Robert C. Hughes

Thin films of Pd/Ni alloys for detection of high hydrogen concentrations

Thin alloy films of Pd and Ni (8%<Ni<20% in at. %) formed by dual‐electron‐beam evaporation techniques have been found to give durable and quickly reversible detectors of high H2 concentrations (pH2 0.1%–100%, 0.7–700 Torr) near 1 atm and 300 K, including accurate determinations of pH2 around the lower explosive limit of 4% in air. The addition of Ni suppresses the α to β phase transition found in pure Pd under these conditions. The measurement of resistivity changes in the thin films along with flatband shifts of metal‐oxide‐semiconductor capacitors on the same Si wafer gives accurate values of pH2 over more than six decades.

Thin‐film palladium and silver alloys and layers for metal‐insulator‐semiconductor sensors

The addition of Ag to Pd in the gate metal of a metal‐insulator‐semiconductor gas sensing diode can improve the performance and change the selectivity of the sensors for a variety of reactions. Data on the response of diodes with 12 different ratios of Ag to Pd in alloys and layers of Pd and Ag to hydrogen and other gases are reported. Diodes with as much as 32% Ag respond very well to H2 gas and the films are much more durable to high hydrogen exposure than pure Pd films. Improvements in the rate of response and aging behavior are found for certain Ag combinations; others give poorer performance. The presence of Ag on the surface changes the catalytic activity in some cases and examples of H2 mixed with O2 and/or NO2, propylene oxide, ethylene, and formic acid are given. Such selectivity forms the basis for miniature chemical sensor arrays which could analyze complex gas mixtures.

Review of Chemical Sensors for In-Situ Monitoring of Volatile Contaminants

Sandia National Laboratories has sponsored an LDRD (Laboratory Directed Research and Development) project to investigate and develop micro-chemical sensors for in-situ monitoring of subsurface contaminants. As part of this project, a literature search has been conducted to survey available technologies and identify the most promising methods for sensing and monitoring subsurface contaminants of interest. Specific sensor technologies are categorized into several broad groups, and these groups are then evaluated for use in subsurface, long-term applications. This report introduces the background and specific scope of the problem being addressed by this LDRD project, and it provides a summary of the advantages and disadvantages of each sensor technology identified from the literature search.

Miniature radiation dosimeter for in vivo radiation measurements

We have made initial characterization measurements on a miniature radiation dosimeter which can be used for in vivo radiation measurements. It consists of a radiation sensing field effect transistor (RADFET) mounted in a 0.8 mm OD plastic catheter. The RADFET acts as a dosimeter by storing trapped charge proportional to absorbed dose. The stored charge signal can be differentiated to give dose rate. We report on the techniques for mounting, a circuit for dose readout, drift of the readings, linearity of response, temperature and angular dependence, and unpowered operation of the device.

Geminate Recombination of X‐Ray Excited Electron—Hole Pairs in Anthracene

A recent controversy is discussed concerning the relative importance of geminate and track recombination of charge carriers excited by ionizing radiation in single crystals of anthracene. Data is presented which strongly supports the geminate recombination hypothesis for bulk excitation by x rays in the 50–500−keV range. In agreement with the earlier work of Kepler and Coppage, it is found that it requires about 2700 eV to create a pair of charge carriers in anthracene at an applied field of 10 kV/cm.

Solid‐State Hydrogen Sensors Using Palladium‐Nickel Alloys: Effect of Alloy Composition on Sensor Response

Thin film alloys of Pd and Ni make good sensors for hydrogen partial pressures (pH[sub 2]) over a very wide range: from 10[sup [minus]3] to 700 Torr. For pH[sub 2] from 1 to 700 Torr the atomic percent of Ni in the alloy has a strong effect on the change in film resistance which is used as the sensing property. The phase transition from the [alpha]- to the hydride [beta]-phase must be avoided to maintain a reversible sensor. The characteristics of alloys with Ni of 3, 5.3, 8, and 15% are reported. The phase transition was found in the 5.3% alloy at about 250 Torr, but alloys of Ni >8% showed no phase change behavior up to 630 Torr (pure H[sub 2]) at ambient temperatures. Other characteristics of the Pd/Ni alloy sensors are also reported, including the effect of temperature, oxygen, and relative humidity, and the speed of response to step changes in pH[sub 2].

Paramagnetic Susceptibilities and Temperature‐Dependent Excitation Energies in Linear Organic Crystals

The pseudospin solutions of the regular and the alternating Heisenberg antiferromagnet are shown to explain quantitatively the temperature dependence of the absolute paramagnetic susceptibilities of organic molecular crystals containing linear arrays of free radicals. The pseudospin solutions also reproduce, for a rigid lattice, the temperature dependence of the singlet—triplet energy gap in triplet exciton crystals. This effect, previously interpreted as evidence for exciton—phonon interactions, is again shown to be a many‐body effect, which is found even in the absence of exciton—phonon coupling. Information about the structures of these organic crystals is obtained. It is shown that triethylammonium—(TCNQ)2 corresponds to a weakly alternating Heisenberg antiferromagnet rather than, as previously suggested, to a very strongly alternating antiferromagnet. The results for the absolute paramagnetic susceptibility of Wursters blue perchlorate confirm the previously proposed dimerization below its phase tra...

Radiation-Induced Conductivity in Polymers: Poly-N-Vinylcarbazoie

Experimental evidence is given which indicates that the yield of charge carriers excited by a given dose of x-rays in thin films of poly-N-vinylcarbazole (PVK) is dominated by geminate (initial) recombination of the ion pairs. The yield as a function of applied field gives a very good fit to the theory of initial recombination developed for gases by Onsager. Two competing mechanisms, the Poole-Frenkel effect and track recombination, may be rejected on the basis of the field dependence. A comparison of the charge transients in PVK with other organic materials is given along with a discussion of the possibility of trap modulation of the mobility in PVK. The role of three different kinds of carrier recombination, bulk, track and geminate, is described in terms of the experimental results expected for each of them.

Magnetic Excitations in Charge‐Transfer Complexes. I. p‐Phenylenediamine–Chloranil

The electron magnetic resonance of single crystals of p‐phenylenediamine–chloranil (PDC) at 160 Mc/sec, 9.5, and 35 Gc/sec is reported. The thermally accessible (activation energy 0.13 ± 0.01 eV) magnetic excitations for the linear chains of exchange‐coupled, alternately PD cation and chloranil anion radicals (S = 12) are described by a single, almost axially symmetric g factor, with g‖ = 2.0024 ± 0.0002, g⊥ = 2.0054 ± 0.0002, and | gx − gy | ∼ 0.0001. Temperature‐dependent g‐factor splittings are observed below 315°K, while a single, strongly exchange‐narrowed line (width ∼ 250 mG) is observed above 315° for any orientation of the crystal. The angular dependence of the splittings below 270°K corresponds to three magnetically inequivalent, independent free‐radical chains related to each other by a threefold axis parallel to the chain axis, with a 6° angle between the chain axis and the normal to the molecular planes of the radicals. The collapse of the splittings between 270° and 315° is explained qualita...

ESR of TMPD–TCNQ: Spin Excitations of the Heisenberg Regular Linear Chain

The detailed electron spin resonance of single crystals of the charge‐transfer complex tetramethylphenylenediamine–tetracyanoquinodimethan (TMPD–TCNQ) at X band and Q band are reported. The ESR spectra are interpreted as resulting from the thermally excited (activation energy, 0.068 eV) spin excitations of the Heisenberg regular antiferromagnetically coupled chain. Although the exact solution to the excited states of the Heisenberg chain with antiferromagnetic coupling is not available, an approximate solution treating the quasiparticles as Wannier spin excitons is very successful at predicting the spin resonance properties of the excitations; fundamental to the Wannier spin exciton model is the absence of spin correlation in the excitations. The spin–spin (T2−1) and spin–lattice (T1−1) relaxation rates are observed as functions of temperature and are determined mainly by spin exchange between excitons. Exchange is proportional to exciton concentration, and the low activation energy in TMPD–TCNQ allows ex...