clean%2c high performance materials

7/28/2019 Clean%2C High Performance Materials

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Challenge the future

DelftUniversity ofTechnology

Clean, high performance materials

Challenges and developments

Peter Morshuis


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Materials used in

Electrical Power Engineering

The building blocks of electrical power components

Metals (conductors, housing)

Dielectrics (electrical insulation, power electronics, support structures, cooling)

Gasses

Liquids

Solids

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Material challenges

(clean, high performance)

Environment friendly during entire life cycle

High efficiency/Low losses

Long life

Smart, self-healing

High energy density

Non-flammable

High operating temperature

In case of fire, low smoke production and non toxicity

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1.MetalsReduction of losses using superconducting materials

Reduction of losses using carbon nanotubes


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Carbon nanotubes

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3D model of three types of single-

walled carbon nanotubes. Source:

wikipedia

Length to diameter ratio > 107

Source: wikipedia


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Ballistic conduction

As the length of a conductor approaches zero, the conductance approaches a finite

value, and not infinity

This conductance is determined by the interface between conductor and contact

pads

If the length of the conductor is much smaller than the mean free path, all resistance

occurs at the interfaces

For a carbon nanotube: 6454 /tube

Closely packed parallel-connected tubes,

4 m long, 1.2 nm diameter: resistivity = 0.88 cm

Copper: resistivity = 1.67 cm


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Ultra low resistivity in practice

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Calculated resistivity of composites of

CNT and copper

8Source: O. Hjortstam,et al, Applied Physics A, Vol. 78, No. 8, 2004


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The importance of size

Take a sphere, with radius R

The surface/volume ratio increases with decreasing R

The effect?


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Ultracapacitors

Ultracapacitors can store only a fraction (5%) of the energy of an equivalent-size

lead-acid or lithium-ion battery.

The recipe for a better ultracapacitor is more surface area.

By using tightly bunched nanotubes, the surface area can be increased drastically.

MIT believes the storage capacity can be increased to between 25 and 50 percent

of a batterys energy. At that point, it becomes a compelling device for many

applications.

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A cross section of an electrode made

with carbon nanotubes. Source: MIT


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The ultracapacitor

increasing active surface area

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Plain Old Plates: In a typical capacitor,electrons are removed from one plate and

deposited on the other. Polarized molecules

in the dielectric concentrate the electric field.

One major factor determining capacitance is

the surface area of the plates.

Plated Packed with Ions: An ultracapacitor

can store more charge than a capacitor

can, because the activated carbon has a

pocked interior, much like a sponge. This

means that ions in the electrolyte can cling

to more surface area.

Enter the Nano-World: With finer

dimensions and more uniform distribution,

carbon nanotubes enable greater energy

storage in ultracapacitors than activated

carbon does.

Source: Joel Schindall, IEEE Spectrum Nov 2007


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2.Electrical insulationDevelopment of green and high performance alternatives


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Gases SF6

(sulphur hexafluoride)

SF6

is used world-wide in electric power industry for

Electrical insulation

Arc interruption

Cooling

Examples: Gas insulated systems, switchgear, transformers, capacitors, cables

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Gases - SF6

In spite of its advantages:

High dielectric strength

Thermal stability

SF6 has significant disadvantages:

Harmful for environment

Decomposes into toxic substances

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SF6

a greenhouse gas

According to the Intergovernmental Panel on Climate Change, SF6

is the most

potent greenhouse gas that it has evaluated, with a global warming potential of

22,200 times that of CO2

when compared over a 100 year period.

However, due to its high density relative to air, SF6

flows to the bottom of the

atmosphere which limits its ability to heat the atmosphere.

Its mixing ratio in the atmosphere is lower than that of CO2

about 6.5 parts per

trillion (1012) in 2008 versus 380 ppm (106) of carbon dioxide, but has steadily

increased (from a figure of 4.0 parts per trillion in the late 1990s). Its atmospheric

lifetime is 3200 years.

15Source: Wikipedia, Earth System Research Laboratory


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Time development CO2

and SF6

concentration in air(http://www.esrl.noaa.gov/gmd/ccgg/iadv/)

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Note the difference in y-axis scales: picomol/mol (left) versus micromol/mol (right)


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Alternatives for SF6

SF6-N

2mixtures (5% ... 20% SF

6), high pressure air insulation, vacuum insulation

Solid dielectric insulation

Fluid insulation (for instance esters, seed-based fluids)

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Liquids

Insulation systems consisting of a solid and a liquid are the most frequently used

systems in high voltage apparatus, where components have to be insulated and

loss heat has to be dissipated.

The requirements on the liquid part of the insulating system are not only the electric

and dielectric performance but also the performance regarding requirements

concerning the environment and low flammability.

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Liquids mineral oil

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Mineral oil is used world-wide in electric power industry for

Electrical insulation

Cooling

Examples: Transformers, cables

W4

W2

W4

W2


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Oil-impregnated insulation

Submarine cable links


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Liquids mineral oil

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High dielectric strength in combination with solids

High thermal conductivity

Proven technology

mineral oil has significant disadvantages:

Harmful for environment

Toxic

Low fire point


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Liquids alternatives for mineral oil

Use ester liquids (vegetable oils) instead of mineral oil

Esters are a class of chemical compounds and functional groups.

Examples: rapeseed oil, sunflower oil, biodiesel (natural); synthetic esters

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A carboxylic acid ester. R and R' denote any

alkyl or aryl group. Source: Wikipedia


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Ester liquid vs mineral oil

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Ester liquid vs mineral oil

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nanofluids

Thermal and electrical properties of liquids can be further improved

by adding small fractions (1%) of nanoparticles

25Source: MIT


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Solids

Oil-impregnated paper is used world-wide as insulation in

Transformers

High voltage cables

Capacitors

Epoxy resin is used in

Insulators

Cable accessories

Switchgear

Polyethylene is used in high voltage cables

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Solids - challenges

Design clean solid insulation materials to

-partly- replace oil insulated systems

Allow the construction of compact, light-weight insulation systems

Increase efficiency

Create self-healing properties

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Solids polymer insulation

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High dielectric strength

Low losses

clean

polymer insulation has significant disadvantages:

Bad heat conductivity

Susceptive to electrical degradation


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Solids designing new/better

properties

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We can add fillers to existing materials to affect properties

but we have reached the limits of what can be achieved

on microscopic to macroscopic level

Move to nano


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Nano dielectrics

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When we shrink the dimensions of a material we notice a change in materialproperties

In dielectric systems in which one of the length scales is below 200 nm, i.e.nanodielectrics, the surfacesand interfacesbetween dielectric elements become

increasingly important.

Take a spherical particle with radius R:

Surface = 4R2

Volume = 4/3R3

Carbon nano tubes

surface/volume-ratio increases

with decreasing R


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Importance of the interfacial zone

31*from: Lewis, Transactions on DEI, October 2004

In one cubic meter of a material containing 10volume % of 10 nm spheres, the total interfacearea is approximately 60 km2

Material properties may change drastically withinthe interfacial zonewhich may even becomemore important than the bulk dielectrics.

S f f i t


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Surface area; from micro to nano

(same volume fraction)

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Nanocomposites

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Nanocomposites consist of a dielectric matrix (host) and a disperse phase (nano-scalefiller)

Small size, 1 nm 100 nm small distance between neighbouring filler particles (forthe same volume fraction)

Very large surface area much more interactionbetween filler and host

New mesoscopic properties (between atomic and macroscopic scale)


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New/improved properties

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Higher mechanical strength

Better thermal properties

Higher electrical breakdown strength

Decreased flammability, gas-tight

Better adhesion Anisotropic character of certain nanomaterials provides new opportunities for

design: smart behavior

As yet, very few explanations for the behavior ofnanocomposites


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3.DevelopmentsBuilding macroscopic structures with nanosized bricks


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Developments


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4.SummaryTheres plenty of room at the bottom1959, Richard Feynman


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Summary

Conductors

Prospect of lower losses using carbon nanotubes

Ultra capacitors using large surface areas

Insulating systems Alternatives for greenhouse gases such as SF6

Alternatives for mineral oil

Prospects of creating clean, high performance materials using polymeric insulation and

nanotechnology

Sensors and smart materials

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