Reaney- MLCCs

Question 1

Uses of capacitors

Answer 1

Blocking and de-coupling
AC-DC separation
Filtering (most important)
Energy storage (growing)

Question 2

Capacitance formula

Answer 2

C=εrε0A/h
Where εr is relative permittivity
A is area of plates (electrodes)
h is separation (thickness)

Question 3

Volumetric efficiency formula

Answer 3

C/V=εrε0/h^2

V is volume

Question 4

How do temperature stability codes works for capacitors

Answer 4

First two characters (like X7) correspond to a temperature range. Last letter (like R) tells you how much the capacitance value can change as a % over this temperature range
Example is X7R can vary by +/-15% over -55 to +125°C

Question 5

Most common codes for BaTiO3 capacitors

Answer 5

X7R, Z5U, Y5V

Question 6

Graph of capacitance variation vs temperature for X7R

Answer 6

ΔC/C25 x100 vs T/°C. From -55 to 125C. Starts near -15% with flat dome shape with peak just above 0% at about 0C. Comes down a bit then plateaus then comes down more to like where it started.
Left side is due to shell, right side due to core

Question 7

Graph of capacitance variation vs temperature for Z5U

Answer 7

Same axes. Region of interest from 10 to 85°C. Starts just under 0% and reaches a peak soon after just above 0%. Curves down fairly steeply to -44% at 85C with bit of inflexion approaching this point

Question 8

Why is pire BaTiO3 unsuitable for use in X7R and Z5U?

Answer 8

Permittivity is relatively low for the temperature range of interest and the temperature stability is poor

Question 9

Doping BaTiO3 with Zr

Answer 9

Target B site (Ti). Is isovalent doping because both form 4+ ions. Cubic to tetragonal phase transition suppressed and temperatures of tetragonal-orthorhombic-rhombohedral phase transitions increased. A single broad peak observed at around 10mol% BaZrO3. See slide 12 for graphs

Question 10

Core shell structure

Answer 10

This is how to satisfy X7R criterion. Within one grain there is a core of undoped BaTiO3 surrounded by a shell rich in rare earths and Mg. Usually singer material with small weight % of rare earth oxides and MgO and sintering aids like SiO2 based glass. Get constantly varying composition as function of distance in the ceramic to give a broad Curie maximum in εr as function of temperature

Question 11

How to increase capacitance. What does MLCC do?

Answer 11

Need a thin dielectric layer, large surface area and high dielectric constant. MLCC has two end terminals with internal electrodes from each one extending out into a dielectric material and are interdigitated with each other. The electrodes don’t quite reach the opposite end terminal

Question 12

How to make multilayers by tapecasting

Answer 12

Thin dielectric layer formed by tapecasting the ceramic. This is carried out from a slurry of oxide powder composed of binders, deflocculants and the constituent oxides. Viscosity, flow characteristics and powder size and distribution determines how slurry can be smeared onto moving plastic tape using a doctor blade. Electrodes then screen printed using metallic inks onto the surfaces of the green body. See dielectric module notes for tapecasting diagram.

Question 13

What happens after tapecasting?

Answer 13

The tape with electrodes are stacked in desired patterns and fired at 400C to remove binder. Resulting body then cut into segments and contact electrodes deposited onto sides.

Question 14

What are BME electrodes and why did interest turn to them?

Answer 14

Base metal electrotechnology electrodes. For MLCC applications. Due to sudden increase in cost of Pd.

Question 15

Problems making BME electrodes

Answer 15

They oxidise in air and become useless. So need entire MLCC to be fired in low P(O2) environment. But then BaTiO3 formulations undergo severe reduction and become semiconducting in these reducing conditions.

Question 16

Why does BaTiO3 become semiconducting in reducing conditions?

Answer 16

Representative equation:
BaTiO3 + xH2 -> BaTiO3-x + xH2O
O sub o ->1/2O2(g) + vo•• + 2e’ (in CB)

Question 17

Solution to reduction of BaTiO3

Answer 17

Make non-reducible BaTiO3 based dielectrics. Used to use acceptor dopants like Cr2O3 and MnO substitute onto the B site. Oxygen vacancies considered to be trapped by regions of lower charge in the lattice according to:
Mn(Ti)’’ + vo•• -> (Mn(Ti)’’vo••)

Question 18

More recent solutions to reduction of BaTiO3

Answer 18

Rare earth dopants best suited to prevent reduction during low P(O2) firing. Also sintering temperature had to be reduced to prevent excessive migration of electrodes during firing. Typically use glass additives like Li2O - SiO2 - CaO glasses to increase grain boundary resistance of BaTiO3 ceramic.

Question 19

How can RE dopants act as acceptor and/or donors?

Answer 19

Depends where they sit in BaTiO3 lattice.
On A site: 2RE2O3 = 4RE(Ba)• + v(Ti)’’’’ + 6Oo
On B site: RE2O3 = 2RE(Ti)’ + 3Oo + vo••

Question 20

Which RE cations exhibit desired behaviour for solving problem of reduction?

Answer 20

Dy3+, Ho3+, Eu3+
Have lower ionic radii
Can compensate for local defect chemistry induced by any given condition which may arise during firing.
These ions particularly good when using thin dielectric layers <5μm.
Improve lifetime of capacitor by orders of magnitude