challenges..Engineered.nanomaterials.are.
being.created.via.multiple.methods,.for.example,.arc.discharge,.laser.ablation,.CVD,.
gas-phase.synthesis,.sol-gel.synthesis,.and.high-energy.ball.milling..These.processes.
can.begin.from.the.“bottom.up,”.assembling.nanomaterials.from.their.components,.
for.example.by.chemical.synthesis.or.phase.change.processes..Other.manufacturing.
methods.begin.with.bulk.materials,.reducing.their.size.via.mass.change.processes.to.
create.nanomaterials.from.the.“top.down.”
The.bottom-up.synthesis.routes.are,.by.far,.the.most.widely.used.for.nanoparti-
cles..While.engineered.nanoparticles.often.are.thought.of.as.precursors.or.raw.mate-
rials.to.be.incorporated.into.higher.value-added.products.via.one.of.the.ive.families.
of. processes. described. previously. in. this. chapter,. the. initial. step. of. synthesizing.
nanoparticles.most.closely.its.within.the.family.of.“phase.change.processes,”.which.
includes.processes.such.as.CVD..The.use.of.top-down.methods.such.as.high-energy.
ball.milling.is.limited.to.larger.diameter.particles.with.less.stringent.monodispersity.
and.purity.requirements..Ball.milling.is.essentially.a.grinding.process.that.would.it.
within.the.machining.processes.of.the.“mass.change.processes.”
As.with.the.other.manufacturing.processes,.the.process-structure-property.inter-
relationships.are.signiicant..For.example,.the.manufacturing.process.can.affect.the.
atomic.structure.of.carbon.nanotubes,.which.in.turn.affects.many.properties,.such.as.
the.electrical.conductivity.(e.g.,.metallic.vs..semiconducting),.thermal.conductivity,.
strength,.and.stiffness..One.relatively.coarse.difference.is.the.production.of.single-
walled.nanotubes.(SWNTs).versus.multi-walled.nanotubes.(MWNTs)..Single-walled.
nanotubes.have.better.conductivity.and.strength.properties.but.are.much.less.reactive.
and.therefore.more.dificult.to.functionalize.(i.e.,.to.create.compatibility.with.other.
materials.for.bonding)..In.general,.the.properties.of.nanoparticles.are.governed.by.
process-induced. factors. such. as. the. size. and. size. distribution,. degree. of. porosity,.
and.surface.reactivity..In.synthesis.processes,.size.and.structure.can.be.controlled.
