Ionic structure, semi- and super-conductors - ThinkChemistry

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Nov 15, 2013 (3 years and 9 months ago)

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Face centred cubic lattice
e.g. NaCl

Body centred cubic lattice

e.g. CsCl

Single unit cell

Single unit cell


Sodium chloride has the
coordination number 6:6, as the
sodium ion has six neighbouring
chloride ions, and visa versa.

Caesium chloride has the coordination
number 8:8, as the caesium ion has 8
neighbouring chloride ions, and visa
versa.


Superconductors are materials
which conduct electricity with no
loss of energy. They have zero

resistance.




Ceramic materials, such as
magnesium diboride.


Superconductors were, in the past,
impractical because they had to be
super cooled by liquid helium which
scientists were complaining was
“Really deer to buy.”


Ceramic superconductor’s critical
temperature for superconductivity isn’t
as close to zero degrees Kelvin, which
means liquid nitrogen can be used in
place of helium. This is less expensive.


What are they?

Superconductors can be
metal elements like
mercury or silver.



Metal alloys such as niobium
-
titanium.


=

In a weak applied field, a superconductor
"expels" all magnetic flux. This allows a
magnetic object to “float” above a
superconductor.


Arrows represent magnetic field lines

Material before
cooling

Material after cooling,
in superconductive
state

Scientists have come to the conclusion
that this effect is “pretty class”.


Power Cables:

Right now, up to 10% of electrical energy
carried by power cables is lost as heat

Using a superconductor could eliminate
this energy loss.

It would also allow power lines to be
hidden underground.

Other uses include:

Magnetic Resonance
Imaging

Superconductors create
high magnetic fields,
this allows doctors to
see inside parts of the
body that hitherto had
not been visible outside
of investigative surgery.

Using superconductors to craft
electronic switches could advance
microprocessor technology
making them extremely fast.

Electronics

Other uses include:

Transport

Trains may be able to hover
over tracks which means
frictionless, high speed
travel like the Maglev Train
in Shanghai, China.

Oh, and that car from I, Robot

Semiconductors are materials that have a
conductivity lying somewhere between
non
-
metals which have no free charged
particles, and metals, which have free
moving electrons.

Examples are silicon
and germanium,
metalloids that have
four valence electrons.

Their conductivity
increases with
temperature and
exposure to light.

P
-
type

N
-
type

In a p
-
type semiconductor,
and impurity from group
three such as gallium or
indium is added, this creates
three bonds with the group
four semiconductor leaving a
gap where an electron should
be. This is known as a
“positive hole” as it contains
no negative electron.

Si

Ga

Ga

Ga

Ga

Electron


Positive hole

Si

Ga

Ga

Ga

Ga

Free electron

In an n
-
type conductor, an
impurity from group five such
as antimony or arsenic is
added to a group four
semiconductor, four bonds are
created leaving one free one
to act as a charge carrier.

When light
energy falls on a
semiconductor,
the conductivity
increases.

The energy a
photon delivers to
an electron
releases it from

n
-
type side of the
junction and the
depletion layer
causes a potential
difference.

P
-
N Junction

N
-
type

P
-
type

Depletion layer


-

-

-

-

-

-

-

+

+

+

+

+

+

+

Which of the following statements referring to

the structures of sodium chloride and caesium

chloride is correct?


A: There are eight chloride ions

surrounding each sodium ion.


B: There are eight chloride ions

surrounding each caesium ion.


C: The chloride ions are arranged

tetrahedrally round the sodium ions.


D: The chloride ions are arranged

tetrahedrally round the caesium ions.

Answer B:
caesium chloride
is a body centred
lattice with a
coordination of
8:8.

Magnesium chloride has the same
coordination number as sodium
chloride. What shape is the
magnesium chloride crystal?

Answer:


Sodium chloride has
a coordination
number of 6:6, like
sodium chloride,
magnesium chloride
is a face centred
lattice.

A superconductor is a material


A : Whose electrical conductivity decreases

with decreasing temperature


B: That does not conduct electricity unless

doped with another material


C: That can conduct electricity with zero

resistance


D: Whose electrical conductivity increases

with increasing temperature.


Answer C:
Superconductors
that are cooled to
extremely low
temperatures past a
critical point can
conduct electricity
with no energy loss.

Explain the Meissner effect.

Answer:

When cooled below a critical point,
a superconductor repels all
magnetic flux, this allows a
magnetic object to “float” above it.

Silicon can be converted into an n
-
type

semiconductor by adding


A: Boron


B: Carbon


C: Arsenic


D: Aluminium.

Answer C:
Using a group
five element
like arsenic to
dope silicon
will leave one
free electron
turning it into
an n
-
type
semiconductor.

How does a photovoltaic cell convert light energy into
electricity?

Answer:

Photons release
electrons, they gather
at the p
-
n junction and
form a depletion layer
creating a potential
difference.