1) Multilayer ceramic capacitors

How do they behave with ac and dc

they block dc but pass ac so can couple ac voltages and decouple dc voltages; their reactance changes as the frequency increases as fewer polarisation effects are able to occur over the required timescale

Define the reactance and give the equation for capacitive reactance

Opposition to current flow that stores energy rather than dissipating it; χ_C = 1/ωC

Give the equation for capacitance

C = ε_rε_0 A/h, ε_r,0 = relative and free space permittivity, A=plate area, h = plate separation; divide by volume to get volumetric efficiency

What is the research goal with BaTiO3 capacitors

increase permittivity while improving temperature stability of device; ε>1000 are based on ferroelectrics

Why is BaTiO3 unsuitable for use in X7R and Z5U devices

relatively low ε in the temperature ranges

X7R

+-15% over -55→125C

Z5U

+22,-56% over +10-85C

Cation substitutions

isovalent doping to change temp of phase transitions which changes temp of max ε

Heterogeneous dielectrics

pure BaTiO3 has ε vary largely with temperature; can core of pure BaTiO3 and shell doped with rare earths which shift and broaden Tc of the shell material; this broadens the change in ε with temperature; this is typically used for X7R

Why not always make X7R capacitors

they have a much lower peak permittivity than Z5U, however they do have better temperature stability

Why use multilayer capacitors

increases the surface area and therefore the capacitance

How are multilayer ceramics made via tape casting

form thin dielectric layers via tape casting using a slurry of binders, deflocculants (helps dispersion) and constituent oxides; viscosity and flow characteristics determine how thin slurry can be smeared onto moving plastic tape with doctor blade; electrodes screenprinted using metallic inks onto the surface of the green body; tape and electrodes are layered (interdigitised) then fired; large body then cut into segments and contact electrodes added to the ends making multiple MLCCs

What is the problem with BMEs such as Ni

readily oxidise in air at sintering temps producing NiO2 (insulator); so have to fire in a low oxygen atmosphere (N2 based which is reducing)

Whats the problem with firing in a N2 (reducing atmosphere)

BaTiO3 is not stable in a reducing atmosphere, O ions leave as oxygen gas producing e which turn it into a (n-type) semiconductor, useless as a dielectric

Why are rare earth dopants used

they (Dy, Ho, Eu) stabilise BaTiO3 against reduction in low P(O2) firing, they can act as donors or acceptors being able to sit on either lattice site and compensate for changes in local defect chemistry; RE with lower ionic radii are better as more easily sit on the Ti4+ site; also had to sinter at lower temp to prevent excessive migration of Ni electrodes