ISM Question Bank

1. 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

These materials can act as sensors but not as actuators or transducers.

Smart Technologies Prospects:

• New sensing materials and devices.

• New actuation materials and devices.

• New control devices and techniques.

Self-detection, self-diagnostic, self-corrective and Self-controlled functions of smart materials/ systems.

Smart Structure: A smart structure is a system that incorporates particular functions of sensing and actuation to perform smart actions in an ingenious way. The basic five components of a smart structure are

1. Data Acquisition: the aim of this component is to collect the required raw data needed for an appropriate sensing and monitoring of the structure.

2. Data Transmission (sensory nerves): the purpose of this part is to forward the raw data to the local and/or central command and control units.

3. Command and Control Unit (brain): the role of this unit is to manage and control the whole system by analyzing the data, reaching the appropriate conclusion, and determining the actions required.

4. Data Instructions (motor nerves): the function of this part is to transmit the decisions and the associated instructions back to the members of the structure.

5. Action Devices (muscles): the purpose of this part is to take action by triggering the controlling devices/ units.

2. 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

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

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.

4. 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.

5. 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

1. 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.
2. 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.
3. 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.

6. What is a smart system? Give an example. What are the components of a smart System?

Ans: Smart systems incorporate functions of sensing, actuation, and control in order to describe and analyze a situation, and make decisions based on the available data in a predictive or adaptive manner, thereby performing smart actions. In most cases the “smartness” of the system can be attributed to autonomous operation based on closed loop control, energy efficiency, and networking capabilities.

Components of a smart system:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

7. Write short notes on electrostrictive materials.

Ans: Electrostrictive materials are a class of materials that exhibit a change in shape or size when subjected to an electric field. Similar to piezoelectric materials, electrostrictive materials undergo a mechanical deformation in response to an applied electric field. However, in contrast to piezoelectric materials, the deformation in electrostrictive materials is primarily due to the reorientation of electric dipoles within the material, rather than the displacement of charges.

8. 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.

9. Write short notes 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.

10. Explain with a neat sketch about metal, polymer and ceramics.

Ans:

Metals, polymers, and ceramics are three broad categories of materials that have different structures and properties.

1. Metals: Metals are typically dense, ductile, and good conductors of heat and electricity. They have metallic bonding, which involves the sharing of electrons between metal atoms, resulting in a highly delocalized electron cloud. This structure gives metals their characteristic properties such as high strength, malleability, and conductivity.
2. Polymers: Polymers are large molecules made up of repeating units called monomers. They have covalent bonding between the monomers, which can be linear, branched, or cross-linked. This structure gives polymers their characteristic properties such as flexibility, low density, and ease of processing. Polymers can be either natural, such as cellulose and DNA, or synthetic, such as polyethylene and polystyrene.
3. Ceramics: Ceramics are typically hard, brittle, and have high melting points. They have ionic or covalent bonding, which results in a strong network of atoms. This structure gives ceramics their characteristic properties such as high hardness, chemical resistance, and thermal stability. Examples of ceramics include pottery, glass, and refractory materials.

11. 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.

12. Explain the mechanism 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.

13. Explain the Metal and Metallization technique used in MEMS application.

Ans:

• 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.

14. 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.

15. Write short notes on Piezoelectric materials.

Ans:

• 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.

16. 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

1. Powder synthesis: This involves the production of ceramic powders by chemical methods such as precipitation, sol-gel processing, or hydrothermal synthesis.
2. 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.
3. 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.
4. Extrusion: This process involves forcing a ceramic paste through a die to create a specific shape, such as tubes, rods, or other complex shapes.

17. Draw the neat sketch for thick film fabrication using the doctor-blade process.

Ans:

18. List out the characteristic of Magneto electric materials and Magnetorheological fluids.

Ans:

Magneto-electric materials:

1. Magneto-electric materials exhibit a coupling between their magnetic and electric properties. This means that an applied magnetic field can induce an electric polarization, and vice versa.
2. They are often composed of materials that are ferromagnetic and ferroelectric in nature, such as bismuth ferrite or lead zirconate titanate.
3. The magneto-electric effect is often weak in these materials, requiring a strong magnetic field to induce a noticeable electric polarization.
4. They have potential applications in various fields, including sensors, data storage, and energy conversion.

Magnetorheological fluids:

1. Magnetorheological fluids are composed of a carrier fluid, such as oil or water, and small magnetic particles, typically on the order of micrometers in size.
2. They exhibit a change in viscosity in response to an applied magnetic field. This means that their flow properties can be controlled using a magnetic field.
3. They can be used in various applications, including vibration damping, shock absorption, and robotics.
4. The strength of the magnetic field required to induce a noticeable change in viscosity depends on the concentration and properties of the magnetic particles in the fluid.

19. Write short notes on Shape memory materials.

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

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

B. Other shape memory alloys include:

1. Ni-Ti (~55% Ni)
2. Ag-Cd 44/49 at. % Cd
3. Au-Cd 46.5/50 at. % Cd
4. Fe-Pt approx. 25 at. % Pt
5. Mn-Cu 5/35 at. % Cu
6. Fe-Mn-Si
7. Pt alloys
8. Co-Ni-Al
9. Co-Ni-Ga
10. Ni-Fe-Ga

20. Write a short note on Inverted cylindrical magnetron (ICM) RF sputtering.

Ans:

• This consists of a water-cooled copper cathode, which houses the hollow cylindrical BST target, surrounded by a ring magnet concentric with the target.
• A stainless steel thermal shield is mounted to shield the magnet from the thermal radiation coming from the heated table.
• The anode is recessed in the hollow-cathode space.
• The latter aids in collecting electrons and negative ions, hence minimizing ‘re-sputtering’ the growing film.
• Outside the deposition chamber, a copper ground wire is attached between the anode and the stainless steel chamber.
• A DC bias voltage could be applied to the anode to alter the plasma characteristics in the cathode/anode space.
• The sputter gas enters the cathode region through the space surrounding the table.

• By using the above set-up, Cukauskas et al. were able to deposit BST films at temperatures ranging from 550 to 800 C.
• The substrate temperature was maintained by two quartz lamps, a type-K thermocouple and a temperature controller.
• The films were deposited at 135W to a film thickness of 7000A and cooled to room temperature at 1 atm of oxygen before removing them from the deposition unit.
• This was then followed by annealing the films in 1 atm of flowing oxygen at a temperature of 780 C for 8 h in a tube furnace.