Bulk sensors

There are two basic ways to look for explosive material. They differ in their point of focus. Some sensors seek the mass of explosive material within a device. These are particularly useful when the device is well sealed and its surface is well cleaned of stray explosive molecules, or when the explosive being used is nonaromatic, that is, it does not readily release molecules from its bulk. We will refer to these as bulk sensors. They include X-ray techniques, both transmission and backscatter neutron activation in several techniques y -ray excitation, in either transmission or backscatter modes and nuclear resonance techniques, either nuclear magnetic resonance (NMR) or nuclear quadrupole resonance (NQR). Bruschini [1] has described these thoroughly. They are also described by the staff of the Jet Propulsion Laboratory [2], The following forms a very brief synopsis. 

1 X-ray Techniques X-ray techniques are familiar because of their use in medical diagnosis. The basic concept is that material of different densities or chemical compositions absorb and scatter X-rays differently. When the X-rays pass through the materials and strike the film or detector, they form a gray-scale image. After proper calibration a bulk charge of explosive may be inferred from this image. 

In many applications, such as seeking buried munitions, it is not possible to place the X-ray source and the detector on opposite sides of the objects being investigated. In this case techniques have been developed to form images from the X-rays that are scattered back toward the source, or backscattered. 

2 Neutron or y Sensors Neutron- or y -based sensors are similar in concept to the X-ray sensors. They use different forms of excitation and different detectors, but the basic forms of transmission or backscatter follow the pattern described above. Both normally rely on extensive computation for signal processing called computed tomography, where the detector signals are combined to synthesize an image of the irradiated object. 

3 Electromagnetic Techniques Sensors based on the related principles of NMR and NQR have been successfully developed. These are active techniques that excite the electromagnetic interactions between the atomic nuclei 

---

Bulk sensors certainly have a role in chemical sensing of explosives, but the subject of this book is the other basic type sensor, one that seeks molecules released from the bulk of the explosive material in an object. We will refer to these as trace chemical sensors. They are sometimes called vapor sensors, but that seems a less accurate description when they are applied to explosive molecules, which may not always be found in a vapor state. As we shall see in Chapter 5, that requires us to understand where and how to look for these molecules. It will become apparent upon a little reflection that the two types of sensors are complementary and are best used in different situations. Furthermore, even when trace sensors are used, in some situations sampling of particles of soil or vegetation or sampling from surfaces may prove to be more productive that vapor sampling. For underwater sources the term vapor sensing is also inappropriate. 

Zakrzewska found that titanium dioxide doped with Nb and Cr should be considered as a bulk sensor. Its performance was governed by the diffusion of point defects, i.e. very slow diffusion of Ti vacancies for Ti02. 9.5 at% of Nb and fast diffusion of oxygen vacancies in the case of HOt. 2.5 at% Cr sensor. The corresponding response times were 55 min for TiOT. 9.5 at% of Nb and 20 s for HOt. 2.5 at% Cr [292]. 

The semiconductor boundary sensor (surface conductivity sensor in Table 7.2) avoids the disadvantage of the high temperatures that were important for the bulk sensors... 

A chemical microsensor can be defined as an extremely small device that detects components in gases or Hquids (52—55). Ideally, such a sensor generates a response which either varies with the nature or concentration of the material or is reversible for repeated cycles of exposure. Of the many types of microsensors that have been described (56), three are the most prominent the chemiresistor, the bulk-wave piezoelectric quartz crystal sensor, and the surface acoustic wave (saw) device (57). 

Bulk-wave piezoelectric quartz crystal sensors indirecdy measure mass changes of the coating on the surface of the sensing device. This change in mass causes changes in the resonant frequency of the device, and measurements ate based on frequency differences. 

The performance characteristics of ceramic sensors are defined by one or more of the foUowing material properties bulk, grain boundary, interface, or surface. Sensor response arises from the nonelectrical input because the environmental variable effects charge generation and transport in the sensor material. 

These lead-based materials (PZT, PLZT, PMN) form a class of ceramics with either important dielectric, relaxor, pie2oelectric, or electrooptic properties, and are thus used for appHcations ia actuator and sensor devices. Resistive properties of these materials ia film form mirror the conduction processes ia the bulk material. Common problems associated with their use are low dielectric breakdown, iacreased aging, and electrode iajection, decreasiag the resistivity and degrading the properties. 

Bulk and surface imprinting strategies are straightforward tools to generate artificial antibodies. Combined with transducers such as QCM (quartz crystal microbalance), SAW (surface acoustic wave resonator), IDC (interdigital capacitor) or SPR (surface plasmon resonator) they yield powerful chemical sensors for a very broad range of analytes. 

Aromatic solvents or polycyclic aromatic hydrocarbons (PAFI) in water, e.g. can be detected by QCM coated with bulk-imprinted polymer layers. Flere, the interaction sites are not confined to the surface of the sensitive material but are distributed within the entire bulk leading to very appreciable sensor responses. Additionally, these materials show high selectivity aromatic solvents e.g. can be distinguished both by the number of methyl groups on the ring (toluene vs. xylene, etc.) and by their respective position. Selectivity factors in this case reach values of up to 100. 

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

FfCURE 13.54 Semiconductor gas sensors (o) tubular, (b) thick film, (e) bulk-type one-electrode sensor where a thin Pt wire spiral is embedded Inside a sintered oxide semiconductor button. ... 

Alternatively the capillary rise can be measured using a bulk acoustic wave sensor [97Che]. (Data obtained with these methods are labelled BAW). 

Lead(II) sulfide occurs widely as the black opaque mineral galena, which is the principal ore of lead. The bulk material has a band gap of 0.41 eV, and it is used as a Pb " ion-selective sensor and IR detector. PbS may become suitable for optoelectronic applications upon tailoring its band gap by alloying with II-VI compounds like ZnS or CdS. Importantly, PbS allows strong size-quantization effects due to a high dielectric constant and small effective mass of electrons and holes. It is considered that its band gap energy should be easily modulated from the bulk value to a few electron volts, solely by changing the material s dimensionality. 

According to Eq. (1) the steady-state current across a micro-ITIES is proportional to the bulk concentration of the transferred species. Thus, the micro-ITIES can function as an amperometric ion-selective sensor. Similarly, the peak current in a linear sweep voltam-mogram of ion egress from the micropipette obeys the Randles-Sevcik equation. Both types of measurements can be useful for analysis of small samples [18a]. 

It should be noted that in a vapour phase the liquid layer on the surface of a sensitive element of the sensor (zinc oxide) must be sufficiently thin, so that it would not produce any influence on the diffusion flux of oxygen through this layer. Possible lack of the film continuity (the presence of voids) does not prevent determination of concentration of oxygen in the bulk of the cell by the vapour - gas method. In this case, one deals with a semi-dry method. On the contrary, the presence of a thick liquid layer causes considerable errors in measuring t, because of different distribution of oxygen in a system gas - liquid layer -semiconductor film (this distribution is close to that in the system semiconductor film - liquid), in addition to substantial slowing down of oxygen diffusion in such systems. 

---