Riassunti di Aerospace Materials

NANOSTRUCTURED MATERIALS

A nanocrystalline material is a single or multi-phase polycrystalline solid with a grain size of few nanometers (<100 nm) in at least one dimension, while ultrafine grain size materials have grain size of 100-300 nm.

In general, without any changes in chemical composition with respect to the bulk non nanostructured material they have:

• higher strength, hardness, ductility and toughness; affinities; thermal expansion coefficient (CTE), magnetic properties and electrical resistivity;
• lower elastic modulus, thermal conductivity.

The increase of lower creep temperature and superplasticity, which both helps in industrial applications because these allow the material to be shaped easily at lower temperature and creating very thin pieces or extremely high deformations up to 600% without the risk of cracking or narrowing.

^{regarding creep see the slides at 25/02/22}

Nanomaterials are classified basing on their dimensions as 0-D, 1-D, 2-D or 3-D, depending on the number of sizes having a non nanoscale dimension (< 100 nm). In general, 0-D, 1-D and 2-D nanomaterials are used as nanofillers to create nano composites made of a matrix that can be polymeric, metallic or ceramic. In general, nanocomposites have a % of reinforcement of about 5% in volume, and this is due to the fact that with respect to conventional composites for the same % of reinforcement, nanostructured material is much higher and therefore a higher tendency to agglomerate, which would lead to a loss of mechanical properties and non-uniform behavior. Despite that, even if the volume % is lower, this instead of 40-50% mechanical properties are much better.

Some fields of application for nanostructured materials are:

• thermal barrier coatings (TBC) (because of the higher CTE (more similar to metal are used for turbine blades alumina, so lower mechanical stresses and the lower thermal conductivity, so the inner temperature is lower and the life-time longer to operating temperature higher);
• against crack propagation, because thanks to an higher presence of grain bounds, even cracks are more detailed;
• antifouling and anti-erosion coatings;
• coatings for anti-icing thanks to an superhydrophobic behavior, so that for drops impacting the surface of the high angle have a rotation movement and goes away, detaching from the surface;
• electrolyte coatings of low w superhydrophobic behavior in order to make droplets with an imposed angle around impurities on a surface.

In general for a nanocrystalline material the volume fraction of the material in the grain boundary is much higher than the one for micro-crystalline one.

For a small mean size (10-15 nm) for example we assume (C≈3) d/λ, where λ is the

grain boundary thickness usually 0.5-1 nm in order to contain 5-10 atom and d is the size of the grain.

Moreover, as the grain size the volume set until till of the lattice increases and then the material takeon different propritites from the bulk one, where because it becomes amotasphous like crystalline since the grain proprieties and the influence of along grain boundaries proprieties for example σ = Y.C.T_m. + energy + impourities** usually

act also as composition, so the lattice structure is different from the bulk material at the same temperature.

- amorphization if the grain size is below a critical size (14 nm for Ca);

- supersaturation because the solids has more space in the lattice because of a higher in volume and because the free energy along grain boundaries is lower, so usually segregates there;

every factor with diffusion process because of the large amount of atoms in grain boundary which gives a lot of short diffusion paths, so there is enhanced diffusion creep and superplasticity.

One of the most important behavior of nanocrystalline materials is the increase in strength and hardness without any change in density in general.

This behavior is explained by the Hall-Petch equation which fits very well experimental data up to grain size of 100 nm σ_y = σ₀ + K_y!√d

where σ₀, σ_y where d: < 100 μm) Then there are several factors:

- grain boundaries are barriers to slip, cause these microscopic length and as d those number increases;.

- pile-up is the process for which the movement of a dislocation creates a stress field that makes other dislocations slip but once the curvature one

along a grains boundaries the other will reach r creating local higher stress.

so the back stress which increase as d with a factor 1/d^1/2

- the high density of triple junction between 2 grain boundaries in which the diffusion coefficient of can be 5 or more orders of magnitude higher than the grain boundary to one.

To understand how nanostructure materials are construed we have to define the ultra fine grained materials (UFG) like those materials having an average grain size of less

In general a SWCNT can be thought as a layer of graphite made of hexagons (it rarely can be defects in...) rolled up closed by two hemispherical caps made of pentagons and hexagons. It has a diameter usually < 2 nm and are made of one layer 10^-8 m of its nanoshell. A TWCNT can be instead be thought as more than one concentric SWCNT, so that its diameter can reach ~ 30-40 nm: moreover, there can be bonds between walls, this can be used as nano-springs or nano-shock absorbers.

Some characteristics of carbon nanotubes are:

• high thermal conductivity ~ 10³ W m^-1k^-1 against 50 ÷ 100 of metals;
• Young modulus 1 TPa, higher than any other material;
• tensile strength of 150 GPa (600 higher if it is specific on weight) then bulk;
• current density allowed up to 10⁷ A/cm² (~100 times than one of copper)
• thermal stability up to 2800 °C

Some very interesting properties are:

• the sensitivity to electric field, which are able to bend the CNT up to 90°. This means that nanomotors or nanomanipulators can be done by controlling this field;
• ballistic conduction: they are able to make electrons pass without being heated. However their conductivity can be rewrote by joining them;
• if the cap ends of CNT is open they are able to store gases thanks to capillarity and large surface/weight ratio;
• chemical sensibility on the environment as conductivity varies a lot with atomic destructivity (useful for electro-chemical sensors).

They can be for example produced by chemical vapor separation but must be purified after their production.Then we are able to diminish the grain size of the mixture and to highly decrease the density and to change the mode of matrix fracture from intergranular to transgranular.

Polymer nanocomposites

The use of PFNC and their development does aim only to enhance mechanical properties of the neat resin, but mainly to provoke new properties without loose the easy processability of the resin or increasing its weight.In the fabrication process of PFNC we always want both good distribution & damage through suspension of aggregates & polymer intrusions (can be done only for soluble polymer, not thermoplastic) but even for small number of incorporation of nanotubes into a polymer solution prepared & constructing in a suitable solvent.To mix CNT magnetic stirring of high shear...

to make the field applied.

piezoelectric materials are able to polarize and produce a ΔV due to a temperature change

the ferroelectric materials are to have reversible and nonreversible polarization.

For a piezoelectric material, it's possible to define a relation between electrical and me-

chanical variables like:

E = c + d · T = k · + · O - from voltage to strain (converse piezoelectric)

D = e · E + k · O - from stress to electric filed (direct piezoelectric)

Where s and t are the mechanical stress and stress tensors, E is the electrical

field, D is electrical displacement and O is absolute temperature. The reversal

pole measure zero stress, zero current, and constant electric field while d me-

ter coefficients are the sources for the effect and are in general reversible in

HCI & D behavior.

function it is very important to know that P is the polarization of the material. then

using E / _p ^θ attain very large oscillations while if E / ^p _θ obtain

heating hear stress and this can occur at a hundreds of Hz instead of the few units of Hz.

case of SMA macheans so they can be used both for embedded sensors and rational il-

cooling control.

the most used piezoelectric materials are ceramics but they have some drawbacks like

the possibility of electrical breakdown of the electric fields 9 higher than the carri-

the consolidation of the material teaching a lack of the properties.

aproping occurs if the electric field E applied to the opposite direction of the polariz-

ation is higher the 0.5 φ electric constant, it lead to a box proprios of

rowth in polarization.

for temperatures higher than Curie Temperature (~600 K) depoling is swore and again

creep are faster.

non linearity and hysteresis up to 4-10% could make T ++ and this is limimating a

fact in frequencies applicalions where electrorheologic materials are better, thanks to

areomophossers.

Even polymer can be a good choes as piezoelectric materials because they have:

major stress constant so they are better sensors,

they are lightweight, rough and easy to manufacture in complex shapes

high strength and impact resistance,

higher dielectric constant.

Even electroactive or magneto active materials can be used like for SMA, to flame