Applications

Lead is one of the most widely used substances in the world, with applications as a pure metal, as an alloying element in other metals, as an additive in organic sub­stances, and as an additive or primary material component in ceramics and glasses. Lead, in metallic form is used in numerous applications, including lead-acid batteries, lead sheet and pipe, sheathing for electrical cable, radiation shielding, and lead shot and weights. As an alloying element, lead is used extensively in lead-tin solders for electronic packaging and other applications. Lead is also an alloying element in bronzes, steels, and aluminum alloys. As an additive in organic substances, lead is used in pigments, paints, polymers, and gasoline. The focus of the remainder of this section, however, is the use of lead in making ceramics and glasses.

The applications for lead and lead compounds, mostly oxides, as used in ceramic and glass applications can be categorized as follows:

Glasses

1. Leaded glass (“crystal”) for household products

2. Glazes and enamels for ceramic whitewares

3. High-index optical and ophthalmic glass

4. Radiation shielding glass

5. High electrical resistance glass for lamps and display technologies

6. Glass solders and sealants for glass-to-glass joining and hermetic glass-to-metal sealing

Electronic ceramics

1. Capacitor dielectrics

2. Piezoelectrics

3. Electrooptic devices

The primary reasons for adding lead to glass are to increase the refractive index of the glass, to decrease the viscosity of the glass, to increase the electrical resistivity of the glass, and to increase the X-ray absorption capability of the glass used for radiation shielding. The primary reason for using lead-based electronic ceramics is to modify the dielectric and piezoelectric properties, such as Curie point and piezoelectric coupling factor.

There are numerous glass products that contain lead. Because lead has two oxidation states (+2 and +4), the lead in glass can act as either a network former by replacing the silicon atom, or a network modifier by causing the formation of nonbridging oxygen atoms [4, 5], as shown in Fig. 1. The presence of lead breaks up the Si-O network and significantly reduces the viscosity of the glass (see Fig. 2). The working point of a high-lead glass, for instance, is reduced to approximately 850°C, compared to ~1,100°C for soda lime glass and >1,600°C for fused silica.

Leaded glass, which is used in houseware applications such as decorative glassware and vases, is commonly (and erroneously) referred to as “crystal” because it exhibits a higher index of refraction than other glasses. Representative values of the index of refraction for various glasses are listed in Table 6. This property results in the glass appearing shinier, brighter, and more colorful than a typical glassware (soda lime silica) glass. Leaded glass for these applications typically use PbO as a raw material, with content ranging from 18 to 38 wt% PbO [10]; a representative value is 24.4 wt% PbO [11].

Glazes for ceramic bodies and porcelain enamels for metallic substrates are coatings that are applied to these surfaces with a variety of purposes: chemical inertness, zero permeability to liquids and gases, cleanability, smoothness and resistance to abrasion and scratching, mechanical strength, and decorative and aesthetic considerations [12].

Applications

Fig. 1 Lead in glass, acting as either a network former or network modifier [6]

Table 6 Refractive indices of various glasses [8,9]

Glass composition

Average refractive index

Silica glass, SiO2

1.458

Vycor glass (96% SiO2)

1.458

Soda lime silica glass

1.51-1.52

Borosilicate glass (Pyrex ™)

1.47

Dense flint optical glasses

1.6-1.7

Lead silicate glasses

2.126

Подпись: Fig. 2 Viscosity versus temperature characteristics for various glass compositions [7]. 1, Fused silica; 2, 96% silica; 3, soda lime (plate glass); 4, lead silicate (electrical); 5, high-lead; 6, borosilicate (low expansion); 10. aluminosilicate

These coatings are applied to numerous products including china, vases, sinks, toilets, and washing machines. Lead, in the form of litharge (PbO), is often added to glazes, but not usually to enamels, because it reduces the viscosity of the glass, which in turn provides a smoother, more corrosion-resistant surface. The higher index of refraction that results is desirable for these applications. Lead-containing glazes typically have a composition between 16 and 35wt% PbO [13-15].

Optical glass includes a wide variety of applications. Of these, lead oxide is most often incorporated into optical flints, although it might also be added to optical crown glass, ophthalmic glass (crown or flint), and optical filter glass [16]. For example, products in which the presence of lead is valued include Cerenkov counters, magne­tooptical switches and shutters, and the cores of fiberoptic faceplates [17]. One of the reasons why lead is added to optical glass is it creates a high index of refraction, which can facilitate total internal reflection. The lead content of optic glasses varies considerably:
6-65 wt% PbO in optical flint glass, 4 wt% PbO in optical crown glass, and 6-51 wt% in ophthalmic glass [4,16].

Radiation shielding glass is used in television and computer monitors that contain cathode ray tubes (CRTs) because CRTs generate X-rays [1]. Exposure of the viewer to these X-rays is undesirable and limited by US Federal Standard Public Law 90-602 (Radiation Control for Health and Safety Act, 1968). X-ray absorption by a given material is dependent upon the wavelength of the radiation and the density, thickness, and atomic number of the material. Because a lead-free glass might exhibit a linear absorption coefficient as low as 8.0 cm-1 [18], lead is often added to CRT glass to provide the required X-ray shielding. The primary glass components of the CRT include the panel (or faceplate), the funnel, and the neck. Representative lead compositions and linear absorp­tion coefficients, for the corresponding glass components, are shown in Table 7.

As a result of the large ionic size of the Pb2+ ion (0.132 nm), the electrical resistivity of leaded glass is orders of magnitude higher than that of lead-free, soda lime glass (direct-current (DC) resistivity at 250°C: 1085 and 1065 ohm-cm, respectively [19]) [20,21]. This characteristic of leaded glass is a primary reason why it is used for the stem and exhaust tube in many light fixtures: incandescent, fluorescent, and high – pressure mercury fixtures, as well as for hermetic seals in electronic devices. A typical composition for leaded glass in lamps is 20-22 wt% PbO [19,22].

As discussed earlier, the presence of lead in glass results in a significant change in its viscosity characteristics (see Fig. 2). Although this is true for all of the lead-containing glasses discussed earlier, it is of particular significance for the applications of glass sol­ders (for joining glass to glass) and sealants (for joining glass to metal), which are almost always made from high-lead glasses. For instance, leaded glass is used to join the panel of a CRT to the funnel, to seal electronic packages, to bond the recording head, and to seal the panel on a flat panel display. Compositions for these high-lead glasses range from 56-77 wt% PbO, with the higher values being more common [4,18,23].

Typical PbO content for the lead-containing glass products described earlier are summarized in Table 8.

Crystalline, lead-containing ceramics generally fall within the category of materi­als called PZTs/PLZTs, which are lead-(lanthanum) zirconate titanates. These materials

Table 7 PbO content and linear absorption coefficient requirements in CRT components for color monitors [18]

Component

PbO content (wt%)

Linear absorption coefficient (cm-1)

Panel

2.2

28.0

Funnel

23.0

62.0

Neck

28.0

90.0

Table 8 Summary of lead oxide content in various glass products

Product

PbO content (wt%)

Leaded glass (“crystal”) for household products

18-38

Glazes and enamels for ceramic whitewares

16-35

High-index optical and ophthalmic glass

4-65

Radiation shielding glass

2-28

High electrical resistance glass

20-22

Glass solders and sealants

56-77

are ferroelectrics, with the perovskite (CaTiO3) crystal structure and unusual dielectric properties [24,25]. They are used in capacitor dielectrics, piezoelectrics, and elec­trooptic devices [26-28]. For capacitor applications, important properties include dielectric constant, capacitance deviation, and maximum dissipation factor. Lead – based compositions for capacitor dielectrics include lead titanate, lead magnesium niobate, lead zinc niobate, and lead iron niobate-lead iron tungstate. For piezoelectric applications such as sensors and actuators, important properties include electromechanical coupling factors, piezoelectric constants, permittivity, loss tangent, elastic constants, density, mechanical quality factor, and Curie temperature. Lead-based compositions for piezoelectrics generally fall into the PZT category [Pb(Zr, Ti)O3], although proprietary compositions generally include dopants such as Mg, Nb, Co, Ni, Mo, W, Mn, Sb, and Sn. For electrooptic applications, important properties include optical transmittance and haze; linear electrooptic effect coefficient (rc), second-order (or quadratic) electrooptic effect coefficient (R), and half-wave voltage; dielectric constant, ferroelectric hysteresis loop characteristics, and piezoelectric coupling constants; and microstructure, grain size, and porosity. The general composition for electrooptic devices is PLZT: Pb1_xLax(Zr^Ti1_^)1_i/4O3. The compositions for all three of these product applications represent a lead content on the order of 55-70 wt%.