The Economist - USA (20212-12-04)

The Economist December 4th 2021 Science & technology 81

proton,possiblybillionsofyearslater,and

youhavedeuterium­freewater.Theresult,

inthesesamples,atleast,wastheequiva­

lentof 20 litresofwaterforeverycubicme­

treofrock.

Doingthesums,DrDalythinksabout

halfofEarth’swaterderivesfromc­typeas­

teroids,withanadmixtureofcomets.The

rest,whichdilutesthedeuteriuminthis,is

theresultofgrainsofweatheredspacedust

falling through Earth’s atmosphere

throughouttheplanet’s history,burning

upastheydidsoandfreeingtheirmicro­

scopicaqueouspayloadstoraindown,lit­

erally,ontheplanet’ssurface.

Thisfinding,moreover,alsohintsthat

watercouldaccumulateanywhereinthe

solarsystem which the solar wind can

reach—thesurfaceoftheMoon,forexam­

ple,aswellasasteroids.Thatisgoodnews

for spaceexplorers. Insuch places, the

weathereddustcouldbea sourceofwater

forastronauts.n

Mobilephones

Through a glass,

brightly

S

ince 2006 , whenCorning,anAmerican

glassmaker,  developed  Gorilla  Glass  to

give  Apple’s  first  iPhone  a  scratch­resis­

tance  screen,  many  other  types  of  tough­

ened glass have appeared for use in hand­

sets. But as rugged as they are, people keep

cracking  and  breaking  them.  Jingwei  Hou

thinks he has found a way to prevent that.

Dr  Hou  and  his  colleagues  at  the  Uni­

versity  of  Queensland,  in  Australia,  are

among  a  number  of  researchers  working

on a group of materials called perovskites.

The  original  perovskite,  calcium  titanium

oxide, is a mineral (see picture) discovered

in  the  Ural  mountains  in  1839  and  named

after Count Lev Perovski, a Russian miner­

alogist. Since then, the name has come to

be  used  for  a  number  of  materials  that

share a similar crystal structure. 

Much  of  the  interest  in  perovskites

comes because those which combine met­

al atoms with chlorine, bromine or iodine

(members  of  a  group  of  elements  called

halogens) are semiconductors. This makes

them potentially useful in a variety of elec­

tronics.  In  particular,  one  promising  fea­

ture  of  metal  halides  is  that  they  can  be

used  to  make  new  types  of  solar  cells

which  are  more  efficient  at  converting

sunlight  into  electricity  than  the  silicon­

based cells currently employed. Oxford pv,

a British company, is bringing some of the

first perovskite solar cells to market.

However,  what  works  in  one  direction

will  often  work  in  the  other,  and  so  it  is

with perovskites. This means that instead

of  converting  sunlight  into  electricity,

metal­halide perovskites can operate in re­

verse,  by  emitting  light  in  response  to  an

electric current. An array of tiny specks of

perovskites  could  therefore  work  like  a

grid of leds in a display screen.

To  produce  such  screens,  Dr  Hou  and

his team developed a way to bind tiny “na­

nocrystals” of lead­halide perovskites into

a specially treated synthetic glass compos­

ite.  As  these  nanocrystals  are  extremely

sensitive,  and  decay  rapidly  if  exposed  to

moisture in the air, being bound up in the

glass  protects  them.  It  also  prevents  lead

ions,  which  are  toxic,  leaching  out  of  the

material.  At  the  end  of  their  lives,  the

screens would be recyclable. 

A study Dr Hou carried out in collabora­

tion  with  teams  from  Britain  and  France,

which  was  published  recently  in  Science,

showed  that  perovskite  screens  made  in

this  way  were  efficient  light  emitters  able

to produce sharp, bright images. The light

could also be tuned to different colours by

altering the nanocrystals’ characteristics.

An  added  bonus  was  that  the  screens

were extremely tough, because of the way

the  nanocrystals  bonded  with  the  glass.

That  was  helped  by  the  mechanical  struc­

ture of the glass (known technically as a ze­

olitic  imidazolate  framework)  having  a

spongelike atomic arrangement. This pro­

vides a degree of flexibility, thus avoiding

breakage problems caused by brittleness. 

In  as  much  as  anything  can  be  consi­

dered unbreakable, perovskite screens just

might  be  that,  reckons  Dr  Hou—and  they

might be used not just for phones, but also

for products ranging from televisions and

computers to lighting. But he does think it

could take five to ten years of development

before this will happen.Inthe meantime,

people should take a bitmorecare with the

screens on their devices.n

Toughening up phone screens

with perovskites

Coming soon, to a screen near you

Electricflight

Whirly birds are go

S

cores of electricallypowered  verti­

cal take­off and landing (evtol) aircraft

are under development, heralding a future

where  air­taxis  whisk  people  from  one

place  to  another.  Provided  their  batteries

are  charged  up  using  renewable  power,

these new flying machines will also travel

in a green, planet­saving way. They there­

fore appear to be the nemeses of noisy, fos­

sil­fuel guzzling helicopters. But not quite

yet.  As  with  all  forms  of  electrically  pro­

pelled  flight,  weight—and  therefore

range—are issues. In the interim, however,

a  touch  of  electrification  looks  likely  to

make conventional choppers not only saf­

er but also more useful.

The idea is to turn them into hybrids so

that, like hybrid cars, they are powered by a

combination  of  an  electric  motor  and  a

combustion  engine  (a  jet  turbine,  in  the

case of most commercial helicopters). This

will make them a bit greener, by improving

fuel  economy  and  thus  reducing  emis­

sions, as happens in a hybrid car. But heli­

copter hybridisation brings other benefits

too, especially when it comes to safety.

To test this out, Airbus Helicopters, part

of  the  eponymous  European  aerospace

group, has fitted a helicopter that it uses as

a flying laboratory with an electrically po­

wered  engine­backup  system.  This  con­

sists of a battery and an electric motor con­

nected to the helicopter’s main gearbox. In

Hybrid power makes helicopters safer

and more productive