Chapter 3 - Properties of Nanomaterials Flashcards
Explain the difference between intensive and extensive properties
intensive - property that does not depend on the size of the amount of material in the system
extensive - additive for independent, non-interacting subsystems. It is proportional to the amount of mass in the system
Dividing two extensive properties can create an intensive property (ex., density made of volume and mass)
How does melting point work?
melting begins at the surface and propagates through the solid as atoms at the surface and grain boundaries are less constrained to vibrate compared to atoms inside the crystal lattice.
How does melting point change for nanoparticles?
- as grain size decreases, the percentage of atoms at the surface increases significantly. Thus, melting temperature decreases with the size
*freestanding nanoparticles, in general, show lower melting points compared to bulk
How does thermal conductivity work?
heat is transported in materials by lattice vibration waves and free electrons. Metals, since the election mechanism is more efficient than phonon processes due to metals having many free electrons
heat transfer occurs inside materials through conduction (based on a temperature gradient)
*in general:
kgas < kliquid< knon-conductive solid < kconductive solid
k also varies with temperature and pressure (most noticeably for gases)
How does thermal conductivity change for nanoparticles compared to bulk?
thermal conductivity is much lower for NPs than for bulk
- however, it increases with temperature for NPs whereas it decreases with temperature for bulk
How does diffusivity work?
diffusivity is the capability of a material to diffuse or a rate of diffusion, a measure of the rate at which particles or heat, ect. can spread
the rate of change of a chemical species depends on the diffusion coefficient and the concentration gradient
diffusion kinetics increases with increasing defect content of the material which provides easy diffusion paths compared to bulk lattice
How does diffusivity change for nanoparticles compared to bulk?
Nanomaterials have increased diffusivity compared to single crystal or conventional polycrystals
How does grain growth work?
Grain boundaries are high-energy regions, as the SA/V reduces, the grains coarsen (the crystal structures realign)
the smaller the crystallite size, the larger the SA/V, thus more grain boundaries
How does grain growth change for nanoparticles compared to bulk?
For nanocrystalline solids, Q (activation energy for grain boundary migration) is lower and n (microcrystalline grains) is higher so get enhanced grain coarsening (grain growth increases)
*at high temperatures, nanocrystalline solids are not stable but can make them stable by pinning/addition/decoration with another phase/metal that can decrease grain coarsening.
How does a nanoparticles ability to hold energy compare to its bulk form?
NPs can hold more energy than conventional coarse-grained materials because of their large grain boundary (surface) area
- thus Nanocrystalline materials are good candidates for separator plates and electrodes in batteries because they have increased energy density
What are the 3 main components of a battery?
- cathode (positive) electrode
- anode (negative) electrode
- electrolyte
How does charge/discharge work for a Li-ion battery?
charging: Li ions flow from positive to negative electrode through electrolyte and intercalated between graphite sheets
discharging: ions flow back through electrolyte from the negative to the positive electrode
*electrons flow through the metal wire to meet ions at the other end
What is charge capacity?
The energy stored in a battery is given in ampere hours (Ah)
What is C-rate
The nominal capacity delivered in x number of hours (ie. the current)
What is energy (for batteries)?
The voltage of the battery together with its charge capacity defines the energy stored in the battery in Watt Hours (Wh).
ex. a 10ah battery operating at 12V can story 120 Wh of electrical energy
What is energy density?
The energy storage capacity per weight or volume of material
Why do nanomaterials improve the performance of Li-ion batteries?
They can hold more energy, thus requiring batteries to require less frequent recharging and thus lasting longer
- have shorter diffusion lengths for the LI-ions
higher electrode-electrolyte contact area rising from inherently high surface area characteristics of the particles
Why is Silicon a desire substitute for graphite anodes and why is it not currently used
Silicon has the potential to have a capacity 10X that of graphite
- however, it experience 300% volume expansion, causing severe capacity fade and battery failure
**Using Si NP with diameters <150nm shows a significant increase in toughness (no cracking or fracture), thus allowing energy to be stored
What is density of states?
a system the describe the number of states per interval of energy at each energy level that are available to be occupied by electrons
- becomes discretized as the material moves from bulk to the quantum well, to the quantum wire, to the quantum dot
How do semiconductors deal with optical properties?
An incident photon with energy greater than the band gap, the electron may be excited to the unfilled conduction band
- photon is absorbed and a hole is left in the valence band, thus when the electron in the conduction band returns to the valence band and recombines with a hole, a photon is released with energy equal to the band gap of the semiconductor
What is an exciton?
An electron that has be sent to the conduction bad and left a hole behind
*excitons are mobile and can move freely through the material
How does electrostatic forces impact the exciton?
The electron and hole are binded together with a Coulombic attractive force.
This electrostatic interaction reduces the energy required for exciton formation, bringing the energy level closer to the conduction band
**exciton radius is in the nanoscale, and thus for nanomaterials the exciton radius is confined
What is Photoluminescence
The light emission from any form of matter after photon absorption
How does photoluminescence change with nanoparticles?
As the particles size decreases, the wavelength reflected back is lower
**get different colours - probably because the exciton radius is reduced
What are magnetic properties?
the strength of a magnet is decided by its coercivity and saturation magnetization values which increase when grain size decreases (SA/V of grains increases)
How does NPs change the magnetic properties?
Enhancement of remanence can be obtained when the nanocrystalline grain size and the degree of coherence across the interphase boundaries are such that exchange coupling occurs between the two phases
**essentially want coupling at the interphase boundaries to enhance remanence
What is remanence?
the magnetization left behand in a ferromagnetic material after an external magnetic field is removed
What is coercivity?
the applied magnetic filed required to reduce the magnetization of that material to zero
How to mechanical properties change on the nanoscale?
They increase immensely (hardness, strength increase and, elastic moduli decrease)
