Chapter 12

Classify each of the following materials as metal, semiconductor, or insulator: (a) GaN (b) B (c) \(\mathrm{ZnO}\) (d) \(\mathrm{Pb}\)

Classify each of the following materials as metal, semiconductor, or insulator: (a) InAs (b) \(\mathrm{MgO}\) (c) \(\mathrm{HgS}\) (d) Sn

State whether each statement is true or false, and why. (a) Semiconductors have a larger band gap than insulators. (b) Doping a semiconductor makes it more conductive. (c) Metals have delocalized electrons. (d) Most metal oxides are insulators.

State whether each statement is true or false, and why. (a) A typical band gap energy for an insulator is \(400 \mathrm{~kJ} / \mathrm{mol}\) (b) The conduction band is higher in energy than the valence band. (c) Electrons can conduct well if they are in a filled valence band. (d) Holes refer to empty atomic sites in a solid crystal.

For each of the following pairs of semiconductors, which one will have the larger band gap: (a) \(\mathrm{CdS}\) or \(\mathrm{CdTe}\) (b) GaN or InP (c) GaAs or InAs?

For each of the following pairs of semiconductors, which one will have the larger band gap: (a) InP or InAs (b) Ge or AlP (c) AgI or CdTe?

If you want to dope GaAs to make an n-type semiconductor with an element to replace Ga, which element(s) would you pick?

If you want to dope GaAs to make a p-type semiconductor with an element to replace As, which element(s) would you pick?

What advantages does silicon have over other semiconductors for use in integrated circuits?

Why is it important for Si crystals to be \(99.999999999 \%\) pure, as opposed to \(99 \%\) pure, for silicon chips?

What material is traditionally used to make the gate in a MOSFET transistor? What material is used in the next generation transistors?

The semiconductor GaP has a band gap of \(2.2 \mathrm{eV}\). Green LEDs are made from pure GaP. What wavelength of light would be emitted from an LED made from GaP?

The first LEDs were made from GaAs, which has a band gap of \(1.43 \mathrm{eV}\). What wavelength of light would be emitted from an LED made from GaAs? What region of the electromagnetic spectrum does this light correspond to: UV, Visible, or IR?

Metals, such as Alor Fe, and many plastics are recyclable. With the exception of many glasses, such as bottle glass, ceramic materials in general are not recyclable. What characteristics of ceramics make them less readily recyclable?

It is desirable to construct automobiles out of lightweight materials to maximize fuel economy. All of the ceramics listed in Table \(12.3\) are less dense than steel. Why do you think ceramic materials are not more widely used in the construction of automobiles?

Why is the formation of very small, uniformly sized and shaped particles important for many applications of ceramic materials?

Describe the general chemical steps in a sol-gel process, beginning with \(\mathrm{Zr}(s)\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)\). Indicate whether each step is an oxidation-reduction reaction (refer to Section 4.4), condensation reaction, or other process.

To what does the term superconductivity refer? Why might superconductive materials be of value?

What are the differences in the electrical and magnetic properties of an excellent metallic conductor of electricity (such as silver) and a superconducting substance (such as \(\mathrm{Nb}_{3} \mathrm{Sn}\) ) below its superconducting transition temperature?

(a) What is the superconducting transition temperature, \(T_{c} ?(\mathrm{~b})\) The discovery by Müller and Bednorz of superconductivity in a copper oxide ceramic at \(35 \mathrm{~K}\) set off a frantic scramble among physicists and chemists to find materials that exhibit superconductivity at higher temperatures. What is the significance of achieving \(T_{c}\) values above \(77 \mathrm{~K}\) ?

An ester is a compound formed by a condensation reaction between a carboxylic acid and an alcohol. Use the index to find the discussion of esters in Chapter 25, and give an example of a reaction forming an ester. How might this kind of reaction be extended to form a polymer (a polyester)?

Write a chemical equation for formation of a polymer via a condensation reaction from the monomers succinic acid \(\left(\mathrm{HOOCCH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\right)\) and ethylenediamine \(\left(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\right)\).

Draw the structure of the monomer(s) employed to form each of the following polymers shown in Table \(12.5\) (a) polyvinyl chloride, (b) nylon 6,6, (c) polyethylene terephthalate.

Write the chemical equation that represents the formation of (a) polychloroprene from chloroprene (Polychloroprene is used in highway-pavement seals, expansion joints, conveyor belts, and wire and cable jackets.); (b) polyacrylonitrile from acrylonitrile (Polyacrylonitrile is used in home furnishings, craft yarns, clothing, and many other items.)

Proteins are polymers formed by condensation reactions of amino acids, which have the general structure In this structure, \(R\) represents \(-\mathrm{H},-\mathrm{CH}_{3}\), or another group of atoms. Draw the general structure for a polyamino acid polymer formed by condensation polymerization of the molecule shown here.

What molecular features make a polymer flexible? Explain how cross-linking affects the chemical and physical properties of the polymer.

What molecular structural features cause high-density polyethylene to be denser than low-density polyethylene?

Are high molecular masses and a high degree of crystallinity always desirable properties of a polymer? Explain.

Briefly describe each of the following: (a) elastomer, (b) thermoplastic, (c) thermosetting plastic, (d) plasticizer.

Neoprene is a polymer of chlorobu tadiene. The polymer can be used to form flexible tubing that is resistant to chemical attack from a variety of chemical reagents. Suppose it is proposed to use neoprene tubing as a coating for the wires running to the heart from an implanted pacemaker. What questions would you ask to determine whether it might be suitable for such an application?

Patients who receive vascular grafts formed from polymer material such as Dacron \(^{\otimes}\) are required to take anticoagulation drugs on a continuing basis to prevent blood clots. Why? What advances in such vascular implants are needed to make this precaution unnecessary?

Several years ago a biomedical company produced and marketed a new, efficient heart valve implant. It was later withdrawn from the market, however, because patients using it suffered from severe loss of red blood cells. Describe what properties of the valve could have been responsible for this result.

Skin cells from the body do not differentiate when they are simply placed in a tissue culture medium; that is, they do not organize into the structure of skin, with different layers and different cell types. What is needed to cause such differentiation to occur? Indicate the most important requirements on any material used.

In what ways are a nematic liquid crystalline phase and an ordinary liquid the same, and in what ways do they differ?

In contrast to ordinary liquids, liquid crystals are said to possess "order." What does this mean?

Describe what is occurring at the molecular level as a substance passes from the solid to the nematic liquid crystalline to the isotropic (normal) liquid phase upon heating.

Liquid crystalline phases tend to be more viscous than the isotropic, or normal, liquid phase of the same substance. Why?

The smectic liquid crystalline phase can be said to be more highly ordered than the nematic. In what sense is this true?

Describe how a cholesteric liquid crystal phase differs from a nematic phase.

CdS has a band gap of \(2.4 \mathrm{eV}\). If large crystals of \(\mathrm{CdS}\) are illuminated with ultraviolet light they emit light equal to the band gap energy. (a) What color is the emitted light? (b) Would appropriately sized CdS quantum dots be able to emit blue light? (c) What about red light?

True or false: (a) The band gap of a semiconductor decreases as the particle size decreases, in the \(1-10-\mathrm{nm}\) range. (b) The light that is emitted from a semiconductor, upon external stimulation, is longer and longer in wavelength as the particle size of the semiconductor decreases.

Gold is a face-centered cubic structure that has a unit cell edge length of \(4.08 \AA\) (Figure 11.34). How many gold atoms are there in a sphere that is \(20 \mathrm{~nm}\) in diameter? Recall that the volume of a sphere is \(\frac{4}{3} \pi r^{3}\).

Cadmium telluride, CdTe, takes the zinc blende structure (Section 11.8) with a unit cell edge length of \(6.49 \AA\). There are four cadmium atoms and four tellurium atoms per unit cell. How many of each type of atom are there in a cubic crystal with an edge length of \(5 \mathrm{~nm}\) ?

One major difference in the behavior of semiconductors and metals is that semiconductors increase their conductivity as you heat them (up to a point), but the conductivity of a metal decreases as you heat it. Suggest an explanation.

What properties of the typical nematic liquid crystalline molecule are likely to cause it to reorient when it is placed in an electrical field that is perpendicular to the direction of orientation of the molecules?

Teflon \(^{\otimes}\) is a polymer formed by the polymerization of \(\mathrm{F}_{2} \mathrm{C}=\mathrm{CF}_{2}\). Draw the structure of a section of this polymer. What type of polymerization reaction is required to form it?

Ceramics are generally brittle, subject to crack failure, and stable to high temperatures. In contrast, plastics are generally deformable under stress and have limited thermal stability. Discuss these differences in terms of the structures and bonding in the two classes of materials.

A watch with a liquid crystal display (LCD) does not function properly when it is exposed to low temperatures during a trip to Antarctica. Explain why the LCD might not function well at low temperature.

Suppose that a liquid crystalline material such as cholesteryl benzoate is warmed to well above its liquid crystalline range and then cooled. On cooling, the sample unexpectedly remains clear until it reaches a temperature just below the melting point, at which time it solidifies. What explanation can you give for this behavior?

In fabricating microelectronics circuits, a ceramic conductor such as TiSi_{2} is employed to connect various regions of a transistor with the outside world, notably aluminum wires. The TiSi is deposited as a thin film via chemical vapor deposition, in which \(\mathrm{TiCl}_{4}(g)\) and \(\mathrm{SiH}_{4}(g)\) are reacted at the Si surface. Write a balanced equation for the reaction, assuming that the other products are \(\mathrm{H}_{2}\) and \(\mathrm{HCl}\). Why might TiSi \(_{2}\) behave better as a conducting interconnect on Si than on a metal such as Cu?