Topic 1.1: Introduction to Ceramics - Insight into Chemical Engineering

Ceramics:

It is an inorganic and non-metallic solid material. The word “ceramic” comes from the Greek word “Keramos” which means potter’s clay.

They are typically hard, brittle, heat-resistant, low thermal expansion, corrosive-resistant and chemically non-reactive.
They may have a crystalline or partly crystalline structure.

The earliest ceramics made by humans were pottery objects made from clay only or a combination of clay with other materials like silica.
They are prepared by molding clay, silica, and water combinations into desired shapes, followed by the application of heat and subsequent cooling.

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Ceramics are now found in a variety of household, industrial, and construction products. Examples are floor tile, bricks, plates, glass, toilets, snow skis, sparkplugs, phone lines, drainage pipes, etc.
Ceramics can be dense or lightweight depending on how they are made.
Ceramics have high melting points so require high temperatures for processing.
Unlike metals, ceramics are always a combination of two or more different elements.
Ceramics are usually a combination of metal/metalloid and non-metal (mainly O, N, C, B) elements. For examples, Al₂O₃, SiO₂, SiC etc.

Note: A metalloid is an element that has properties that are intermediate between those of metals and nonmetals. Metalloids can also be called semimetals. Examples are B, As, Si, Sb, Ge etc.

Combination of (Metal + Non-metal) form an ionic bond. For example, MgO, BaTiO₃.
Combination of (Metalloid + Non-metal) form a covalent bond. For example, BN, SiC.
Ceramics can be grouped into different categories based on the elements they contain. For examples, Oxides: Metal + Oxygen (Al₂O₃); Carbides: Metal + Carbon (SiC); Nitrides: Metal + Nitrogen (BN); Borides: Metal + Boron (SiB₃)

Question: Why are ceramics brittle?

Explanation: They are brittle due to their atomic bonding and crystal structure.

Atomic Bonding: Ceramics usually have ionic or covalent bonds, which are strong but brittle.

These bonds are formed by the sharing or transfer of electrons between atoms, resulting in a strong bond that is difficult to break.
These types of bonds do not allow plastic deformation, which is the ability of a material to change shape without breaking.
Therefore, a material held together by either type of bond will tend to fracture before any plastic deformation occurs, resulting in low toughness.

Crystal Structure: The crystal structure of ceramics is often highly ordered and regular, with a repeating pattern of atoms.

This regular structure makes it more difficult for defects, such as cracks or voids, to move through the material without causing a fracture.

Porous Nature: These materials are porous, the pores and other microscopic imperfections act as stress concentrators, further decreasing toughness and tensile strength.

When a force is applied to a ceramic material, these strong bonds, ordered crystal structure, and porous nature make it more likely that the material will break rather than deform plastically. This is why ceramics are generally brittle and prone to fracture when subjected to stress or strain.

Question: Why are ceramics brittle (hard and liable to break easily) and metals ductile (capable of being drawn into wire) in nature?

Explanation:

Ceramics:

Ceramics typically have directional ionic or covalent bonds, which involve the sharing or complete transfer of electrons between two atoms.
These bonds are strong and hold the atoms rigidly in place, making them very resistant to deformation.
Examples are Al₂O₃, ZrO_2, etc.

Metals:

In contrast, metals have metallic bonds, which involve the sharing of electrons among many atoms.
A metallic bond involves a “sea” of delocalized electrons shared by all the atoms in the metal lattice. The metal atoms are held together by a “sea” of electrons floating around.

Delocalized electrons are not confined to specific bonds but rather move freely throughout the lattice.
Under stress, these delocalized electrons allow the metal atoms to slide past each other more readily.
Examples are Al, Cu, etc.

In summary, the delocalized nature of electrons in metallic bonding allows for greater atomic mobility compared to the rigid, directional nature of ionic and covalent bonds in ceramics. So, metals are ductile and ceramics are brittle.

Question: Why are ceramics generally porous in nature?

Explanation: Unlike metals that can be melted and poured into molds, most ceramics are formed by shaping powders. These powders undergo compaction, shaping, and firing at high temperatures to produce a dense, solid structure. However, achieving a complete non-porosity in ceramic poses a challenge due to several factors:

Firstly, the powder particles cannot be perfectly packed together during the compaction stage, resulting in voids within the compacted powder. This inability to achieve perfect particle packing contributes to the porous nature of ceramics.
Additionally, binding agents or other additives are commonly employed during ceramic processing to aid in shaping and consolidation. However, these additives evaporate at high temperatures, leaving voids within the ceramic structure.

Properties of Ceramics:

The various properties of ceramic materials are given below.

Physical Properties:

Density:

It is a measure of mass per unit volume. It tells how much mass is contained in a given volume.

Most ceramics are lighter than metals.
They have an intermediate density between polymers (lower) and metals (higher).
The density ranges from 2 to 6 g/cm³.
Non-crystalline materials are less dense than crystalline ones.
The density of ceramic has an impact on its strength.

As\;\rho\uparrow,\;strength\;of\;ceramic\uparrow

The theoretical density of ceramic can be computed using the formula given below.

\rho=\frac{n(\sum A_c+\sum A_a)}{V_cN_A}

Where, n = No. of formula units in unit cell; $\sum A_c$ = Sum of atomic weights of cations; $\sum A_a$ = Sum of atomic weights of anions; V_c = Unit cell volume; N_A = Avogadro’s number.

Porosity/Void fraction:

It is a measure of the void or empty spaces in a material. It is the fraction of the volume of voids over the total volume.

\phi=\frac{V_{void}}{V_{total}}

The majority of ceramics are porous in nature. Ceramics can be porous or non-porous in general.

As\;\phi\uparrow,\;strength\;of\;ceramic\downarrow

As\;\phi\uparrow,\;density\;of\;ceramic\downarrow

The porosity of ceramic can be controlled during the synthesis of ceramic.

Color:

Most ceramics are transparent.

Color depends on the interaction of light with ions in the main ceramic or pigments that are added to the ceramic as a secondary phase.

Mechanical Properties:

Ceramic mechanical properties are crucial in structural and construction materials.

Hardness:

It is the ability of a material to resist deformation induced by mechanical indentation.

Ceramics are hard and rigid in nature. Hardness is measured by the hardness test.

Hardness Test Method: In this method, an indenter is used to force into the sample surface. Then, the depth of indentation is measured.

Brittleness:

It is the ability of a material to fracture without undergoing plastic deformation when subjected to stress.

Ceramics are brittle in nature. Brittleness is measured by the fracture toughness test.
These properties depend both on temperature and the amount of crystallinity.
At low temperatures, both crystalline and non-crystalline phases are brittle.
At high temperatures, crystalline phases are brittle but non-crystalline phases are ductile.

NOTE: Crystalline materials have a melting point (T_m), whereas non-crystalline materials have a glass transition temperature (T_g).

Fracture Toughness Test: Fracture toughness is the ability of a material to resist fracture when a crack is present.

In this method, an initial crack is made in the specimen then testing is accompanied by loading the specimen in tension.

The load and crack opening displacement is recorded until failure.
Material with a large value of fracture toughness undergoes ductile fracture, whereas a material with a low value of fracture toughness undergoes brittle fracture.
This test is used to determine the energy needed to cause failure within the material.

Question: How do the brittle-ductile characteristics of a ceramic material with 50% crystalline and 50% non-crystalline phases vary with temperature?

Explanation: Below Tg, both phases are brittle. Above Tg, the crystalline phase is brittle but the non-crystalline phase is ductile. Above T_m, both phases melt and the material is entirely ductile.

Compressive Strength:

It is the ability of a material to resist compressive forces.

Ceramics have high compressive strength. It is measured by the compressive test.

Compressive Strength Test: In this test, a compressive load is applied over the specimen.

This test is used to determine the material’s behavior under applied crushing loads.
The compressive strength of ceramics is 10 times higher than their tensile strength. So, they are used in applications where the load is compressive in nature.

Tensile Strength:

It is the ability of a material to withstand a tensile (pulling) force. Stress-strain curves can be generated using a tensile strength test.

Ceramics are brittle in nature. So, the stress-strain behavior of ceramic is not usually determined by a tensile test. It is done using the flexural test.

Flexural Test: It is used to determine the bending properties of a material. Flexural strength is the ability of a material to resist deformation under load.

It can be done using a 3-point or 4-point bending load method.

In the 3-point method, the specimen with a rectangular cross-section is placed on two parallel supporting pins. The loading force is applied in the middle.

Impact Strength:

It is the ability of a material to resist a sudden applied load.

They have poor impact strength.

Thermal Properties:

Thermal Conductivity:

It is the ability of a material to conduct heat.

They have low thermal conductivity due to ionic & covalent bonding which does not have free electrons.
They have the ability to withstand high temperatures. For example, Zirconia is used for kiln walls.

Thermal Expansion:

It is the ability of a material to change its shape, area, volume, and density in response to a change in temperature.

Ceramics has the highest melting point (600 to 4000 ⁰C) but their coefficient of thermal expansion is much less as compared to metals.
These properties are governed by the bond strength between atoms.

Specific Heat Capacity:

It is the amount of heat required to raise the temperature of the unit mass of a substance by one degree Celsius.

They have higher specific heat capacity than that metal.

Thermal Shock Resistance:

It is the ability of a material to withstand sharp changes in temperature. They have high thermal shock resistance.

Electrical Properties:

Electrical Conductivity:

It is the ability of a material to conduct an electrical current.

They are usually electrical insulators although some exhibit semiconducting and conducting behavior also.

Piezoelectric Property:

It is the ability of certain materials to generate an electric charge in response to applied mechanical stress.

Some ceramics especially quartz exhibit piezoelectric behavior under which a mechanical loading generates potential difference across its surfaces.
For example, Barium titanate (BaTiO₃) and Lead zirconate titanate (Pb[Zr_xTi_1-x]O₃; where x lies between 0 to 1) are used for manufacturing transducers, actuators, and medical ultrasound equipment.

Question: Differentiate between piezoelectric & pyroelectric effects.

Explanation:

Piezoelectric Effect: It is the ability of certain materials to generate an electric charge in response to applied mechanical stress. Examples are Barium titanate, PZT (Lead zirconate titanate).

Pyroelectric Effect: It is the ability of certain materials to generate a temporary voltage when they are heated or cooled.

The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the materials changes.
This polarization change gives rise to a voltage change across the crystal.
An example is Gallium nitride (GaN).

Chemical Properties:

They have good chemical resistance to weak acids and weak bases.

They are soluble in certain strong acids (Hydrofluoric acid) and strong bases. Usually, non-crystalline phases dissolve first.

Advanced Ceramics Materials:

Advanced ceramics are high-performance materials that exhibit superior properties such as high strength, hardness, wear resistance, corrosion resistance, and thermal stability. They are typically made up of non-metallic, inorganic materials such as oxides, nitrides, carbides, and borides.

Aluminum Nitride (AlN):

It is formed by reacting molten aluminum with nitrogen.

Al+1/2N_2\rightarrow AlN

It has a high thermal conductivity of up to 321 W/(m.K).
It has very high electrical resistivity. So, it is an electrical insulator.
It is used in power electronics, aeronautical systems, railways, microwaves, and military applications.
It is used in many electronic applications such as in electrical circuits operating at a high frequency.

Aluminum Oxide/Alumina (Al₂O₃):

It is formed by reacting molten aluminum with air.

2Al+3/2O_2\rightarrow Al_2O_3

It has good stiffness and strength, good resistance to wear, and high hardness.
It is used for a variety of applications such as X-ray tubes, electron tubes, laser devices, aerospace devices, high vacuum applications, flow meters, pressure sensors, cutting tools, wear components, electronic components, and biomedical implants.

Silica (SiO₂):

It is the most widely used ceramic material.

It is used in thermal insulation, abrasives, laboratory glassware, optical fibers, etc.
Fine particles of silica are used in tires, paints, etc.

Silica Carbide (SiC):

It is a semiconductor and is often used in high-temperature electronics. It is considered one of the best materials for very high-temperature applications such as furnace components.

It has low density, high strength, high hardness, high thermal conductivity, wear resistance, and excellent chemical resistance
It is used as coatings on other materials for protection from extreme temperatures.
It is also used as an abrasive material.
It is used as a reinforcement in many metallic and ceramic-based composites.

Silica Nitride (Si₃N₄):

It has excellent strength, toughness, and thermal shock resistance.

It is used in bearing balls, cutting tools, valves, and welding jigs.
It is used in high-temperature applications such as gas turbines and automotive engines.

Zirconium dioxide/Zirconia (ZrO₂):

It is a white powdered material commonly used to produce dental frameworks.

It has excellent wear resistance, toughness, and high strength.
It is used in producing many other ceramic materials. It is used as an additive in many electronic ceramics.
It is also used in making oxygen gas sensors.
It is used in the manufacture of knives. The blade of a ceramic knife retains its sharpness for far longer than that of a steel knife.
It is used in dental implants, cutting tools, and fuel cell components.

Diamond (C):

It is the hardest material known to be available in nature.

It is used in jewelry.
It is used as cutting tools, abrasion-resistant coatings, etc.

Titanium oxide (TiO₂):

It is mostly found as a pigment in paints.

It is used to make other ceramics like BaTiO₃.

Lead zirconium titanate (Pb[Zr_xTi_1-x]O₃):

It is the most widely used piezoelectric material and is used as gas igniters, ultrasound imaging, in underwater detectors.

Titanium boride (TiB₂):

It exhibits great toughness properties and hence found applications in armor production.

It is also a good conductor of both electricity and heat.

Advanced ceramics are used in a variety of industries such as aerospace, automotive, electronics, biomedical, and energy due to their superior properties and performance characteristics.