Lead salt detectors

Chemical bath deposition is a technique in which thin semiconductor films are deposited on substrates immersed in dilute solutions containing metal ions and a source of hydroxide, telluride, sulfide, selenide, etc., ions. One of the first chemically deposited semiconductors, reported in 1869, was a PbS thin film [26]. During the ensuing 140 years, CBD has been used to deposit films of metal sulfides, selenides, and oxides, and various other compounds. While it is a well-known technique in a few specific areas (notably photoconductive lead salt detectors, photoelectrodes, and, more recently, thin-film solar cells), it is by and large an under-appreciated technique. 

Lead Salt Detectors The lead salt detectors PbS and PbSe are IV-VI compounds, and were among the earliest photon detectors. They are grown by precipitation onto a substrate that has pre-delineated contacts. Some representative spectral values for the lead salt detectors are provided in Figure 5.1 and Table 5.2. 

All practical lead salt detectors to date are PC, although at least one PV nano-crystal structure has been reported - see Choi et al. (2009). 

Desired Spectral Rejection. The near-zero transmittance of Ge and Si in the visible and ultraviolet can be used to advantage to protect InSb and lead salt detectors from undesirable effects of exposure to UV. 

Fig. 9. Spectral sensitivity of detectors where the detector temperatures in K are in parentheses, and the dashed line represents the theoretical limit at 300 K for a 180° field of view, (a) Detectors from near uv to short wavelength infrared (b) lead salt family of detectors and platinum siUcide (c) detectors used for detection in the mid- and long wavelength infrared. The Hg CdTe, InSb, and PbSnTe operate intrinsically, the doped siUcon is photoconductive, and the GaAs/AlGaAs is a stmctured supedattice and (d) extrinsic germanium detectors showing the six most popular dopants.

Detector elements are prepared either by sublimation in the presence of a small partial pressure of O2 or by chemical deposition from alkaline solution containing a lead salt and thiourea or selenourea (63). Lead sulfide and lead selenide deposit from solutions as mirror-like coatings made up of cubic crystallites 0.2—1 p.m on a side. The reaction may nominally be represented by the following ... 

Infrared detectors -lead sulfide m [LEAD COMPOUNDS - LEAD SALTS] (Vol 15) -mercury compounds m [MERCURY COMPOUNDS] (Vol 16) -metal tellundes as [TELLURIUM AND TELLURIUM COMPOUNDS] (Vol 23)... 

Potentiometric detectors typically measure the potential difference (A ) across a membrane, which originates from the difference in analyte concentration in the eluent versus an internal reference solution. The most common potentiometric measuring device is a pH electrode, in which a glass membrane responds to hydronium ion concentration in the test solution. Other ion-selective, or indicator, electrodes are also available commercially. The attribute of an indicator electrode to be highly selective for a particular species is also its drawback, in that a different electrode is needed for each type of ion. Halides and sulfates can be monitored using silver/silver salt and lead/lead salt electrodes, respectively [50]. 

NIR analyzers do not use flammable diluents, reagents, or carrier gases, but the light source, which in most instruments is an incandescent tungsten-halogen lamp operated at high temperature. The lead-salt photoconductive NIR detectors require up to 200 VDC, which represents electrical spark hazard if the instrument enclosure is not designed appropriately. 

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

Operation of detectors with their associated alarm panels should be checked and calibrated after installation. Detector performance can be impaired in a hostile environment by blockages to the detector (i.e., ice, salt crystals, wind blown particles, water or even fire fighting foam, or by inhibition of the catalysts by airborne contaminants such as compounds of silicon, phosphorus, chlorine or lead. It is essential that detectors and alarm panels be checked and re-calibrated on a routine basis. 

From Table 3.2, it can be found that most mobile phases are acidified with acetic acid, formic acid, or phosphoric acid. The pH of the mobile phase is 2 to 4 during ruiming. Although 2-propanol is used as a mobile phase modifier, the amount added should be below 2% due to the high viscosity of 2-propanol, which can lead to high pressure in the system. Inorganic mobile phase modifiers, and ammonium and phosphate salts, are normally used in the HPLC method with an electrochemical detector to increase the detector sensitivity. Also, formic acid is better than acetic acid to be used as a mobile phase modifier in an HPLC system with an MS detector. 

The adoption of an eluent having a very low conductivity, which can be passed directly through the conductometric detector. Typical eluents used are benzoate, phthalate, or other aromatic acid salts, with low limiting equivalent conductances (leading to direct detection) or potassium hydroxide eluent, with high conduc-... 

The potentiometric detector operates on the same principles as ion-selective electrodes. An indicating electrode measures a change in the potential in the presence of certain sample ions. Schmuckler et al. [47] explored the use of potenliometric-type detectors for ion chromatography. Halides and pseudohalides were detected with a sil-ver/silver salt indicator electrode. Other indicating electrodes were also suggested, for example, lead/lead salicylate to detect sulfate type anions. Other workers have reported potentiometric detection [48-52]. 

Piezoelectricity links the fields of electricity and acoustics. Piezoelectric materials are key components in acoustic transducers such as microphones, loudspeakers, transmitters, burglar alarms and submarine detectors. The Curie brothers [7] in 1880 first observed the phenomenon in quartz crystals. Langevin [8] in 1916 first reported the application of piezoelectrics to acoustics. He used piezoelectric quartz crystals in an ultrasonic sending and detection system - a forerunner to present day sonar systems. Subsequently, other materials with piezoelectric properties were discovered. These included the crystal Rochelle salt [9], the ceramics lead barium titanate/zirconate (pzt) and barium titanate [10] and the polymer poly(vinylidene fluoride) [11]. Other polymers such as nylon 11 [12], poly(vinyl chloride) [13] and poly (vinyl fluoride) [14] exhibit piezoelectric behavior, but to a much smaller extent. Strain constants characterize the piezoelectric response. These relate a vector quantity, the electrical field, to a tensor quantity, the mechanical stress (or strain). In this convention, the film orientation direction is denoted by 1, the width by 2 and the thickness by 3. Thus, the piezoelectric strain constant dl3 refers to a polymer film held in the orientation direction with the electrical field applied parallel to the thickness or 3 direction. The requirements for observing piezoelectricity in materials are a non-symmetric unit cell and a net dipole movement in the structure. There are 32-point groups, but only 30 of these have non-symmetric unit cells and are therefore capable of exhibiting piezoelectricity. Further, only 10 out of these twenty point groups exhibit both piezoelectricity and pyroelectricity. The piezoelectric strain constant, d, is related to the piezoelectric stress coefficient, g, by... 

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