---
title: "Ceramic Material - MML218"
tags : "SEM4, MME"
---
# Ceramic Material - MML218
Radio and Television set
YaBaCuO
Levitating Train
Dr. Atul Ramesh Ballal
ballal.atul@gmail.com
(+91) 98603 82899 (M)
0712-280 1509 (Office)
[Course Website](https://sites.google.com/view/arballal/Home/cm-22)
UK-REC
Pls inform in advance if you may not be able to join the class
Objective
- Structure
- Defects
- Processing, Characterisation
- Properties and applications
- Advances in ceramic
Ceramic Materials
Science - Rules, Laws
Engineering - Design, Blueprints
Technology - Manufacturing, Hardware
Materials Engineering - Design Material to suit a desire
Material - Solids with engineering applications
In Materials Engineering we are taking about solids with engineering applications.
Stents
- Biomedical Implants
- They should not corrode.
- They shoud have same modulous of elasticity as the blood vessels.
Types of Materials
- Metal/Alloys
- Ceramics
- Polymers
| Property\Material | Polymers | Ceramics |
| ---------------------------- | --------------------- | ------------------------------- |
| Electrical Conductivity | Good Insulators | Good Insulators/Good Conductors |
| Hardness/Strength | Low | Very High |
| Ductility | High | Low |
| Thermal Conductivity | Low | Low |
| How do they look from inside | | |
| Structure |Van der Waal's forces| Covalent ionic|
## Applications of ceramics and definition
New/Advanced
- Cutting tools
- Hard
- High melting point
- ex- WC,Diamond,B~4~C
- Electrical
- Insulators
- Semicondusctors
- Electrolytes
- Conductors
- Superconductors
(The properties can be tuned according to the need)
- Nuclear Fuels
- UO~2~/Thoria(ThO~2~)
Ceramics aren't melted - their processing methods differ from that of Metals/Alloys and Polymers
Structure - Property
Ceramics
- Ceramics are **solid compunds** formed by application of temperature or sometimes temperature and pressure comprising at least
- 1 Metal + Non Metallic Elemental Solids
- 1 Metal + 1 Non-metal - CuO, Fe~2~O~3~, CaO
- 2 Non Metallic Elemental Solids - SiC
- 2 Non Metallic Elemental Solids + 1 Non-Metal
- MgO, SiO~2~, SiC, ZrB~2~, Rocks, dust, clay, mud, cement, bricks
- NMES - elements are covalently bonded solids that, at room temperature, 2 Fundamentals of Ceramics are either insulators or semiconductors
- Applications
- Sanitary ware
- Fine chinas
- Procelains
- Glass Products
## Crystalline/Amorphous
Solids - Long range order(in nm or Angstrom) - Crystalline
Periodicity lacking - Amorphous glassy, non-crystalline
Periodicity is much greater than bond lengths
Most ceramics are crystalline in nature
Amorphous - Glass
## Bonding in Ceramics
Properties of any solids and the way its atoms are arranged are determined primarily by nature and directionality of the inter-Atomic bonds
Ceramics are either covalently bonded or ionically bonded but neither purely covalent nor ionically bonded
```Covalently bonded materials are harder and have higher melting points. eg -SiC```
Fraction Ionic Character = 1 - exp$\left(\frac{-(X_a - X_b)^2}{4}\right)$
X - Electronegativity
$\Delta x > 1.7$ - Ionic - Conductors
$\Delta x < 1.7$ - Covalent -Insulators
### Levels of Sturcture
- Macro - eye - 0.1mm
- Micro - Optical Microscope - 1µm
- Sub-Structure - SEM, TEM
- Crystal - Transmission Electron Microscopy, X-Ray Diffraction Microscopy - Unit cell - nm
- Atomic - XRD, Spectroscopy
- Nuclear - Nuclear Magnetic Resonance - Angstrom
Porosity - µm
Vacancy - nm
## Crystallography
Unit Cell - The smallest space which when repeated describes the three-dimensional pattern of the atoms of a crystal
Lattice - an indefinitely extending arrangement of points
### Crystal Structure of Ceramics
Arrangement of ions/atoms in crystalline ceramics
Properties are sensitive to structure
Ceramics crystal structure are complicated -
- 2+ components
- Wider varieties than metals
- Ceramics structures after first mineral for which it was first decoded
Packing
- Interstitials
- Tetrahedral co-ord
- Octahedral Co-ord
Coordination number is the number of oppositely charged ions that surround the ion in question
Structure us based on
Co-ordination Number/ radius Ratio
- To achieve the state of lowest energy the cations and the anions will then to maximize attractions and minimize repulsions
- Attractions are maximized when each cation surrounds itself with as many anions as possible with provision neither cations nor anions touch
- For a stable coordination the bonded cation and anion must be in contact with each other
- If cation is larger than the ideal radius ration - the cation
#### Electrical Neutrality
Any crystal has to be electrically neutral, sum of positive charges must balance the negative charges, reflected in chemical formula of compounds
The portion of the charge of each cation is equal to the valency of the cation divided by the coordination no of the cation
$\dfrac {|V_a|}{CN_a} = \dfrac{|V_c|}{CN_c}$
Ratio is called bond strength??
##### Notation
[crystal structure] - Composition
[fluorite] - CaF~2~
[spinel] - MgAL~2~O~4~
[rock salt] - NaCl
- AX - Type Structures
- Includes Rocksalt, CsCl, Zinc Blende, and wurtzite structures
- AX~2~ - Type Structures
- Calcium Fluorite, Rutile
- A~m~B~n~X~p~
- Spinel
Coordination no
FeAl~2~O~4~ $\rightarrow Fe^{[4]}Al_2^{[6]}O_4^{[4]}$
$\sum (\{[\text{subscripts}] {[\text{superscripts}]}\}_{\text{cation}}) = \sum (\{[\text{subscripts}] {[\text{superscripts}]}\}_{\text{anion}})$
### Types of Crystal structures
#### [Rock Salt] $A^{[6]}X^{[6]}$
Combination of FCC of cation and FCC of anion
- Nearly 50% of ceramics crystallize in this structure
- Example - KCl, LiF, KBr, Oxides(MgO,CaO,etc) , Sulfides
- Alkaline earth - sulfides
- Largely ionic especially monovalent ion composition
#### [Nickel Arsenide] $A^{[6]}X^{[6]}$
- Same size range of NaCl
- Anion in **HCP**
- Nickel - Octahedral sites
#### [Cesium Chloride] $A^{[8]}X^{[8]}$
- Cation size - increased (compared to [Rock salt])
- It is too large to fit into octahedral interstices
- Cesium can occupy more ions as compared to sodium
- CsCl, CsBr, CsI
#### [Zinc Blende] and [Wurtzite] $A^{[4]}X^{[4]}$
- Zinc Blende - ZnS
- Wurtzite - FeO
- Cations are too small
- To fit into octahedral interstices - unstable configuration
- Fits into tetrahedral interstices
- Only half of the tetrahedral sides are used
#### [Fluorite] $A^{[8]}X_2^{[4]}$
- Fluorite - CaF~2~
- Large cations fit into BCC of anions SCC
- Anions fit into tetrahedral spots of Cations FCC
- Cations have twice the charge of anions only half of the cation sites occupied
- CaF~2~, ThO~2~, UO~2~, CeO~2~
#### Antifluorite $A_2^{[8]}X^{[4]}$
Anion in FCC,cations in tetrahedral voids
Ex - Li~2~O
#### [Rutile] $A^{[6]}X_2^{[3]}$
- Rutile - TiO~2~
- Medium sized carions with charge of 4+
- TiO~2~, PbO~2~, GeO~2~, SnO~2~, VO~2~, NbO~2~
- Distorted close packed structure
- Cations fill hald of the octahedral
2 Ti-O - 1.988 Å
4 Ti-O - 1.944 Å
#### Covalent [SiO~2~]/[Silica] $A^{[4]}X_2^{[2]}$
#### Ternary Ceramics [Perovskite] $A^{[12]}B^{[6]}X_3^{[6]}$
BaTiO~3~
- Many are cubic
- Distorted - Tetreagonal, Orthorhombic, rhombohedral
- Contains Large cation (close to anion size) and small cation
- Larger cation joins anion
- Smaller cation fills 1/4 of octahedral interstices
Line Dislocations
- Half of the atomic plane is missing/extra
Structural imperfection/defects in ceramics
- Deviation or imperfection
- Vibrations
## Atomic Defects
- Frenkel
- Attom Lattice - Interstitial
- Schottky
- Anion/Cation pair
Point defects - any lattice point not occupied by proper ion/atom needed to preserver long range periodicity
Linear - Dislocation
Planar - surface imperfection domains of different orientation, Grain Boundary/twin boundaries
3D - Pores, Cracks and Inclusions
### Defect Symbols/Notation
$\text{(Symbol of species)}_\text{location of defect}^\text{effective charge}$
V - Vacancy
Effective electric charge = Present - perfect
Kroger-Wink Notation
- Main Symbol
- Species
- V for Vacancy - V~m~, V~x~
- Subscript
- Crystallographic position of species
- i for interstitial
- Superscript
- Effective charge on the defect defined
- Diffrence between the real charge and defect species and that of species that would have occupied that site in a perfect crystal
- Negative Charge - Prime
- Positive Charge - dot
- Zero Charge - x
In semiconductors boron is likely to go to sillicon side.Imabalance is created so that holes are formed
`ABC for cations XYZ for anaions`
### Defects Reaction
Defects and concentration - energies of formation
- Defects can be treated as chemical entities
Rules for balanced defect reaction:
- Mass Balance
- Can't be created/destroyed
- Vacancies have zero mass
- Electronegativity/Charge balance(Superscript):
- Both sides of defect reaction must have total effective charge
- Presevation of regular site ratio
- NaCl - Site ratio - 1:1
- No. of M sites in M~a~X~b~ must always be in correct proportion to the no. of X sites
<!---| | Orignal | Occupied |Doped |Occupied after doping
| -------- | -------- | -------- |-------|--------------
|Cation-Na | 4 | 4 |4+2 |4+1
|Anion-Cl | 4 | 4 |4+2 |
|Ratio | 4:4 | 4 |--->
### Stoichiometery Defect Reaction
- Chemistry of the crystal does not change as a result
- No mass transferred across the crystal boundries
Example: Frenkel defect Ion leaves it's lattice position and moves to interstitial site
1. Frenkel Trivalant cation moves
M~m~^x^--->V~M~'''+M~i~^...^
O^x^--->V~o~^..^+O~i~''
2. Schottky
Grains - µm range
$\Delta g_s$ - Gibbs free energy is given out
3. Antistructure/ Misplaced Atoms
Mostly in Covalent ceramic SiC
C~C~ + Si~Si~ $\rightarrow$ Si~C~ + C~Si~
### Non-Stoichiometric Defect Reaction
- Composition of crystal changes as the result of reaction
- Mass is transferred across crystal boundaries
- At low oxygen pressure
$O_O^\times \rightarrow \frac 12O_2+V_O^\times$
- Oxygen ion leaves as a neutral species
- Leaves 2e^-^
- Localized electron at vacant site defect charge is zero
- Weakly bound
<br>
- Reduction - Outward Mass Transfer
## Extrinsic defects
- Till now pure crystal
- Impurity cannot be avoided
- various components arise
Gudelines
- Impurities usually substituted for the host ions of electronegetive nearest their own, even if ionic sizes differ
i.e. Cations $\rightarrow$ Cations, Anions $\rightarrow$ Anions
- In covalent compunds $\Delta E$ may be negligible ,size may play an important role
- Write the host crystal - Solvent
- Write solute(dopent) crystal
-
# Processing of Ceramics
Processing of Metals
- Minerals/Ore
- Benification/mineral dressing
- Conentration
- Extraction
- Pyro - Fe
- Hydro - Cu, Zn
- Electro - Al
- Molten Metal
- Casting - Slab
- Shaping
- Forging
- Welding
Processing of Ceramics
:::info
No melting is invovled
:::
- Powder Synthesis
- Output - Particles
- Consolidation/Shaping
- By applying pressure
- Output - Green Body
- Sintering
- Apply temperature
```flow
st=>start: Start
PS=>operation: Powder Synthesis
SH=>operation: Shaping
ST=>operation: Sintering
st->PS->SH->ST->
```
- Grain size will affect the mechanical properties like
- Toughness-ENERGY absorbed before fracture
- Smaller Grain size - Higher Toughness
- Lower Temperature during Sintering to oppose grain growth
## Ceramic Products (perception of products)
- Dense
- Hard
- Bulk Density/Crystallographic Density
- As per specified dimensions
- Homogeneous
- Chemistry
- MicroStructure
- Grain size
- Heating Difference - Ceramics are generally insulating - Core recieves less heat
- Grain shape
- Phase distribution
- porosity
- Prorosity/Density
- Minimized Processing time
- Minimum cost
- lower temperature
Yeild Strength is $\propto \dfrac {1}{ \sqrt{D}}$
D is diameter of grain
### TiO~2~ Syntesis
| Process | Conventional Values | Precipitation Values |
| ------- | -------- | -------- |
| Powder Particle Size (microns) | 1 | 0.1 |
| Sintering Temperature (°C) | 1400 | 800 |
| Sintering time (hours)| 10 | 1.5 |
| Final Relative Density(%) | 93(7% porosity) | >99 |
| Final Grain Size (micron) | 50-100 | 0.15 |
- Preciptition saves energy as lower temperatute is required for lesser time
- Precipitaion also gives a better product with smaller grain size (greater yeild stregth and density)
- Bricks are baked (Sintered) to remove the air bubbles and hence reducing their volume and increasing their density. As high density bricks are required for construction pourpose.
- The unbaked bricks are known as green bricks.
<!---
--->
## Powder Synthesis
Desirable Properties - Powders
- Particle size - < 1µm - Finer
- High surface area better bonding
- Higher Reactivity
- Densification $\propto \dfrac 1{\text{Particle Size}}$
- Particle size distribution i.e. range - Narrow
- Shape - Spherical/equiaxial
- State of agglomeration - No agglomeration or soft
- Chemical composition High purity
- Impurities may introduce liquid phase, sites for chemical reaction
- Phase Composition- Single Phase
> Mid Semester Examination
>>Even
>>>Q1. Rutile
>>>Diagram
>>>Asymmetric structure
>>>Coordination number
>>
>>>Q4. Coordination number - radius ratio
>>>Electrical Neutrality
>>
>>Odd
>>>Q1. Relative sizes
>>>Configuration
>>>Coordination numbers
>>>Diagram
>>
>>>Q3.Plastic Defromation
>>>Slip - movemnt of dislocations - covalently bonded
>>>Ionic - presence of dislocation - charge
>>
>>>Q4. Homogeneous
>>>High density
### Characterization
- Characterization
- Physical
- Size/Distribution
- Shape
- Agglomeration
- Surface Area
- Density and Porosity
- Chemical
- Minor Impurities
- Traces
- Phase
- Crystallinity
- Phases
- To scan the powder using electron microscope do the coating
- Do the gold or palladium coating
- So that they do not fly with the air and damage the lens while creating vaccum
#### Physical
- Types Of Particles
- Powder
- Colloidal Particles - 1nm to 1µm
- Coarse Particles - 1 to 100µm
- Granules - 100µm to 1 mm
- Sand
- Aggregate - greater than 1mm
- Particle Size and Distribution
- Irregular Particle
- Size = Diameter of sphere of equal volume
- May vary with instruments,different values with different instruments like XRD Microscope
- Mean Size
$\bar D = \dfrac{\sum D_i}{N}$
- Spread
$S = \sum$
- Unimodal, Narrow
- Histogram
:::info
Diffrential density should not be there or the material will crack
:::
- Characterization of Particle Size/Distribution
- Microscopy
- Optical - > 1µm
- SEM - >0.1µm
- TEM - >0.001µm - Nano particles
- Sieving - 5 - 1000 µ
- Sedimentation - 0.1-100µm
- Light Scatter - 0.1-1000µm
- X-Ray Diffraction - Less than 0.1µm
Microscopy
- Dilute suspension - Agitate - Evaporate
- Agitation - to break aglomeration
Coating - SEM
Sieving
- Using meshes
Sedimentation
- Based on Settling/Terminal Velocity
- $v = \dfrac{D(\rho_s - \rho_l)g}{18\eta}$
- Cannot be used for colloidal particles
Light Scattering
- Particle Size = f(intensity of scattered Light)
- $\text{sin } \theta = \dfrac{1.22\lambda}{D}$
X-Ray Broadening
- Bragg's law
$\lambda = 2d\text{ sin }\theta$
d - Interplanar Distance
$\lambda$ - Wavelength
- Sample - Powder/Solid
- In terms of Å
- Diffrent planes in diffrent directions (family of planes)
- Intensity vs Theta is measured
- The peaks represent planes
- The data is fed into directory or JCPDS card
- Diffraction angels are fed(signature XRD pattern)
- Powder colour
- Material identification
- Constrains on XRD
- Agricultural products
- The materials other than crystalline won't work
- No peaks only humps(noise)are seen in case of polymers and glass(amorphous material)
- Uses of XRD
- Identifying presence of Trace Elements
- Progress of phase transformation - MgCO~3~ - MgO
- Crystallinity
- Distinct peaks - for planes
- Non-Crystallinity - Hump - Amorphous
- Phase
- Chemical Composition
- Crystal Structure
- Physically Identifiable
- Peaks will diffuse if we keep on reducing the size by crushing the material, reason they start becoming amorphous - Peaks widen
Scherrer's formula
$D = \dfrac{0.9\lambda}{\beta \text{cos }\theta}$
D - Crystallite size
$\beta$ - FWHM Full Width Half Maxima
$\lambda$ - Wavelength
- Crystallite size is inversely proportional to the width
- Defraction takes place on the Crystallite - hence D is Crystalite size and not particulate size
Particle Shape
- If particals are irregular the flowability will be less
- Affecting the shaping process
- Shape Factor - Deviation from idealized geometry
- Eleongated
- Aspect Ratio = $\dfrac{\text{longest}}{\text{shortest}}\text{dimension}$
More the surface area more the internal energy less internal energy is required
S/C Area
- Gas Adsorption
- V~m~ = Amount of gas required to cover powder s/c with a monolayer
- s/c area calculated
Porosity
- Capilary action
- Mercury going into pores
- Applying pressure
- Volume~intruded~ vs Pressure
Phase Composition by CRD
S/C structure by
- STM - Scanning Tunnel Microscopy
- LEED - Low Energy Electron Diffraction
S/C chemistry by
- AES - Auger Electron Spectroscopy
- X-ray Photoeletron Spectroscopy
- SIMS - Secondary Ion Mass Spectroscopy
#### Chemical Composition
Optical Atomic Spectroscopy - 1-1000ppm - Liquid
XRF - >0.1% - Solid
SIMS - <1ppm - Solid
### Powder Synthesis Technique
#### Mechanical Alloying
##### Principle
- It occurs by repeated cold welding and fracture of the powder particles until the final composition of the powders coressponds to the percentages of the respective constituents in the inital charge
- Mixing of powders in the right proportion and loading the powder nto the mill along with the grinding medium
- This mix is then milled for the desired length of time until a stady state is reached
##### Process Variables
- Type of Mill
- The materials of milling tool
- Types of milling media
- Milling atmosphere
- Milling environment
- Milling Media to powder weight ratio
- Milling Temperature
- Milling Time
##### Powder Characterization
- Size of particle is relatively unimportant
##### Advantages
- Production of fine dispersion of second phase particles
- Extension of solid solubility limits
- Refinement of grain sizes down to nanometer range
- Synthesis of novel crystalline and quasi-crystalline phases
- Development of amourphous phases
- Disordering of ordered intermetallics
- Possibility of ordered intermetallics
- Possibility of alloying of difficult to alloy elements
#### Atomisation
##### Principle
- Breaking up of liquid into fine droplets
- Forcing a stream of molten metal through a small orifice and bombarding it with a stream of compressed gas
- This causes metal to disintigrate and solidify into finely divided powder particles
##### Process Variable
- Metal Flow (Single Nozzle) - 1 to 500 kg/min
- Water flow rate - 20 to 200 L/min
- Water velocity (At exit) - 10 to 500 m/s
- Water Pressure (At exit) - 5 to 150MPa
- Metal superheat 75 to 150°C - above melting point
##### Powder Charateristics
- Smalled than 150µm
- Free of fine porosity - Reletively compact with high packing densities and low surface areas
- Particle size controlled by pressure of th ewater jets
##### Advantages
- High production rate
- Preallyed powders can only be produced by atomization
#### Solid State Synthesis
Diffusion controlled
Fick's 1st law
$J = -D(\frac{dc}{dx})$
##### Principle
- Consists of heating non-volatile solids which react to form required product
1. Select starting materials
2. Weigh
3. Mix in Agate mortar and pestle
4. Ball Mill
5. Pelletize
6. Calcine
7. Grind product
##### Process Variables
- Fine grain powders to maximize surface area
- Ractive starting reagents are better than inert
- Will defined compositions
- Reactivity, strength, cost, ductility
- Atmosphere is also critical
##### Powder Characteristics
- Particle size of the product depends on the inital particle size
##### Advantages
- The method can be used to prepare an extremely large number of compounds
#### Sol-Gel Synthesis
##### Principle
- Process is a wet-chemical technique for fabrication of materials (typically metal oxide) starting from a chemical solution (sol) which acts as a precursor for an integrated network(gel) of either distrete particles or network polymers
- Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significnt amount of shrinkage and densification
##### Process Variables
- The rate at whcih the solvent can be removed is ultimately determined by the distribution of porosity in the gel
##### Powder Characteristics
- The crystallinity of the final product -obtained after removal of solvent and residuals from the pores by aging, drying, and annealing - highly dependent on the experimental conditions used
##### Advantages
- Cheap and low-temperature preparation of both non crystalline solids and crystalline ceramics
- Allows fine control on the product's chemical composition
- Produce products with small crystallites/high surface
#### Spray Pyrolysis
<!---
##### Principle
##### Process Variables
##### Powder Characteristics
##### Advantages
--->
##### Principle
- Aerosol process - atomizes a solution that heats the droplets to produce solid particles
##### Process Variables
- Solute Concentration
- Atomization technique
- Temperature, Temperature gradient
- Residence time in furnace
- Carrier Gases
##### Powder Characteristics
- Conventional spray pyrolysis - multiple nanosized crystallites, but these crystallites are virtually inseparable due to the formation of a three-dimensional network
##### Advantages
- Does not require high quality targets or substrates
- Does not require a UHV sytem
- Continuously produces the material
#### Precipitation
##### Principle
- Precursors such as nitrates, carbonates can be used as starting materials instead of oxides
- It involves taking stoichiometric mixture of soluble salts of the metal and precipitating them as hydroxides, citrates, oxalates or formates
- Mixture is filtered, dried, heated
##### Process Variables
- Additives added
- Media Used
##### Powder Characteristics
- The use of additives and control of nucleation, particle growth, and particle agglomeration allow the production of powders with a wide range of particle sizes, particle density, particle shape with specific, surface areas from less than 1 m^2^/g to 8^2^/g
- Mixing starting reagents in solution leads to smaller particles size, decreased diffusion distance - Lower temperature of final reaction
##### Disdvantages
- Expensive Reagents
- Precipitation rate of reagent may be different
- Difficult to fin reagents with comparable solubilites
#### Combustion Synthesis
- Burning
- Fuel + Oxygen
- Fuel
- Citric Acid
- Glycine
- Urea
- Oxygen - Metal Precursor
- NiNO~2~
- ZrOCl~2~
- CeNO~3~
- Explode
- Fuel + Nitrate Solution - Viscous - F+N Organic Complex - Gel - Mixing at molecular level
- Advantage - Produce Oxide powders
- Fluffy - cotton wool like
##### Principle
- Metathesis reaction
- Self-propagating high temperature synthesis
- It involves
- Mixing of individual nitrates with an organic fuel in stoichiometric ratio
- Drying of the mix leaduing to formation of precursor
- Ignition/Combustion of the mix
- Breaking down of the prorous form to obtain dine powders
- Characterisation of as-burnt powders
##### Process Variables
- Orgaic Amount of fuel used
- Heating temperature
##### Powder Characteristics
- Output - Powder Particles
- Fine
- Chemically homogeneous and pure, single phase powders in the as synthesized consition
- Particle size can be controlled by varying precursor solution concentration
- Multi-component particles can also be obtained by adding different salts into the solution
##### Advantages
- High reaction temperature can volatize low boiling point impurities and result in higher purity products
- Simple exothermic nature of reaction avoids the need for expensive processing
- Low operating time and processing costs
- Environmentally clean by products
- Larger amount of gas evolved - porous product - agglomerates formed are easily crushed and ground into a fine powder
## Shaping
- Compacting
- Differential Pressure - undesirable
- Powder Compaction
- Dry pressing, hot pressing, cold isostatic pressing, hot isostatic, pressing, etc
### Powder Compaction
- Mechanical Compaction - dry or semidry powders
- Applied pressure is not transmitted uniformly - friction between particles
- Stress variations lead to density variation
- Very rarely have spherical particles - rarely smooth
- Rough Particles suffer from agglomeration
### Isostatic Pressure
- Cermic powder is loaded into a flexible chamber and pressure is applied outside the chamber with hyraulic fluid
- Spark plug insulators, carbide tools
- Uses hydrostatic pressure to compact the ceramic powders from all directions
- Avoids the problem of non-uniform density in the final product that is often observed in conventional uniaxial pressing
### Casting
Casting ceramics - room temperature - suspended in a liquid - slurry
- Slurry is poured into a poroud mold that removes the liquid and leaves a particulate compact in the mold
## Sintering
### Defination
- coherent,soild structure mass transport
### Other Points
- Temperature
- Bonding - Between particles - Atomic
- Thermal treatment - to bond particles into a coherent, predominantly solid structure - mass transport events - atomic scale
- Bonding leads to improved strength and a lower system energy
Solid-Vapour interface
System wants to lower - solid-vapour interface energy
Curvature - More atoms are exposed to the interface
System tried to remove curvature - Flattening
Ultimate result the material flattens
Interfacial energy = Specific energy $\times$ Surface Area
Sintering is associated with formation of inter-particle bonds
Bonds grow by various mechanisms that occur at atomic level
- Many - throught solid state diffusion
LPS - liquid provides bonding, contribute capillary force and enhances mass transport
HIP - Yielding and diffusion at a time (application of pressure)
- Newer techniques involve reactions
- as pore structure changes shrinkage occurs - as vapour phase is removed
### Driving Force of Sintering
- Reduction of the total interfacial energy
The total interfacial energy of a powder compact (after shaping) = $\gamma A$
$\gamma$ - Specific Surface (interface) energy
A - Total Surface area of the compact
$\Delta(\gamma A) = A\cdot \Delta\gamma + \gamma\cdot\Delta A$
Change in interface energy ($\Delta\gamma$) - Densification
Change in interface Area ($\Delta A$) - Grain Coarsening
HIP - Hot isostatic pressing
Sintering Cycle
- Horizontal stages are called binder burnout Stage - 600-800°C
- 30 minutes break is given so that CO~2~ and H~2~O comes out
- Porosity reduces heavily beyond this point
Variables affecting sinterability and microstructure
- Related to Raw Materials
- Powder
- Shape
- Size
- Size Distribition
- Agglomeration - Differential Sintering occours (undesireable)
- Chemistry
- Composition
- Impurity
- Non-Stoichimetry
- Homogeneity
- Related to Sintering condition
- Temperature
- Time
- Pressure
- Atmosphere
- Heating
- Cooling Rate
Surface energy is assesd by surface area
Chemical potential diffrence between flat and sphere
$\Delta U = \dfrac{4\gamma \Omega}{D}$
$\gamma$ - Surface energy
$\Omega$ - Atomic Volume
Stages of sintering
Necking observed
Pores=incomplete sintering
Pores shrink and they get closed hence vaccum needs to be applied to remove the moisture completely
Starting point
- Contact in Particles
- Initial Bonds - Point contacts 0 Highly Deformed
- Contact grow - Reduction i Surface Area
- pores structure becomes rounded and discrete particles are less evident
Intermediate
- Tubular,rounded pore structue that is open to compacted surface
- gas can penetrate
- many sinteres structures are sintered to this stage only to preserve desirable pore structure.
- many sintered structured structures are sinteres to this stage only to preserve desirable pore structure.
As pores shrink,density increases and pore spherodize and no longer connected to compact surface.
- 
#### Sintering Mechaisms (Mass Transfer)
Evaporation Condensation
- Evaporation at Convex site
- Condensation at Concave site
- Atoms from the surface evaporates and condenses at neck resulting in neck growth
- **NOT SHRINKAGE**
Viscous/Plastic Flow
- Plastic flow of atoms - Area of high stress to low stress region
- Application of Stress is important - HIP
- Leads to densification
Diffusion
- Surface
- Can cause only neck growth
- Grain Boundary
- Causes Neck Growth and Shrinkage
- Volume
- Causes Neck Growth and Shrinkage
Volume Diffusion
- Grain boundary -> Neck
- Not only neck growth but also a centre to centre approach of the two particles i.e. shrinkage occurs because material is removed from the contact area of the particles
Covex surface -> Neck
- As the source of material is particle surface there is no shrinkage even though the nexk growth
Grain Boundary -> Neck
### Sintering Diagrams
- Identifies dominant Sintering mechanism depeng on various Experimental Conditions
- The boundary lines indicate the experimental conditions under which the contribution of two different mechanisms to sintering are same
- Silver particles with a radius of 38µm are sintered at 0.8T~m~
- Contour of constant time in diagram show the neck sizes found after sintering for those periods of time at various temperature.
# Properties of Ceramic Materials
## Physical Properties
Density and Melting Temperature
- Atomic Bonding
- Crystal Structure
- Phase Equilibrium
### Density
Factors affecting
- Size
- Atomic Weight
- Tightness of packing of atoms
- Amount of Porosity
Crystallographic - Ideal Density - Chemical Compostion and interatomic spacing from XRD
Theoretical - Zero microstructural porosity - multiple phases, defect structure, solid solution
Bulk/Sintered - Measured Density - lattice defects, porosity, phases
Specific Gravity - density of equal volume of water at 4°C - Based on Crystallographic pr theoretical
#### Crystallographic Density
Affected by
- Atomic Mass of Constituents
- Stacking of ions or atoms in a structure
- Ionic ceramics have high denisty
- Higher Temperatue polymorph - lower density (more open structure)
- ZrO~2~ is an exception to this (density increses from room temp to higher temp.)
#### Theoretical Density
Pycnometery
### Open Porosity
- Reduce strength
- Allow permeability
- Alter electrical characteristics - optical behaviour
- Mercury Porosimetry
UHTC - Ultra High Temperature Ceramic
## Mechanical Properties
Ceramics also have properties like Elasticity, Ductility, Wear strength, fatigue strength
- Materials which show only linear portion(in Stress Strain Curve)are Rigid in nature
- Range of Y for ceramics 100-500GPa
Why creamics dont show plasticity
as no slip are present as no dislocations are there hence no plasticity
- Application of force to any solid Result in reversible elastic strain followed by either fracture or fracture preceeded by plastic deformation
- Theoretical Ceohesive strength = E/10
- presence of flaws reduces it
### Fracture Toughness
Small Pores left in sample after sintering can be reffered as micro-cracks.
Force is being distributed evenly in the six columns
Microscracks may be preset as shown above
Stresses at the region surrounding microcrack are very high. The presence of flaw amplifies applied stress at the crack tip ($\sigma_{\text{tip}}$)
c - crack length
$\rho$ - Radius of curvature
$\sigma_f \approx \dfrac{Y}{20} \sqrt{\dfrac{\rho}{c}}$
$\sigma_f$ - Stress at fracture
a is crack tip radius
Cylindrical sample - cup and cone fracture
sheet sample - 45° fracture
$\sigma_{\text{tip}} < \sigma_{\text{max}}$ - Crack doesn't Propagate
$\sigma_{\text{tip}} > \sigma_{\text{max}}$ - Fracture propogates at the speed of sound
due to which catasrophic faliure is ther
Cracking:
It takes time to Nucleate
But it takes no time to propogate
Why is ceramics stronger in Compression than in tensile
- When compressed the cracks are closed
Thats why virat compressive strength is said
***Long time to nucleate the crack and no time to propogate the crack***
The energy in tensile testing 80 - 90% in heat and 10% in elongation of bonds in addition of vibrational energy
In fracture matrial is creating new surfaces
### Griffith energy criterion for fracture
- Strain energy
- all bonds stretch and elestic energy is strored
- Surface energy
- Energy consumed in formation of surface = elastic energy released
- Critical condition - release rate> consumed rate
- Total energy = U~0~+V~0~.U~ela~
In presence of crack length c some volume will relax(bond relax -> lose strain energy) relaxed volume by shaded portion
While calculating V~0~=l~0~ x b~0~ x t~0~
we have to reduce
the energy of volume reduced
that is the area of the shaded portion where there is no stress due to crack
Subtracting the ideal stress in the shaded region.
**Surface Energy**
till c< c~critical~ material will absorb energy after c > c~critical~ material will release energy(creation of new surfaces due to fracture)
If you get a ceramic do radiographic test and check for crack
if the crack for 20 microns and in case you have to do the test reduce the stress to do the test
**Conclusions**
$\sigma_{app}$ < $\sigma_{yeild}$
$\sigma.\sqrt{\pi.c}=K_1$ Stress internsity factor
$\sqrt{2.\gamma.E}=K_{1c}$ fracture toughness - critical stress intensity factor
when K~1~ $\geq$ K~1c~
***imp asked in many objective exams especially units are imp***
#### Compression
- Cracks tend to propogate & twist out of thier orignal orientation to prapogate parallel to compressive axis
- Fracture , NOT by UNSTABLE propagation of single crack , but slow extension and linking of many cracks --> croshed zone
- Not the size of single largest crack
- But average crack size is important
### Toughness Mechanisms
$K_{Ic} = \sqrt{YG_c}$
G~c~ - Toughness of the material in joules per squre meter
Toughness limit - G~c~ = $2\gamma$
Delay the propagation/growth of the crack as much as possible
- Increase the energy to extend the crack as much as possible for tough material
- Best way to do it is by having more number of grain boundaries
- Smaller grain size is desireable
#### Crack deflection
- Fracture Toughness of polycrystalline ceramics is higher than single crystal of same composition
- Polycrystalline - Crack delfected around weak grain boundaries - average stress intensity at uits tip is reduced - stress is no longer always normal to the crack plane
#### Transformation Toughening
Volume of alpha is grater
After phase transformation volume increases due to which adjoining grains distort(under stress)
- Fine tetragonal ZrO~2~ dispersed
- During Careful cooling -> fine particles maintained
- Metastable tetragonal phase
- Approaching crack front is the catalyst that triggers transformation & puts the region ahead of compression.
- Extra Energy is required to external crack through compression layer, increases both toughness & strength.
## Optical Properties
- Interaction of electromagnetic radiation with the materials
- transmission
- reflection
- diffraction
- absorption
- refraction
- radiation emitted by the material
Metals
- Opaque
- electron jumps to higher energy levels
Ionic Ceramics
- Filled electron cells
- No Energy levels available
- Transparent to electromagnetic radiation
Semiconductors
- have small band gap
- transmit under some conditions
- become opaque as soon as enough energy is present for the electrons to enter the conduction band
Good Insulators - Large bandgap - Transmit
Adsorption, transparency = f(scattering by flaws, pores, inclusions)
Applications - Strategic and military, radars, spacecrafts, high energy lasers
MgF~2~,ZnS,ZnSe
Mostly single crystals are used as the light may bet scattered due to grain boundaries
Colour - selective absorption
- Types
- Internal Transitions - Incomplete electron shell
- Charge transfer - Electron transfered from ion to ion
- Electron Transition - crystal imperfections
- Bandgap - Intrinsic Colouration
- Bond Field and Oxidation state
- Colour is the complementary colour of the absorbed spectral colour
Pls play with band gaps , all colors are valuable.
Phosphorescence
- Excitation of light resulting from excitation of material by appropriate energy scource.
- TV screens, Photocopy
Optical Fibres, Lasers
Electro-Optics, acousto-optics
## Electrical Properties
Ohm's Law - V = IR
Drift Velocity
$\sigma =n\,q\,\mu$
n - number of charge carriers
q - Charge
$\mu$ - Mobility
Electronic conductivity = f(Temperature, Impurity, Solid Solution, Plastic Deformation)
e^-^ - Require less activation energy to conduct
Ceramic - Ionic Conduction - Require more activation energy - Movement of ions - Thermal Energy
If you keep on increasing the temperature the connductivity of metals decreases due to increase in lattice vibrations.
### Ionic Conductivity
Ionic Conduction increases with increase in temperature as the number of charge carriers increase.
Ionic Conductivity = f(Movement of ions, Energy Hill Concept) = f(Doping, Temperature, Vacancies)
Transference number - Indicates the charge carrier
t~+~ = 1
- 100% of Conduction Takes place because of C^+^ because of smaller size
- Eg - NaCl,KCl,AgCl
t~-~ = 1
- 100% of Conduction Takes place because of C^-^
- Fluorite structure,BaF~2~, PbF~2~, Zirconia
Transference number - Fraction of total conductivity contributed by a type of carrier
t~+~, t~-~, t~e~, t~h~
t~e~ = 1
FeO, VO, TiO~2~ - 100% electronic conductivity
Ceria to Zirconia -> electronic + ionic
Glasses -> Ionic Diffusion
Applications
- Oxygen Senors
- SOx - NOx decomposition, SOFC, Sodium ion conductor in batteries in automotive and satallites, Li ion conductors
- Intrinsic and extrinsic
#### Superconductors
$\sigma=\infty$
$\rho=0$
Resistance is cause by thermal vibrations
By liquid helium the temperature was reduce and at a pirticular temperature the resistance came out to be zero That temperature is known as *****critical temperature*****
- Superconductor has to be diamagnetic.
Countries have ***wasted*** money on research on superconductiong materials.
- Mercury was cooled to 4 degree and the resistance came out to be near zero
- Critical Temperature - Resistance drops to zero
- BCS theory electrons team togather in pairs and move in phase in sync
Example : In a hall once class gets over all the sudents move towards the exit in syncronized manner and in chaotic manner
### Dielectrics/insulators
- Ceramics which are good insulators are refered as dielectrics
- Although they are not conducting electricity, they are not inert to Electric fields.
<br>
- Difference in valence band and conduction band is huge
- There is slight shift of charges when placed in electric fields
- Formation of electric dipole is called polarisation.
Contribution
- Electron - atoms
- Orientations - Molecules
- Space-charge Polarization - µ
- Atomic ionic polarization - unit cell
#### Electron Polarisation
- Electrons shift towards the positive and nucleus towards negative
- Disappear on removal of field
- Small Displacment
- Electronic polarisation is insensetive to Temperature
#### Orientation Polarisation
- Observed in non- Symmetric molecules
- H~2~O --> H^+^ + OH^-^
- HCl, H~2~O, HF
- Inversely proportional to Temperature
#### Space Charge
- Least understood
- Space charge are random charges caused by cosmic radiations thermal deterioration.
#### Atomic/Ionic
- Polarization effects
- Ferroelectricity
- Pyroelectricity
- Piezoelectricity
- Temperature - Directly affected
- Dielectric constant increases with temparature.
#### Dielectric Constant
- Degree of polarizability or charge storage capability
$K'=\frac{K_\text{material}}{K_\text{vacuum}}$
- Dielectric Constants
- NaCl 5.9
- LiF 9
- MgO 9.6
- BaO43
- TiO 15-170
- BaTiO~3~ - 1600
Low K' - insulators - Al~2~O~3~, BeO, AlN, Polymers
High K' - Capacitors
K' = f(temperature)
K' = f(freq of applied field)
#### Dielectric Strength(V/mm)
- For a unit thickness how much voltage it can tolerate without breaking down(allow passage of current)
- Capability to withstand an electric field without breaking down or allowing current to pass
- Organic - high Strength
- Rubber - 2000 V/mil
- Mica - 5500V/mil
mil = $\dfrac 1{1000}$inch
Dielectric Loss
- Ideally - only displacement of charge via polarization must occur
- Current-voltage - phase 90°
- Actual - Current Lags behind by δ
- Amount tan(δ) = loss
#### Capacitance
- when placed in electric fields, Capactity of a dielectric to store electric charge
- Higher polarizability means Higher dielectric constant therefore more charge can be stored
Q = CV
Q - Charge
C - Capacitance
V - Voltage
$C = ε_0\dfrac{K'A}{t}$
#### Piezoelectricity
- Polarization when Pressure applied
- Pressure -> Measurable electric potential
- In transducer, Ultrasonic devicec , microphones
- Triclininc will the best to get maximum polarization
- Anisotropic crystal with no centre of symmetry are piezoelectric
#### Pyroelectricity
- Electricity released by heating
- Spotaneous polarization at high temperature
- Wurtzite, Rochelle Salt, BaTiO~3~, Li~2~(SO~4~)~3~
- LiTiO~3~ --> 690°C sensitivity 10^-6^per°C
##### Barium Titanate
Large Size of Ba ions - octahedral interstial position is quite large compared to size of Ti ions
Minimum energy position - off-centre in the directon of each of the size oxygen ions
Spontaneous polarization for each Ti ion - degree of polarization is high
Electrical field - Titaium ions shift - High bulk polarization and high dielectric constant
Temperature influences Crystal Structure ---> polarisation characteristics . Above 120°C ---> cubic and the behaviour described above prevails . The thermal vibrations is high enough to result in random orientation .
Temperature < 120°C ---> tetragonal, Octahedral site ----> distorted wit Ti ion in off centre positio ---> permanent dipole.
Polarization characteristics can be modified by alterations of the crystal structure
Ba and Pb ions are very large -> large octahedral site in which the titanium ions can readily move
Cooling in presence of electric field - Maximum alignment of domains
Beyond curie temperature - Ferroelectric - Paraelectric
Solid solution additions can change the currie temperature and alsothe shape of the hystersis loop
Shape of Hystersis loop varies for different temperature - higher temperature - smaller the loop
Polarization remaining after material has been fully polarized and then had the field removed is remnan polarization(Pr)
#### Ferroelectricity
- Spontaneous Polarization
- Retention after removal of electric field
- Large - Hysterisis Loop
- Region in crystal where dipoles are aligned - Domains
## Magnetic Properties
- Memory units, Bio-imaging
Sources
- Intrinsic magnetic properties detected by
- Electronic Structure
Odd number of electrons - net magnetic movement - Unparied electrons
Paired electrons cancel each other's spin
In oxide ceramics
Al^3+^, Mg^+2^, Zn^+2^, Ti^+2^ - Paired electron and hence no net magnetic
- Magnetic cermaic - ferrites
- Cubic Ferrites
- Spinel - MFe~2~O~4~ (Ni, Mn, Mg, Zn, Cu)
- Garnet - R~3~Fe~5~O~12~ (Y/Gd)
- Hexagonal - BaFe~12~O~19~, Ba~2~MFe~12~O~22~
- Orthogonal perovskite - RFeO~3~ (Ni, Mn, Cr, Co, Al)
Ferrites - Easy magnetize/demagnetize - Soft
Permanent Magnetization/ difficult demagnetization - Hard
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