Liquid Phase sintering

Question 1

When is liquid phase sintering used?

Answer 1

~ for ceramics that are not intended for high temp service

~ sintering is done at lower temps, porosity is more completely eliminated

Question 2

Where does the liquid come from?

Answer 2

~ a less refractory minor additive, or an additive that forms a low temp eutectic with the main phases

Question 3

How must the liquid be effective?

Answer 3

~ the liquid must wet the particles: replacements of solid-vapor interfaces with solid-liquid and liquid-vapor interfaces must lower overall SURFACE ENERGY

~ γSL + γLVcos(θ) = γSV
~ θ: contact angle

Question 4

What are important values of theta?

Answer 4

~ <90 degrees: partially wetting
~ 0 degrees is wetting

Question 5

cos(θ) =

Answer 5

γSV - γSL / γLV

~ as γSL decreases, θ decreases, resulting in improves wetting: oxide liquids are generally wetting to oxide solids

Question 6

What liquids have low γSL and θ?

Answer 6

~ liquids that react with (form eutectics with) the solid, or where the solid is partially soluble in the liquid generally have low γSL and θ

Question 7

What happens in the absence of vapor?

Answer 7

~ γGB = 2γSL*cos(Φ/2)
~ γGB: grain boundary interfacial energy
~ Φ: dihedral angle

Question 8

What happens if γGB/γSL > 1.848?

Answer 8

~ yields Φ/2 < 22.5; the condition for which liquid phase fully penetrates between particles

~ a wetting liquid between solid particles will tend to pull the particles together, as the finite liquid spreads over more solid surface area

Question 9

What happens for sub-micron particles?

Answer 9

~ capillaries of 0.1-1μm develop suction pressures of up to 12 MPa (1740 psi) for silicate liquids

~ for around 1μm powder, less than 1vol% liquid is needed to coat all particles

Question 10

What is the process of liquid phase sintering?

Answer 10

~ liquid forms and spreads by capillary flow, pulling particles together

~ lubricated particles rearrange into tighter packing configurations

~ assuming the solid is soluble in the liquid, for points of contact, if they dissolve into the liquid to precipitate out onto particles elsewhere, liquid is freed up to wet more distant particles, while particles reshape for better accomodation

~ liquid can penetrate spaces between agglomerates and break them up. this decreases the probability for abnormal grain growth