ISM Question Bank Unit 1-2

1. What do you understand by smart materials; give some examples?

Ans: Smart materials are materials that have one or more properties that can be significantly altered in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields.

Examples:
1. Piezoelectric materials
2. Shape memory alloys
3. Magnetic shape memory alloys
4. Shape memory Polymers
5. PH sensitive polymers

1. Piezoelectric materials are materials that produce a voltage when stress is applied. Since this effect also applies in the reverse manner, a voltage across the sample will produce stress within the sample. Suitably designed structures made from these materials can therefore be made that bend, expand or contract when a voltage is applied.

2. Shape memory alloys and shape memory polymers are thermoresponsive materials where deformation can be induced and recovered through temperature changes, an example is Nitinol (Nickel Titanium).

3. Magnetic Shape Memory alloys are materials that change their shape in response to a significant change in the magnetic field.

4. Shape-memory polymers (SMPs) are polymeric smart materials that have the ability to return from a deformed state to their original shape induced by an external stimulus (trigger), such as temperature change.

5. PH-sensitive polymers are materials which swell/collapse when the pH of the surrounding media changes.

2. Describe components of smart systems; classify smart materials.

Ans: same as Question 4 and 22.

3. What do you understand by Metallization? explain any one physical vapor deposition (PVD) process with a suitable sketch.

Ans: Metallization is a process whereby metal films are formed on the surface of a substrate. These metallic film sare used for interconnections, ohmic contacts, etc.

Hence, their continuity, uniformity and surface properties are critical in the device performance. Metal films can be formed by using various methods, with the most important being physical vapor deposition (PVD).

Sputtering is a physical phenomenon involving the acceleration of ions via a potential gradient and the bombardment of a ‘target’ or cathode. Through momentum transfer, atoms near the surface of the target metal become volatile and are transported as a vapor to a substrate.

A film grows at the surface of the substrate via deposition. Sputtered films tend to have better uniformity than evaporated ones and the high-energy plasma overcomes the temperature limitations of evaporation.

Most elements from the Periodic Table can be sputtered, as well as inorganic and organic compounds.

Refractory materials can be sputtered with ease. In addition, materials from more than one target can be sputtered at the same time.

This process is referred to as ‘co-sputtering’ and can be used to form ‘compound thin films’ on the substrate.

The sputtering process can, however, be used to deposit films with the same stoichiometric composition as the source and hence allows the utilization of alloys as targets [14].

Sputtered thin films have better adhesion to the substrate and a greater number of grain orientations than evaporated films.

4. Explain the classification of smart materials.

Ans: Actively Smart:

They possess the capacity to modify their geometric or material properties under the application of electric, thermal or magnetic fields, thereby acquiring an inherent capacity to transduce energy.

• Piezoelectric

• Magneto strictive

• Shape memory alloys

• Electro-Rheological fluid, etc.

They can be used as force transducers and actuators

Passively Smart: Those smart materials that are not active are called passively smart materials. Although smart, they lack the inherent capability to transduce energy.

• Optic fibers

5. Provide the properties and applications of Ceramic Materials.

Ans: Properties:

Ceramics have high hardness.
They are brittle and have poor toughness.
They have a high melting point.
They have poor electrical and thermal conductivity.
They have low ductility.
They have a high modulus of elasticity.
They have high compression strength.
They show optical transparency to a variety of wavelengths

Application:

Silicon carbide and tungsten carbide are technical ceramics that are used in body armor, wear plates for mining, and machine components due to their high abrasion resistance.
Uranium oxide (UO2) is a ceramic that is used as a nuclear reactor fuel.
Zirconia is a ceramic that is used to make ceramic knife blades, gems, fuel cells, and oxygen sensors.
Barium titanate is a ceramic that is used to make heating elements, capacitors, transducers, and data storage elements.
Steelite is a ceramic that is used as an electrical insulator.

6. Explain various steps involved in the processing of semiconductor materials.

Ans:

7. Explain stepwise processing of traditional ceramics with a suitable sketch.

Ans: Traditional ceramic processing typically involves several steps. Here is a stepwise breakdown of the process:

1. Raw Material Preparation: The first step is to select and prepare the raw materials for the ceramic production. This may involve mining and extracting the necessary minerals, such as clay, silica, feldspar, and various additives. The raw materials are then crushed, ground, and mixed to achieve a consistent composition.

2. Shaping: Once the raw materials are prepared, the next step is shaping the ceramic material into the desired form. There are various techniques for shaping ceramics, including:

– Hand-Forming: The ceramic material can be shaped manually by skilled craftsmen using techniques like molding, coiling, or slab construction.

– Wheel-Throwing: This method involves using a potter’s wheel, where the ceramic material is placed on the wheel and shaped by hand or using tools.

– Slip Casting: In slip casting, a liquid clay mixture called slip is poured into a plaster mold. The mold absorbs moisture from the slip, forming a solid layer against the mold surface. Excess slip is poured out, and the remaining material is allowed to dry and harden.

3. Drying: After shaping, the ceramic objects need to be dried to remove excess moisture. This is typically done gradually to prevent cracking or warping. The drying process can be natural or accelerated using temperature and humidity-controlled drying chambers.

4. Firing: Once dried, the ceramic objects are fired in a kiln to achieve the desired hardness, strength, and shape retention. Firing involves subjecting the ceramics to high temperatures, which can range from several hundred to over a thousand degrees Celsius. There are two main types of firing:

– Bisque Firing: This initial firing is done at a lower temperature to remove any remaining water and organic materials from the clay. It transforms the ceramic into a porous, but still fragile, state called bisqueware.

– Glaze Firing: After the bisque firing, a glaze—a glassy coating—can be applied to the ceramic surface to enhance its appearance and functionality. Glazing involves applying a mixture of minerals and chemicals onto the ceramic, which melts and fuses during the glaze firing, creating a smooth, protective layer.

5. Finishing: The final step involves any additional finishing touches, such as sanding, polishing, or painting, depending on the desired outcome. This step is crucial for achieving the desired aesthetics and functionality of the ceramic objects.

8. With an example explain Magnetostrictive materials.

Ans:

9. Classify and explain shape memory alloys.

Ans:

An alloy that remembers” its original, cold-forged shape. By heating it returns back to the re-deformed shape.
SMAs are materials which can revert back to original shape & size on cooling by undergoing phase transformations.
Shape memory alloys (SMA’s) are metals, which exhibit pseudo-elasticity and the shape memory effect.
Examples: NiTiNOL (thermal), NiMnGa, Fe-Pd, Terfenol-D (Magnetic) CuZnSi, CuZnAl, CuZnGa, CuZnSn(actuator)
The shape change involves a solid-state phase change involving a molecular rearrangement between Martensite and Austenite.
A temperature change of only about 10⁰ C is necessary to initiate this phase change

Shape Memory Alloys
A. Cu-based Alloys

Cu-Al-Ni with 14/14.5 wt.% Al and 3/4.5 wt.% Ni
Cu-Sn approx. 15 at. % Sn
Cu-Zn 38.5/41.5 wt.% Zn

B. Other shape memory alloys include:

Ni-Ti (~55% Ni)
Ag-Cd 44/49 at. % Cd
Au-Cd 46.5/50 at. % Cd
Fe-Pt approx. 25 at. % Pt
Mn-Cu 5/35 at. % Cu
Fe-Mn-Si
Pt alloys
Co-Ni-Al
Co-Ni-Ga
Ni-Fe-Ga

10. Write a short note on Magnetorheological fluids.

Ans: A MR fluid is a smart fluid which usually consists of 20-40 percent iron particles, suspended in mineral oil, synthetic oil, water or glycol.

MRF also contains a substance which prevents the iron particles from setting.
When subjected to a magnetic field, the magnetic particles inside increase the fluid’s viscosity, rendering it viscoelastic solid.
“OFF” position – the MR fluid is not magnetized & the particles inside, distributed randomly, allow the fluid to move freely, acting like a damper fluid.
“ON” position – the particles become energized and align into fibrous structures and restricts the movement of the fluid.

11. Define polymers and classify the types of polymers.

Ans: A polymer is a large molecule or a macromolecule, which essentially is a combination of many subunits. The term polymer in Greek means ‘many parts’. Polymers can be found all around us, from the strand of our DNA, which is a naturally occurring biopolymer, to polypropylene which is used throughout the world as plastic.

There are three types of classification under this category, namely, natural, synthetic, and semi-synthetic polymers.

Natural Polymers

They occur naturally and are found in plants and animals. For example, proteins, starch, cellulose and rubber. To add up, we also have biodegradable polymers called biopolymers.

Semi-synthetic Polymers

They are derived from naturally occurring polymers and undergo further chemical modification. For example, cellulose nitrate and cellulose acetate.

Synthetic Polymers

These are human-made polymers. Plastic is the most common and widely used synthetic polymer. It is used in industries and various dairy products. For example, nylon-6, 6, polyether, etc.

12. Write short notes on Piezoelectric materials and shape-memory materials.

Ans: Piezoelectric Material

Materials that produce a voltage when stress is applied. (An applied mechanical stress will generate a voltage)
Example: Quartz, BaTiO3, GaPO4
The piezoelectric effect describes the relation between a mechanical stress and an electrical voltage in solids.
In physics, the piezoelectric effect can be described as the link between electrostatics and mechanics.
An LED is wired to a piezoelectric transducer. The LED briefly lights when the device is flicked & shows that electricity has been generated by stress and strain.
The second generation of piezoelectric applications was developed during World War II. It was discovered that certain ceramic materials, known as ‘ferroelectrics’, showed dielectric constants up to 100 times larger than common-cut crystals and exhibited similar improvement in piezoelectric properties.

Reverse Piezoelectric effect

An applied voltage will change the shape of the solid by a small amount (up to a 4% change in volume).
Quartz watches, Piezoelectric US oscillator

Application of Piezoelectric effect

In lighters or portable sparkers with a piezo fuze a sudden and strong pressure is used to produce a voltage. The spark then ignites the gas.
A piezo motor is based on the change in mechanical shape of a piezoelectric material when an tension is applied. The material produces ultrasonic or acoustic vibrations and produces a linear or rotary motion.

Shape-memory materials:

13. What are monomers and polymers? give suitable examples.

Ans: Monomers:

The word monomer comes from mono-(one) and -mer (part). Monomers are small molecules which may be joined together in a repeating fashion to form more complex molecules called polymers. Monomers form polymers by forming chemical bonds or binding supramolecularly through a process called polymerization

A monomer is a small molecular sub unit that can be combined with similar sub units to form larger molecules. In living systems, like our own bodies, these larger molecules include carbohydrates, lipids, nucleic acids and proteins. Following diagram represents a number of monomers.Example of Monomers are:- 1). Monosaccharides 2). Amino-acids 3). Nucleotides

Polymer is a large molecule made up of chains of repeating basic molecular units called monomers. Many polymers are named by their basic monomer unit with the prefix poly. Polyvinyl chloride (PVC) is the polymer of vinyl chloride.Proteins and DNA are natural polymers where amino acids are the monomer units.Example of Polymers are:-1). Polysaccharides 2). Polypeptides and proteins 3). Nucleic acids

14. Explain with a neat sketch the silicon crystal growth by the Czochralski crystal puller method.

Ans:

• To demonstrate the methods of growing semiconductors, we will consider the crystal growth of silicon in detail first.

• Basically, the technique used for silicon crystal growth from the melt is the Czochralski technique.

• This starts from a pure form of sand (SiO2), known as quartzite, which is placed in a furnace with different carbon-releasing materials, such as coal and coke.

• Several reactions then take place inside the furnace and the net reaction that results in silicon is as follows:

• The silicon so-produced is known as metallurgical-grade silicon (MGS) which contains up to 2% of impurities.

• Subsequently, the silicon is treated with hydrogen chloride to form trichlorosilane (SiHCl3):

SiHCl3 is a liquid at room temperature. Fractional distillation of the SiHCl3 removes the impurities and the purified liquid is reduced in a hydrogen atmosphere to yield electronic- grade silicon (EGS) by the following reaction:

• EGS is a polycrystalline material of remarkably high purity and is used as the raw material for preparing high quality Si wafers. The Czochralski technique employs the apparatus shown in Figure 2.1.

• To grow a crystal, the EGS is placed in the crucible and the furnace is heated above the melting temperature of silicon. An appropriately oriented seed crystal (e.g. [100]) is suspended over the crucible in a seed holder.

• The seed is then lowered into the melt. Part of it melts but the tip of the remaining seed crystal still touches the liquid surface. The seed is next gently withdrawn, and progressive freezing at the solid–liquid interface yields a large single crystal.

• Absolute control of temperatures and pull rate is required for high quality crystals. A typical pull rate is a few millimeters per minute.

15. Explain the epitaxial growth of semiconductors.

Ans: In many situations, it may not be feasible to start with a silicon substrate to build a smart system.

Instead, one could start with other possibilities and grow silicon films on the substrate by epitaxial deposition to ‘build the necessary electronics’.
The method for growing a silicon layer on a substrate wafer is known as an epitaxial process where the substrate wafer acts as the seed crystal.
Epitaxial processes are different from crystal growth from the melt in that the epitaxial layer can be grown at a temperature much lower than the melting point.
Among various epitaxial processes, vapor phase epitaxy (VPE) is the most common.
A schematic of the VPE apparatus and shows a horizontal susceptor made from graphite blocks.
The susceptor mechanically supports the wafer and being an induction-heated reactor it also serves as the source of thermal energy for the reaction.
Several silicon sources can be used, e.g. silicon tetrachloride (SiCl4), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3) and silane (SiH4).

The typical reaction temperature for silicon tetrachloride is 1200 C.
The overall reaction, in the case of silicon tetrachloride, is reduction by hydrogen, as follows:
SiCl₄(gas) + 2H₂(gas) –>Si (solid) + 4HCl
A competing reaction which occurs simultaneously is:
SiCl₄ (gas) + Si (solid) –> 2SiCl₂(gas)
silicon is deposited on the wafer, silicon is removed (etched).
Therefore, if the concentration of SiCl4 is excessive, etching rather than growth of silicon will take place.

16. Explain the different methods of polymerization techniques.

Ans: Polymerization is the process by which monomers (small molecules) are linked together to form long-chain polymers. There are two main types of polymerization techniques: addition polymerization and condensation polymerization.

1. Addition Polymerization: This technique involves the reaction of unsaturated monomers with themselves, with other monomers or with a co-monomer in the presence of a catalyst.

a. Free radical polymerization: This technique involves the use of a radical initiator to initiate the reaction of a vinyl monomer with itself, forming a polymer chain.

b. Anionic polymerization: This technique involves the use of a strong base to initiate the reaction of an anion with a monomer, forming a polymer chain.

c. Cationic polymerization: This technique involves the use of a strong acid to initiate the reaction of a cation with a monomer, forming a polymer chain.

2. Condensation Polymerization: This technique involves the reaction of two or more different monomers with the elimination of a small molecule such as water, methanol, or ammonia.

a. Polycondensation: This technique involves the reaction of a di-functional or multi functional monomer with itself or with another monomer, forming a polymer chain with the elimination of a small molecule.

b. Step-growth polymerization: This technique involves the reaction of two or more different functional groups on different monomers.

17. Explain the mechanics of UV curing of polymers.

Ans:

UV curing is based on photoinitiated polymerization, which is mediated by photo- initiators.
These absorb UV light and convert the (light) energy into chemical energy in the form of reactive intermediates, such as free radicals and reactive cations, which subsequently initiate the polymerization.
Typical photopolymer formulations contain a photo-initiator system, monomers and oligomers (or a polymer or polymers) to provide specific physical and/or processing properties.
They may also contain a variety of additives to modify the physical properties of the light-sensitive compositions or the final properties of the cured photopolymers.
The photopolymerization reactions fall into two categories, i.e. radical photopolymerization and cationic photopolymerization.
Generally, acrylates are associated with free-radical polymerization while epoxies are typical of cationic curing.
The most commonly used reactive monomeric materials are low-molecular-weight unsaturated acrylate or methacrylate monomers that can be made to cross-link with the use of a radical-generating photo-initiator.
The practical applications of cationic initiated cross-linking of monomeric materials with epoxy and/or vinyl ether functionalities have significantly increased with the development of new UV sensitive, high-efficiency photo-initiators which generate cationic species.
Once the photo-initiator (PI) absorbs light, it is raised to an electronically excited state, PI*.
The lifetimes of the PI* states are short, generally less than 106 s.

18. Explain briefly smart materials and their applications.

Examples:
A. Piezoelectric materials
B. Shape memory alloys
C. Magnetic shape memory alloys
D. Shape memory Polymers
E. PH sensitive polymers

Smart switches & actuators (NiTi-long life)
Safety device, fuse, alarms (CuZnAl-reliability)
Artificial limbs, blood vessels & muscles (SM Polyurethane -bio compatibility)
Adhesive tapes/bands (time bound adhesive property /painless removal/healing property)
Food packaging industry-wrappers (adoptability)
Smart spoons (Temperature sensitive polymers)
Smart nose & tongue (recognition characteristics)
Smart clothes (Adaptive to temperature changes)
An “animated lamp” designed by Romolo Stanco that uses shape-memory alloy to change its Shape whenever it’s turned on and off.

19. How are polymers classified on the basis of their structure?

Ans: • Polymers can be classified, based on their structure (linear, branched or cross-linked), by the method of synthesis, physical properties and by end-use.

• A linear polymer is made up of identical units arranged in a linear sequence.

• This type of polymer has only two functional groups.

• Branched polymers are those in which there are many side-chains of lined monomers attached to the main polymer chain at various points.

• These side-chains could be either short or long (Figure 2.6).

• When polymer molecules are linked with each other at points, other than their ends, to form a network, the polymers are said to be cross-linked (Figure 2.7).

• Crosslinked polymers are insoluble in all solvents, even at elevated temperatures.

• Based on their physical properties, polymers may be classified as either thermoplastic or thermoset.

• Once they are molded in to their shape, usually by applying heat and pressure, these materials become very hard.

• This process of the polymer becoming an infusible and insoluble mass is called ‘curing’.

• When a polymer is vulcanized into rubbery materials, which show good strength and elongation, it is used as an elastomer.

• Fibers are polymers drawn into long filament-like materials, whose lengths are at least 100 times their diameters.

20. Define Metal. Explain in detail metallization techniques.

Ans: Any of a class of substances characterized by high electrical and thermal conductivity as well as by malleability, ductility, and high reflectivity of light is called metal.

Metals are used in MEMS and microelectronics due to their good conductivities, both thermal and electrical.
Metals are somewhat strong and ductile at room temperature and maintain good strength, even at elevated temperatures. Hence, they could also be used to form useful structures.
While thin metal films have been used in IC chips for a long time thick metal film structures are required for some MEMS devices.
Thick metal films are generally used as structural materials in MEMS devices or as mold inserts for polymers in ceramic micro-molding.
Nickel, copper and gold can be electroplated to form these thick films, while three- dimensional stainless steel micro-parts can be fabricated by a process known as photo- forming.
Metallization is a process whereby metal films are formed on the surface of a substrate.
Metal films can be formed by using various methods, with the most important being physical vapor deposition (PVD).
The latter is performed under vacuum by using either an evaporation or sputtering technique.
In evaporation, atoms are removed from the source by thermal energy while in sputtering, the impact of gaseous ions is the cause of such removal.
In addition to several elemental metals, various alloys have also been developed for MEMS.
CoNiMn thin films have been used as permanent magnet materials for magnetic actuation.
NiFe permalloy thick films have been electroplated on silicon substrates for magnetic MEMS devices, such as micromotors, micro-actuators, microsensors and integrated power converters.
TiNi shape memory alloy (SMA) films have been sputtered onto various substrates in order to produce several well known SMA actuators.
Similarly, TbFe and SmFe thin films have also been used for magnetostrictive actuation.

21. Explain the properties and application of polymers, ceramics, and metals in MEMS applications.

Ans: Polymers, ceramics, and metals are commonly used materials in Microelectromechanical Systems (MEMS) applications due to their unique properties and suitability for various purposes. Here’s an overview of their properties and applications

Polymers:
- Properties: Polymers are organic compounds made up of long chains of repeating units. They exhibit excellent flexibility, low density, good chemical resistance, and electrical insulation properties.
- Applications: Polymers find applications in MEMS as flexible membranes, gaskets, coatings, and encapsulation materials. They are used in microfluidics devices, bioMEMS sensors, and actuators due to their compatibility with biological systems and ease of fabrication.
Ceramics:
- Properties: Ceramics are inorganic, non-metallic materials characterized by their high hardness, excellent thermal stability, low thermal expansion, and electrical insulating properties. They are brittle and have high compressive strength.
- Applications: Ceramics are commonly used in MEMS applications where high-temperature stability, chemical resistance, and electrical insulation are required. They are used in pressure sensors, accelerometers, gyroscopes, and resonators. Ceramics such as silicon dioxide (SiO2) and silicon nitride (Si3N4) are frequently employed as structural materials and protective coatings.
Metals:
- Properties: Metals possess high electrical conductivity, good thermal conductivity, high mechanical strength, and ductility. They can be easily formed, machined, and joined.
- Applications: Metals are widely used in MEMS for various applications. They are utilized in microfabricated devices such as microactuators, microcantilevers, and microvalves. Common metals used include gold (Au), aluminum (Al), copper (Cu), and nickel (Ni). Metal alloys like nickel-titanium (NiTi) shape memory alloys are employed for their unique shape-changing properties.

22. What are the components of smart systems? Explain in brief.

Ans: Components of a smart system:

Sensors: Smart systems rely on sensors to collect data from the environment. Sensors can include temperature sensors, humidity sensors, motion sensors, light sensors, or even more advanced ones like cameras or biometric sensors.
Actuators: Actuators are responsible for carrying out actions or operations based on the decisions made by the smart system. Examples of actuators include motors, valves, switches, or even robotic arms.
Connectivity: Smart systems often have connectivity features to enable communication between different components of the system or with external devices. This can include wireless technologies like Wi-Fi, Bluetooth, or Zigbee, allowing remote control and access.
User Interface: Smart systems provide a user interface for interaction and control. This can be a mobile application, a web portal, voice commands, or even touch interfaces embedded in devices.
Decision-Making: Smart systems utilize algorithms and decision-making techniques to process the collected data and generate appropriate actions or responses based on predefined rules or learning algorithms.

23. Define Ceramics and explain processing techniques.

Ans: Ceramics are non-metallic, inorganic materials that are typically made from a mixture of metal or non-metallic oxides, carbides, nitrides, or silicates.

The processing techniques used to produce ceramics

Powder synthesis: This involves the production of ceramic powders by chemical methods such as precipitation, sol-gel processing, or hydrothermal synthesis.
Powder compaction: The ceramic powder is pressed or compacted into a specific shape and size using hydraulic or mechanical presses. This process is known as dry pressing.
Slip casting: This involves mixing the ceramic powder with a liquid to form a slurry, which is then poured into a mold. The mold is slowly drained, leaving a solid ceramic shell behind.
Extrusion: This process involves forcing a ceramic paste through a die to create a specific shape, such as tubes, rods, or other complex shapes.

24. Write short notes on Electrorheological fluids

Ans: Electrorheological (ER) fluids are suspensions of extremely fine non-conducting but electrically active particles (up to 50 micrometres diameter) in an electrically insulating fluid.

The apparent viscosity of these fluids changes reversibly by an order of up to 100,000 in response to an electric field.

The main problem for the heterogeneous ER fluids is particle sedimentation, and the homogeneous ER fluids are promising in this regard, though they sometimes do not show a strong enough transmitted shear stress and have a relatively high viscosity at zero field.

Anhydrous ER fluids could work in a wide temperature range, and are thought to be superior to the hydrous ER fluids.

Three major ER effects are discussed in this chapter, including the positive ER effect, the negative ER effect and photo-ER effect.

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