(c) Parametric Design
Parametric design involves finding values for the features that characterize the object being studied.
Consider a simple example –
We want to design a cylindrical storage tank that must hold 4 m3 of liquid.
The volume is given by
V = r2 l
The tank is described by the parameters, radius 'r', and length l.
Given V = 4 m3 = r2 l
r2 l = 1.273
We can see a number of values for the radius and length, that will satisfy this equation। Each combination-values of r and l gives a possible solution for the design problem.
(d) Original Design
As described in an earlier section, an original design in the development of an assembly or component that did not exist before।
(e) Redesign
The redesign is a modification of an existing product to meet new requirements। It is same as adaptive design. Most design problems solved in industry are for the redesign of an existing product. Suppose a manufacturer of hydraulic cylinders makes a product that is 0.25m long. If the customer needs a cylinder 0.3m long, the manufacturer might lengthen the outer cylinder and the piston rod to meet this special need.
2.3. On the basis of the objective or strategy the designs are of following main types.
A. Production Design
B. Functional Design
C. Optimum डिजाईन
A. Production Design
In production design, the designer designs something in such a way that the cost of producing the product is minimum। That is, the first responsibility of the designer is reduction of production cost. Hence, a production designer is concerned with the ease with which something can be produced, and that at a minimum cost.
B. Functional Design W93
In functional design, the aim is at designing a part or member so as to meet the expected performance level.
Functional design is a way of achieving given requirements।- but the same may the unproducible or costly to produce. A good designer, then, has to consider the production aspects also. A product designed without keeping all these aspects into account, wastes time, money and efforts.
C। Optimum Design [W 95]
It is the best design for given objective function, under the specified constraints।
2.4 On the basis of the field/ area or the domain of design, the following types are important.
1. Mechanical Design
2. Machine Design
3. System Design
4. Assembly/sub-assembly design
5. Computer aided डिजाईन
1. Mechanical Design
It means use of scientific principles, technical information and imagination in the design of a structure,or machine to perform prescribed functions with maximum economy and efficiency।
2. Machine Design
It is the process of achieving a plan for the construction of a machine।
3. System Design
System Design is an iterative decision making process to conceive and implement optimum systems, to solve problems and needs of society।
4. Assembly/sub-assembly design [S 93]
In the design of Assembly/sub-assembly the major criterion is the fulfillment of functional requirements. The assembly has to be designed to meet broad technical parameters and purpose for which it was meant.
The characteristic features are:
¬ The total number of parts used in the design must be minimum.
¬ Sub-assemblies should be capable of being built separately in order to give maximum manufacturing flexibility.
¬ Standard parts may be used.
¬ Flexible parts should be avoided, as they are easily damaged during handling and assembly।
5. Computer aided design [CAD]
It is a design methodology in which the designs take the advantages of digital computer to draw concepts, analyze and evaluate data etc. Computers are largely used in a design office for simulation and prototype study. In modern design, computers have become an indispensable tool.
Other types of designs are
Probabilistic Design
Industrial Design
Probabilistic Design [S 96]
It is a design approach in which design decisions are made using statistical tools. Generally, the external load acting on a body, the properties of materials etc are liable to vary. In probabilistic design, the designer takes into account the variations of such parameters.
Industrial Design [W 93]It is the design made by considering aesthetes, ergonomics and production aspects
Tuesday, April 8, 2008
CHAPTER 2
TYPES OF DESIGNS
2.1 The design can be classified in many ways. On the basis of knowledge, skill and creativity required in the designing process, the designs are broadly classified into three types
(i) Adaptive Design (W 95, 97, 98 ‘00)
(ii) Variant Design (S 97, 99)
(iii) Original Design
(i) Adaptive Design
In most design situations the designer’s job is to make a slight modification of the existing design। These are called adaptive designs. This type of design needs no special knowledge or skill. E.g. converting mechanical watches into a new shape.
(ii) Variant Design
This type of design demands considerable scientific training and design ability, in order to modify the existing designs into a new idea, by adopting a new material or a different method of manufacture. In this case, though the designer starts from the existing designs, the final product may be entirely different from the original product.
E।g. converting mechanical watches into quartz watches. Here a new technology is adopted.
(iii) Original Design
Here the designer designs something that did not exist previously। Thus, it is also called new design or innovative design. For making original designs, a lot of research work, knowledge and creativity are essential. A company thinks of new design when there is a new technology available or when there is enough market push. Since this type of design demands maximum creativity from the part of the designer, these are also called creative designs.
2.2 On the basis of the nature of design problem, design may be classified as
(a) Selection design
(b) Configuration design
(c) Parametric design
(d) Original design
(e) Re-design
(a) Selection Design.
It involves choosing one or more items from a list of similar items. We do this by using catalogues.
Eg. -Selection of a bearing from a bearing catalogue
-Selection of a fan for cooling equipment
-Selecting a shaft।
(b) Configuration / Layout / Packaging Design (W 97, S'02)
In this type of problem, all the components have been designed and the problem is how to assemble them into the completed product. This type of design is similar to arranging furniture in a living room.
Consider the packing of electronic components in a laptop computer. A laptop computer has a keyboard, power supply, a main circuit board, a hard disk drive, a floppy disk drive and room for two extension boards. Each component is of known design and has certain constraints on its position. For example, the extension slots must be adjacent to the main circuit board and the keyboard must be in front of the machine.
The different components are shown above. The designer’s aim is to find, how to fit all the components in a case? Where do we put what? One method for solving such problems is to – select a component randomly from the list and position it in the case so that all the constraints on that component are met.
Let's take keyboard first. It is placed in the front. Then we select and place a second component. This procedure is continued until we reach a conflict, or all the components are in the case. If a conflict arises, we back up and try again. Two potential configurations are shown above.
2.1 The design can be classified in many ways. On the basis of knowledge, skill and creativity required in the designing process, the designs are broadly classified into three types
(i) Adaptive Design (W 95, 97, 98 ‘00)
(ii) Variant Design (S 97, 99)
(iii) Original Design
(i) Adaptive Design
In most design situations the designer’s job is to make a slight modification of the existing design। These are called adaptive designs. This type of design needs no special knowledge or skill. E.g. converting mechanical watches into a new shape.
(ii) Variant Design
This type of design demands considerable scientific training and design ability, in order to modify the existing designs into a new idea, by adopting a new material or a different method of manufacture. In this case, though the designer starts from the existing designs, the final product may be entirely different from the original product.
E।g. converting mechanical watches into quartz watches. Here a new technology is adopted.
(iii) Original Design
Here the designer designs something that did not exist previously। Thus, it is also called new design or innovative design. For making original designs, a lot of research work, knowledge and creativity are essential. A company thinks of new design when there is a new technology available or when there is enough market push. Since this type of design demands maximum creativity from the part of the designer, these are also called creative designs.
2.2 On the basis of the nature of design problem, design may be classified as
(a) Selection design
(b) Configuration design
(c) Parametric design
(d) Original design
(e) Re-design
(a) Selection Design.
It involves choosing one or more items from a list of similar items. We do this by using catalogues.
Eg. -Selection of a bearing from a bearing catalogue
-Selection of a fan for cooling equipment
-Selecting a shaft।
(b) Configuration / Layout / Packaging Design (W 97, S'02)
In this type of problem, all the components have been designed and the problem is how to assemble them into the completed product. This type of design is similar to arranging furniture in a living room.
Consider the packing of electronic components in a laptop computer. A laptop computer has a keyboard, power supply, a main circuit board, a hard disk drive, a floppy disk drive and room for two extension boards. Each component is of known design and has certain constraints on its position. For example, the extension slots must be adjacent to the main circuit board and the keyboard must be in front of the machine.
The different components are shown above. The designer’s aim is to find, how to fit all the components in a case? Where do we put what? One method for solving such problems is to – select a component randomly from the list and position it in the case so that all the constraints on that component are met.
Let's take keyboard first. It is placed in the front. Then we select and place a second component. This procedure is continued until we reach a conflict, or all the components are in the case. If a conflict arises, we back up and try again. Two potential configurations are shown above.
Tuesday, February 26, 2008
REMAINIG PART OF CH-1
1.4 Definition of Design: (S ‘94)
Designing is such a vast field that it is defined in several ways. Various definitions of designing as pronounced by well-known designers are
“Design is that which defines solutions to problem which have previously been solved in a different way”
“Design is the conscious human process of planning physical things that display a new form in response to some pre-determined need”.
“Design is an act of collecting all pertinent information for the production of goods and services to meet some human need”.
The design of any component includes two things,
(i) Product design
(ii) Process design
The product design involves the development of specification for a product that will be functionally sound, good in appearance, and will give satisfactory performance for an adequate life.
The process design involves developing methods of manufacture of the products so that the component can be produced at a reasonably low cost.
1.5 History of Design Process
(i) Design by Single Person
(ii) Over-the-wall design
(iii) Simultaneous Engineering
(iv) Concurrent Engineering
(v) Integrated design and Manufacture.
In olden times one person could design and manufacture an entire product. Even for a large project such as the design of a ship or a bridge, one person had sufficient knowledge of the Physics, Materials and manufacturing processes to manage all aspects of the design and construction of the project. This period is referred to as the period of design by single person in the history of design.
By the middle of the 20th century products and manufacturing processes became so complex that, one person could not handle all aspects of design and manufacturing. This situation led to over-the-wall design process.
In this method each functional departments were separated from others, as shown by wall. There was only one-way communications between Customer, Marketing, Engg. Design and production department. The customers ‘throw’ their needs to marketing department. The marketing department may throw the customer needs to the design department, in many instances, orally. The Engg. Design department may conceive a design and hands it over to the manufacturing sections. The manufacturing department interprets that design and makes the product according to what they think suitable. Unfortunately, often what is manufactured by a company using over-the-wall process is not what the customers had in mind. This is due to lack of interaction between the different departments. Thus, this single direction over-the-wall approach is inefficient and costly and may result in poor quality products.
By the early 1980’s the concept of simultaneous engineering emerged. This philosophy emphasized simultaneous development of the manufacturing process- the goal was the simultaneous development of the product and the manufacturing process. This was accomplished by assigning manufacturing representatives to be members of design team, so that they could interact with the design engineers throughout the designs process.
In the 1980’s the simultaneous design philosophy was broadened and called concurrent engineering. A short definition of concurrent engineering is the simultaneous progression of all aspects, at all stages of product development, product specification, design, process and equipment etc. In concurrent engineering the primary focus is on the integration of teams of people having a stake in the product, design tools, and techniques and information about the product and the processes used to develop and manufacture it. Tools and techniques connect the teams with the information. Although many of the tools are computer-based, much design work is still done with pencil and paper. In fact, concurrent engineering is 80% company culture and 20% computer support.
With the advent of computer technology, drastic changes have taken place in the field of design and manufacturing. The result was a completely integrated design and manufacturing system. This system makes a good use of technologies such as CAD/CAM, FMS etc. The computer integrated manufacturing systems (CIMS) moves towards the ‘Factory of the future’. CIMS is necessary for better quality, efficiency and productivity.
Designing is such a vast field that it is defined in several ways. Various definitions of designing as pronounced by well-known designers are
“Design is that which defines solutions to problem which have previously been solved in a different way”
“Design is the conscious human process of planning physical things that display a new form in response to some pre-determined need”.
“Design is an act of collecting all pertinent information for the production of goods and services to meet some human need”.
The design of any component includes two things,
(i) Product design
(ii) Process design
The product design involves the development of specification for a product that will be functionally sound, good in appearance, and will give satisfactory performance for an adequate life.
The process design involves developing methods of manufacture of the products so that the component can be produced at a reasonably low cost.
1.5 History of Design Process
(i) Design by Single Person
(ii) Over-the-wall design
(iii) Simultaneous Engineering
(iv) Concurrent Engineering
(v) Integrated design and Manufacture.
In olden times one person could design and manufacture an entire product. Even for a large project such as the design of a ship or a bridge, one person had sufficient knowledge of the Physics, Materials and manufacturing processes to manage all aspects of the design and construction of the project. This period is referred to as the period of design by single person in the history of design.
By the middle of the 20th century products and manufacturing processes became so complex that, one person could not handle all aspects of design and manufacturing. This situation led to over-the-wall design process.
In this method each functional departments were separated from others, as shown by wall. There was only one-way communications between Customer, Marketing, Engg. Design and production department. The customers ‘throw’ their needs to marketing department. The marketing department may throw the customer needs to the design department, in many instances, orally. The Engg. Design department may conceive a design and hands it over to the manufacturing sections. The manufacturing department interprets that design and makes the product according to what they think suitable. Unfortunately, often what is manufactured by a company using over-the-wall process is not what the customers had in mind. This is due to lack of interaction between the different departments. Thus, this single direction over-the-wall approach is inefficient and costly and may result in poor quality products.
By the early 1980’s the concept of simultaneous engineering emerged. This philosophy emphasized simultaneous development of the manufacturing process- the goal was the simultaneous development of the product and the manufacturing process. This was accomplished by assigning manufacturing representatives to be members of design team, so that they could interact with the design engineers throughout the designs process.
In the 1980’s the simultaneous design philosophy was broadened and called concurrent engineering. A short definition of concurrent engineering is the simultaneous progression of all aspects, at all stages of product development, product specification, design, process and equipment etc. In concurrent engineering the primary focus is on the integration of teams of people having a stake in the product, design tools, and techniques and information about the product and the processes used to develop and manufacture it. Tools and techniques connect the teams with the information. Although many of the tools are computer-based, much design work is still done with pencil and paper. In fact, concurrent engineering is 80% company culture and 20% computer support.
With the advent of computer technology, drastic changes have taken place in the field of design and manufacturing. The result was a completely integrated design and manufacturing system. This system makes a good use of technologies such as CAD/CAM, FMS etc. The computer integrated manufacturing systems (CIMS) moves towards the ‘Factory of the future’. CIMS is necessary for better quality, efficiency and productivity.
CHAPTER 1 DEFINITION OF ENGINEERING DESIGN
Introduction:
The economic future of India depends on our ability to design, make and sell competitive products. Excellent design and effective manufacture are the pre-requisites of a successive industry. There is a general impression that the quality of Indian products can still be improved. The fact that consumers have lost their confidence on Indian-made products cannot be denied. This problem can be solved only by designing and manufacturing better products through improved methodology. Keeping this in view, the subject “Design and manufacturing” purpose to present the methods and procedures of design and manufacture.
Although engineers are not the only people who design things, the professional practice of engineering is largely concerned with design. It is usually said that design is the essence of engineering.
The ability to design is both a science and an art. The science can be learned through procedures developed by eminent scholars. But the art can be learned only by doing desi
2. Shopping goods
These are expensive and people buy it less frequently.
E.g. Jewellary garments etc.
3. Specialty goods
These are purchased, taking extra pain.
E.g. Rare objects like stamps.
4. Industrial goods.
These are items used in the production of other items.
Eg. Raw materials.
Another way of classifying products is into,
(a) Continuous Products, and
(b) Discrete products
The continuous products are those which are produced in a continuous fashion. For example, plates, sheets, tubes and bars etc are produced in very long lengths, and then these are cut into desired lengths.
On the other hand, discrete products are produced one after another, each in separate units.
On the basis of the output product, the Industry is usually named as continuous industry and discrete industry.
1.3 Requirements in a good product
1. Customer Satisfaction
2. Profit
How to achieve customer satisfaction?
-The product should function properly.
-It must have desired accuracy
-It must have desired reliability
-It must be easy to operate
-It must be serviceable
-It must make minimum space utilization
-It must withstand rough handling
-Pleasant appearances.
-Reasonable price.
How can it be profitable?
-It must be easy to manufacture
-The raw material must be cheap and easily available
-The manufacturing process has to the decided on the basis of quantity to be produced
-It must use standard parts
-It must be easy to pack and distribute.
The economic future of India depends on our ability to design, make and sell competitive products. Excellent design and effective manufacture are the pre-requisites of a successive industry. There is a general impression that the quality of Indian products can still be improved. The fact that consumers have lost their confidence on Indian-made products cannot be denied. This problem can be solved only by designing and manufacturing better products through improved methodology. Keeping this in view, the subject “Design and manufacturing” purpose to present the methods and procedures of design and manufacture.
Although engineers are not the only people who design things, the professional practice of engineering is largely concerned with design. It is usually said that design is the essence of engineering.
The ability to design is both a science and an art. The science can be learned through procedures developed by eminent scholars. But the art can be learned only by doing desi
Types of Products
A product is the tangible end result of a manufacturing process and is meant for satisfying human needs. The product can be classified as follows: -
1. Convenience goods
These are less expensive and are clustered around shops and restaurants.
A product is the tangible end result of a manufacturing process and is meant for satisfying human needs. The product can be classified as follows: -
1. Convenience goods
These are less expensive and are clustered around shops and restaurants.
These can be purchased at consumer’s convenience.
E.g. Cigarette, Candy, Magazines etc.
E.g. Cigarette, Candy, Magazines etc.
2. Shopping goods
These are expensive and people buy it less frequently.
E.g. Jewellary garments etc.
3. Specialty goods
These are purchased, taking extra pain.
E.g. Rare objects like stamps.
4. Industrial goods.
These are items used in the production of other items.
Eg. Raw materials.
Another way of classifying products is into,
(a) Continuous Products, and
(b) Discrete products
The continuous products are those which are produced in a continuous fashion. For example, plates, sheets, tubes and bars etc are produced in very long lengths, and then these are cut into desired lengths.
On the other hand, discrete products are produced one after another, each in separate units.
On the basis of the output product, the Industry is usually named as continuous industry and discrete industry.
1.3 Requirements in a good product
1. Customer Satisfaction
2. Profit
How to achieve customer satisfaction?
-The product should function properly.
-It must have desired accuracy
-It must have desired reliability
-It must be easy to operate
-It must be serviceable
-It must make minimum space utilization
-It must withstand rough handling
-Pleasant appearances.
-Reasonable price.
How can it be profitable?
-It must be easy to manufacture
-The raw material must be cheap and easily available
-The manufacturing process has to the decided on the basis of quantity to be produced
-It must use standard parts
-It must be easy to pack and distribute.
SYLLABUS OF FUNDAMENTALS OF DESIGN AND MANUFACTURING
Group A
Engineering design process and its structure. Identification and analysis of need, product design specifications, standards of performance and constraints.
Searching for design concepts; morphological analysis, brainstorming. Evaluation of design concepts for physical reliability, economic feasibility and utility.
Detailed design; design for manufacture, assembly, shipping, maintenance, use, and recyclability.
Design checks for clarity, simplicity, modularity and safety. Standardization and size ranges.
Reliability and robust design. Design organisation and communication, . technical reports, drawings, presentations and models.
Concept of manufacturing; classification of manufacturing processes. Fundamentals of casting. Basic understanding of commonly used casting processes (sand casting, investment casting and permanent mould casting processes).
Fundamentals of metal forming; hot and cold working; basic understanding of primary metal forming processes (rolling, forging, extrusion and drawing processes, punching and blanking).
Fundamentals of metal cutting; tool-work interaction for production of machined surfaces. Classification of machining processes. Basic machining operations (turning, shaping, planning, drilling and milling processes).
Group B
Fundamentals of grinding and finishing; overview of unconventional machining processes; fundamentals of welding processes; introduction to primary welding and allied processes; selection of manufacturing processes. Design for manufacturability.
Need for integration-commercial, economic and technological perspective; basic tools of integration; concept of a system. introduction to information technology and its elements.
Introduction to group technology; introduction to simulation and database management systems.
Elements of integration:-eontrol1ers, sensors, robots, automated machines; AGVs, AS, RS, etc.
Product and process design- for integration; design for economic manufacturing; design for manufacturing integration.
Introduction to computer aided process planning; selection of machine tools.
Engineering design process and its structure. Identification and analysis of need, product design specifications, standards of performance and constraints.
Searching for design concepts; morphological analysis, brainstorming. Evaluation of design concepts for physical reliability, economic feasibility and utility.
Detailed design; design for manufacture, assembly, shipping, maintenance, use, and recyclability.
Design checks for clarity, simplicity, modularity and safety. Standardization and size ranges.
Reliability and robust design. Design organisation and communication, . technical reports, drawings, presentations and models.
Concept of manufacturing; classification of manufacturing processes. Fundamentals of casting. Basic understanding of commonly used casting processes (sand casting, investment casting and permanent mould casting processes).
Fundamentals of metal forming; hot and cold working; basic understanding of primary metal forming processes (rolling, forging, extrusion and drawing processes, punching and blanking).
Fundamentals of metal cutting; tool-work interaction for production of machined surfaces. Classification of machining processes. Basic machining operations (turning, shaping, planning, drilling and milling processes).
Group B
Fundamentals of grinding and finishing; overview of unconventional machining processes; fundamentals of welding processes; introduction to primary welding and allied processes; selection of manufacturing processes. Design for manufacturability.
Need for integration-commercial, economic and technological perspective; basic tools of integration; concept of a system. introduction to information technology and its elements.
Introduction to group technology; introduction to simulation and database management systems.
Elements of integration:-eontrol1ers, sensors, robots, automated machines; AGVs, AS, RS, etc.
Product and process design- for integration; design for economic manufacturing; design for manufacturing integration.
Introduction to computer aided process planning; selection of machine tools.
Group B
Heat treatment . Iron-carbon system. Annealing, normalising, hardening,. critical cooling rate, hardenability, age hardening, surface hardening, tempering.
Thermal properties . High temperature materials; materials for cryogenic application, thermally insulating materials. (Specific heat, thermal conductivity, thermal expansion).
Ceramic materials and polymers . Silicon structures, polymerism . in glass, electrical properties of ceramic phases, rocks, building stones, refractories.
Polymerisation mechanism , structural properties of polymer, thermoplastics, thermosets, elastomer, resins, composites, particles and fibre reinforced composite. Composite material including nano material.
Electronic properties . Magnetism, diamagnetism, paramagnetism, ferromagnetism, magnetic energy, zone theory of solids, zones in conductors and insulators.
Heat treatment . Iron-carbon system. Annealing, normalising, hardening,. critical cooling rate, hardenability, age hardening, surface hardening, tempering.
Thermal properties . High temperature materials; materials for cryogenic application, thermally insulating materials. (Specific heat, thermal conductivity, thermal expansion).
Ceramic materials and polymers . Silicon structures, polymerism . in glass, electrical properties of ceramic phases, rocks, building stones, refractories.
Polymerisation mechanism , structural properties of polymer, thermoplastics, thermosets, elastomer, resins, composites, particles and fibre reinforced composite. Composite material including nano material.
Electronic properties . Magnetism, diamagnetism, paramagnetism, ferromagnetism, magnetic energy, zone theory of solids, zones in conductors and insulators.
Syllabus of Material Science And Engineering
Group A
Introduction to materials . Metal and alloys, ceramics, polymers and semi conducting materials-introduction and application as engineering materials.
Defects in solids . Point, line and surface defects. Diffusion in solids.
Phase diagrams . Mono-component and binary systems, non-equilibrium system, phase diagram and. application in crystalline and non-crystalline solids.
Mechanical properties . Tensile strength, yield strength, elastic and viscoelastic properties, creep, stress relaxation and impact. Fracture behaviour. Ductile fracture, Griffith theory, effect of heat treatment and temperature on properties of metals.
Deformation of metals. Elastic and plastic deformation, slip, twin, dislocation theory, critical resolved shear stress, deformation in polycrystalline materials; season cracking, Bachinger's effect, strengthening mechanics; work hardening recovery, crystallization and grain growth, cold and hot working. .
Introduction to materials . Metal and alloys, ceramics, polymers and semi conducting materials-introduction and application as engineering materials.
Defects in solids . Point, line and surface defects. Diffusion in solids.
Phase diagrams . Mono-component and binary systems, non-equilibrium system, phase diagram and. application in crystalline and non-crystalline solids.
Mechanical properties . Tensile strength, yield strength, elastic and viscoelastic properties, creep, stress relaxation and impact. Fracture behaviour. Ductile fracture, Griffith theory, effect of heat treatment and temperature on properties of metals.
Deformation of metals. Elastic and plastic deformation, slip, twin, dislocation theory, critical resolved shear stress, deformation in polycrystalline materials; season cracking, Bachinger's effect, strengthening mechanics; work hardening recovery, crystallization and grain growth, cold and hot working. .
remainig part of chapter-17
Dielectric Materials
Capacitors require dielectrics of high e that can function at high frequencies (small relaxation times). Many of the ceramics have these properties, like mica, glass, and porcelain). Polymers usually have lower .
Ferroelectricity
Ferroelectric materials are ceramics that exhibit permanent polarization in the absence of an electric field. This is due to the asymmetric location of positive and negative charges within the unit cell. Two possible arrangements of this asymmetry results in two distinct polarizations, which can be used to code "0" and "1" in ferroelectric memories. A typical ferroelectric is barium titanate, BaTiO3, where the Ti4+ is in the center of the unit cell and four O2- in the central plane can be displaced to one side or the other of this central ion
Piezoelectricity
In a piezolectric material, like quartz, an applied mechanical stress causes electric polarization by the relative displacement of anions and cations.
Capacitors require dielectrics of high e that can function at high frequencies (small relaxation times). Many of the ceramics have these properties, like mica, glass, and porcelain). Polymers usually have lower .
Ferroelectricity
Ferroelectric materials are ceramics that exhibit permanent polarization in the absence of an electric field. This is due to the asymmetric location of positive and negative charges within the unit cell. Two possible arrangements of this asymmetry results in two distinct polarizations, which can be used to code "0" and "1" in ferroelectric memories. A typical ferroelectric is barium titanate, BaTiO3, where the Ti4+ is in the center of the unit cell and four O2- in the central plane can be displaced to one side or the other of this central ion
Piezoelectricity
In a piezolectric material, like quartz, an applied mechanical stress causes electric polarization by the relative displacement of anions and cations.
remainig part of chapter 17
Conduction in Ionic Materials
In ionic materials, the band gap is too large for thermal electron promotion. Cation vacancies allow ionic motion in the direction of an applied electric field, this is referred to as ionic conduction. High temperatures produce more vacancies and higher ionic conductivity.
At low temperatures, electrical conduction in insulators is usually along the surface, due to the deposition of moisture that contains impurity ions.
Electrical Properties of Polymers
Polymers are usually good insulators but can be made to conduct by doping. Teflon is an exceptionally good insulator.
Dielectric Behavior
A dielectric is an electrical insulator that can be made to exhibit an electric dipole structure (displace the negative and positive charge so that their center of gravity is different).
Capacitance
When two parallel plates of area A, separated by a small distance l, are charged by +Q, –Q, an electric field develops between the plates
E = D/ee0
where D = Q/A. e0 is called the vacuum permittivity and e the relative permittivity, or dielectric constant (e = 1 for vacuum). In terms of the voltage between the plates, V = E l,
V = Dl/ee0 = Q l/Aee0 = Q / C
The constant C= Aee0/l is called the capacitance of the plates.
Field Vectors and Polarization
The dipole moment of a pair of positive and negative charges (+q and –q) separated at a distance d is p = qd. If an electric field is applied, the dipole tends to align so that the positive charge points in the field direction. Dipoles between the plates of a capacitor will produce an electric field that opposes the applied field. For a given applied voltage V, there will be an increase in the charge in the plates by an amount Q' so that the total charge becomes Q = Q' + Q0, where Q0 is the charge of a vacuum capacitor with the same V. With Q' = PA, the charge density becomes D = D0 E + P, where the polarization P = e0 (e–1) E .
Types of Polarization
Three types of polarization can be caused by an electric field:
Electronic polarization: the electrons in atoms are displaced relative to the nucleus.
Ionic polarization: cations and anions in an ionic crystal are displaced with respect to each other.
Orientation polarization: permanent dipoles (like H2O) are aligned.
Frequency Dependence of the Dielectric Constant
Electrons have much smaller mass than ions, so they respond more rapidly to a changing electric field. For electric field that oscillates at very high frequencies (such as light) only electronic polarization can occur. At smaller frequencies, the relative displacement of positive and negative ions can occur. Orientation of permanent dipoles, which require the rotation of a molecule can occur only if the oscillation is relatively slow (MHz range or slower). The time needed by the specific polarization to occur is called the relaxation time.
Dielectric Strength
Very high electric fields (>108 V/m) can free electrons from atoms, and accelerate them to such high energies that they can, in turn, free other electrons, in an avalanche process (or electrical discharge). This is called dielectric breakdown, and the field necessary to start the is called the dielectric strength or breakdown strength.
In ionic materials, the band gap is too large for thermal electron promotion. Cation vacancies allow ionic motion in the direction of an applied electric field, this is referred to as ionic conduction. High temperatures produce more vacancies and higher ionic conductivity.
At low temperatures, electrical conduction in insulators is usually along the surface, due to the deposition of moisture that contains impurity ions.
Electrical Properties of Polymers
Polymers are usually good insulators but can be made to conduct by doping. Teflon is an exceptionally good insulator.
Dielectric Behavior
A dielectric is an electrical insulator that can be made to exhibit an electric dipole structure (displace the negative and positive charge so that their center of gravity is different).
Capacitance
When two parallel plates of area A, separated by a small distance l, are charged by +Q, –Q, an electric field develops between the plates
E = D/ee0
where D = Q/A. e0 is called the vacuum permittivity and e the relative permittivity, or dielectric constant (e = 1 for vacuum). In terms of the voltage between the plates, V = E l,
V = Dl/ee0 = Q l/Aee0 = Q / C
The constant C= Aee0/l is called the capacitance of the plates.
Field Vectors and Polarization
The dipole moment of a pair of positive and negative charges (+q and –q) separated at a distance d is p = qd. If an electric field is applied, the dipole tends to align so that the positive charge points in the field direction. Dipoles between the plates of a capacitor will produce an electric field that opposes the applied field. For a given applied voltage V, there will be an increase in the charge in the plates by an amount Q' so that the total charge becomes Q = Q' + Q0, where Q0 is the charge of a vacuum capacitor with the same V. With Q' = PA, the charge density becomes D = D0 E + P, where the polarization P = e0 (e–1) E .
Types of Polarization
Three types of polarization can be caused by an electric field:
Electronic polarization: the electrons in atoms are displaced relative to the nucleus.
Ionic polarization: cations and anions in an ionic crystal are displaced with respect to each other.
Orientation polarization: permanent dipoles (like H2O) are aligned.
Frequency Dependence of the Dielectric Constant
Electrons have much smaller mass than ions, so they respond more rapidly to a changing electric field. For electric field that oscillates at very high frequencies (such as light) only electronic polarization can occur. At smaller frequencies, the relative displacement of positive and negative ions can occur. Orientation of permanent dipoles, which require the rotation of a molecule can occur only if the oscillation is relatively slow (MHz range or slower). The time needed by the specific polarization to occur is called the relaxation time.
Dielectric Strength
Very high electric fields (>108 V/m) can free electrons from atoms, and accelerate them to such high energies that they can, in turn, free other electrons, in an avalanche process (or electrical discharge). This is called dielectric breakdown, and the field necessary to start the is called the dielectric strength or breakdown strength.
remainig part of chapter 17
Electrical Characteristics of Commercial Alloys
The best material for electrical conduction (lower resistivity) is silver. Since it is very expensive, copper is preferred, at an only modest increase in r. To achieve low r it is necessary to remove gases occluded in the metal during fabrication. Copper is soft so, for applications where mechanical strength is important, the alloy CuBe is used, which has a nearly as good r. When weight is important one uses Al, which is half as good as Cu. Al is also more resistant to corrosion.
When high resistivity materials are needed, like in electrical heaters, especially those that operate at high temperature, nichrome (NiCr) or graphite are used.
Intrinsic Semiconduction
Semiconductors can be intrinsic or extrinsic. Intrinsic means that electrical conductivity does not depend on impurities, thus intrinsic means pure. In extrinsic semiconductors the conductivity depends on the concentration of impurities.
Conduction is by electrons and holes. In an electric field, electrons and holes move in opposite direction because they have opposite charges. The conductivity of an intrinsic semiconductor is:
s = n e me + p e mh
where p is the hole concentration and mh the hole mobility. One finds that electrons move much faster than holes:
me > mh
In an intrinsic semiconductor, a hole is produced by the promotion of each electron to the conduction band. Thus:
n = p
Thus, s = 2 n e (me + mh) (only for intrinsic semiconductors).
Extrinsic Semiconduction
Unlike intrinsic semiconductors, an extrinsic semiconductor may have different concentrations of holes and electrons. It is called p-type if p>n and n-type if n>p. They are made by doping, the addition of a very small concentration of impurity atoms. Two common methods of doping are diffusion and ion implantation.
Excess electron carriers are produced by substitutional impurities that have more valence electron per atom than the semiconductor matrix. For instance phosphorous, with 5 valence electrons, is an electron donor in Si since only 4 electrons are used to bond to the Si lattice when it substitutes for a Si atom. Thus, elements in columns V and VI of the periodic table are donors for semiconductors in the IV column, Si and Ge. The energy level of the donor state is close to the conduction band, so that the electron is promoted (ionized) easily at room temperature, leaving a hole (the ionized donor) behind. Since this hole is unlike a hole in the matrix, it does not move easily by capturing electrons from adjacent atoms. This means that the conduction occurs mainly by the donated electrons (thus n-type).
Excess holes are produced by substitutional impurities that have fewer valence electrons per atom than the matrix. This is the case of elements of group II and III in column IV semiconductors, like B in Si. The bond with the neighbors is incomplete and so they can capture or accept electrons from adjacent silicon atoms. They are called acceptors. The energy level of the acceptor is close to the valence band, so that an electron may easily hop from the valence band to complete the bond leaving a hole behind. This means that conduction occurs mainly by the holes (thus p-type).
The Temperature Variation of Conductivity and Carrier Concentration
Temperature causes electrons to be promoted to the conduction band and from donor levels, or holes to acceptor levels. The dependence of conductivity on temperature is like other thermally activated processes:
s = A exp(–Eg/2kT)
where A is a constant (the mobility varies much more slowly with temperature). Plotting ln s vs. 1/T produces a straight line of slope Eg/2k from which the band gap energy can be determined. Extrinsic semiconductors have, in addition to this dependence, one due to the thermal promotion of electrons from donor levels or holes from acceptor levels. The dependence on temperature is also exponential but it eventually saturates at high temperatures where all the donors are emptied or all the acceptors are filled.
This means that at low temperatures, extrinsic semiconductors have larger conductivity than intrinsic semiconductors. At high temperatures, both the impurity levels and valence electrons are ionized, but since the impurities are very low in number and they are exhausted, eventually the behavior is dominated by the intrinsic type of conductivity.
Semiconductor Devices
A semiconductor diode is made by the intimate junction of a p-type and an n-type semiconductor (an n-p junction). Unlike a metal, the intensity of the electrical current that passes through the material depends on the polarity of the applied voltage. If the positive side of a battery is connected to the p-side, a situation called forward bias, a large amount of current can flow since holes and electrons are pushed into the junction region, where they recombine (annihilate). If the polarity of the voltage is flipped, the diode operates under reverse bias. Holes and electrons are removed from the region of the junction, which therefore becomes depleted of carriers and behaves like an insulator. For this reason, the current is very small under reverse bias. The asymmetric current-voltage characteristics of diodes is used to convert alternating current into direct current. This is called rectification.
A p-n-p junction transistor contains two diodes back-to-back. The central region is very thin and is called the base. A small voltage applied to the base has a large effect on the current passing through the transistor, and this can be used to amplify electrical signals (Fig. 19.22). Another common device is the MOSFET transistor where a gate serves the function of the base in a junction transistor. Control of the current through the transistor is by means of the electric field induced by the gate, which is isolated electrically by an oxide layer.
The best material for electrical conduction (lower resistivity) is silver. Since it is very expensive, copper is preferred, at an only modest increase in r. To achieve low r it is necessary to remove gases occluded in the metal during fabrication. Copper is soft so, for applications where mechanical strength is important, the alloy CuBe is used, which has a nearly as good r. When weight is important one uses Al, which is half as good as Cu. Al is also more resistant to corrosion.
When high resistivity materials are needed, like in electrical heaters, especially those that operate at high temperature, nichrome (NiCr) or graphite are used.
Intrinsic Semiconduction
Semiconductors can be intrinsic or extrinsic. Intrinsic means that electrical conductivity does not depend on impurities, thus intrinsic means pure. In extrinsic semiconductors the conductivity depends on the concentration of impurities.
Conduction is by electrons and holes. In an electric field, electrons and holes move in opposite direction because they have opposite charges. The conductivity of an intrinsic semiconductor is:
s = n e me + p e mh
where p is the hole concentration and mh the hole mobility. One finds that electrons move much faster than holes:
me > mh
In an intrinsic semiconductor, a hole is produced by the promotion of each electron to the conduction band. Thus:
n = p
Thus, s = 2 n e (me + mh) (only for intrinsic semiconductors).
Extrinsic Semiconduction
Unlike intrinsic semiconductors, an extrinsic semiconductor may have different concentrations of holes and electrons. It is called p-type if p>n and n-type if n>p. They are made by doping, the addition of a very small concentration of impurity atoms. Two common methods of doping are diffusion and ion implantation.
Excess electron carriers are produced by substitutional impurities that have more valence electron per atom than the semiconductor matrix. For instance phosphorous, with 5 valence electrons, is an electron donor in Si since only 4 electrons are used to bond to the Si lattice when it substitutes for a Si atom. Thus, elements in columns V and VI of the periodic table are donors for semiconductors in the IV column, Si and Ge. The energy level of the donor state is close to the conduction band, so that the electron is promoted (ionized) easily at room temperature, leaving a hole (the ionized donor) behind. Since this hole is unlike a hole in the matrix, it does not move easily by capturing electrons from adjacent atoms. This means that the conduction occurs mainly by the donated electrons (thus n-type).
Excess holes are produced by substitutional impurities that have fewer valence electrons per atom than the matrix. This is the case of elements of group II and III in column IV semiconductors, like B in Si. The bond with the neighbors is incomplete and so they can capture or accept electrons from adjacent silicon atoms. They are called acceptors. The energy level of the acceptor is close to the valence band, so that an electron may easily hop from the valence band to complete the bond leaving a hole behind. This means that conduction occurs mainly by the holes (thus p-type).
The Temperature Variation of Conductivity and Carrier Concentration
Temperature causes electrons to be promoted to the conduction band and from donor levels, or holes to acceptor levels. The dependence of conductivity on temperature is like other thermally activated processes:
s = A exp(–Eg/2kT)
where A is a constant (the mobility varies much more slowly with temperature). Plotting ln s vs. 1/T produces a straight line of slope Eg/2k from which the band gap energy can be determined. Extrinsic semiconductors have, in addition to this dependence, one due to the thermal promotion of electrons from donor levels or holes from acceptor levels. The dependence on temperature is also exponential but it eventually saturates at high temperatures where all the donors are emptied or all the acceptors are filled.
This means that at low temperatures, extrinsic semiconductors have larger conductivity than intrinsic semiconductors. At high temperatures, both the impurity levels and valence electrons are ionized, but since the impurities are very low in number and they are exhausted, eventually the behavior is dominated by the intrinsic type of conductivity.
Semiconductor Devices
A semiconductor diode is made by the intimate junction of a p-type and an n-type semiconductor (an n-p junction). Unlike a metal, the intensity of the electrical current that passes through the material depends on the polarity of the applied voltage. If the positive side of a battery is connected to the p-side, a situation called forward bias, a large amount of current can flow since holes and electrons are pushed into the junction region, where they recombine (annihilate). If the polarity of the voltage is flipped, the diode operates under reverse bias. Holes and electrons are removed from the region of the junction, which therefore becomes depleted of carriers and behaves like an insulator. For this reason, the current is very small under reverse bias. The asymmetric current-voltage characteristics of diodes is used to convert alternating current into direct current. This is called rectification.
A p-n-p junction transistor contains two diodes back-to-back. The central region is very thin and is called the base. A small voltage applied to the base has a large effect on the current passing through the transistor, and this can be used to amplify electrical signals (Fig. 19.22). Another common device is the MOSFET transistor where a gate serves the function of the base in a junction transistor. Control of the current through the transistor is by means of the electric field induced by the gate, which is isolated electrically by an oxide layer.
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