Depletion-interaction effects on the tunneling conductivity of nanorod suspensions.

We study by simulation and theory how the addition of insulating spherical particles affects the conductivity of fluids of conducting rods, modeled by spherocylinders. The electrical connections are implemented as tunneling processes, leading to a more detailed and realistic description than a discontinuous percolation approach. We find that the spheres enhance the tunneling conductivity for a given concentration of rods and that the enhancement increases with rod concentration into the regime where the conducting network is well established. By reformulating the network of rods using a critical path analysis, we quantify the effect of depletion-induced attraction between the rods due to the spheres. Furthermore, we show that our conductivity data are quantitatively reproduced by an effective-medium approximation, which explicitly relates the system tunneling conductance to the structure of the rod-sphere fluid.

Tunneling conductivity in composites of attractive colloids.

In conductor-insulator nanocomposites in which conducting fillers are dispersed in an insulating matrix, the electrical connectedness is established by inter-particle tunneling or hopping processes. These systems are intrinsically non-percolative and a coherent description of the functional dependence of the conductivity σ on the filler properties, and in particular of the conductor-insulator transition, requires going beyond the usual continuum percolation approach by relaxing the constraint of a fixed connectivity distance. In this article, we consider dispersions of conducting spherical particles which are connected to all others by tunneling conductances and which are subjected to an effective attractive square-well potential. We show that the conductor-insulator transition at low contents φ of the conducting fillers does not determine the behavior of σ at larger concentrations, in striking contrast to what is predicted by percolation theory. In particular, we find that at low φ the conductivity is governed almost entirely by the stickiness of the attraction, while at larger φ values σ depends mainly on the depth of the potential well. As a consequence, by varying the range and depth of the potential while keeping the stickiness fixed, composites with similar conductor-insulator transitions may display conductivity variations of several orders of magnitude at intermediate and large φ values. By using a recently developed effective medium theory and the critical path approximation, we explain this behavior in terms of dominant tunneling processes which involve inter-particle distances spanning different regions of the square-well fluid structure as φ is varied. Our predictions could be tested in experiments by changing the potential profile with different depletants in polymer nanocomposites.

Tunneling and percolation transport regimes in segregated composites.

We consider the problem of electron transport in segregated conductor-insulator composites in which the conducting particles are connected to all others via tunneling conductances, thus forming a global tunneling-connected resistor network. Segregation is induced by the presence of large insulating particles, which forbid the much smaller conducting fillers from occupying uniformly the three-dimensional volume of the composite. By considering both colloidal-like and granular-like dispersions of the conducting phase, modeled respectively by dispersions in the continuum and in the lattice, we evaluate by Monte Carlo simulations the effect of segregation on the composite conductivity σ, and show that an effective-medium theory applied to the tunneling network reproduces accurately the Monte Carlo results. The theory clarifies that the main effect of segregation in the continuum is that of reducing the mean interparticle distances, leading to a strong enhancement of the conductivity. In the lattice-segregation case the conductivity enhancement is instead given by the lowering of the percolation thresholds for first and beyond-first nearest neighbors. Our results generalize to segregated composites the tunneling-based description of both the percolation and hopping regimes introduced previously for homogeneous disordered systems.

Load sensing surgical instruments.

Force and pressure sensing technology applied to smart surgical instruments as well as implants allow to give a direct feedback of loads to the surgeon lead to better reliability and success of surgical operations. A common technology used for sensors is low-cost piezoresistive thick-film technology. However, the standard thick-film firing conditions degrade the properties of medical alloys. In order to avoid this problem, the solution is to decrease the firing temperature of thick films. This work presents the development and characterisation of low-firing thick-film systems (dielectrics, resistors and conductors), formulated to achieve chemical and thermal expansion compatibility with an austenitic stainless steel medical alloy. Adherence tests and results on electrical properties of these systems: resistance, temperature coefficient of resistance (TCR) are presented. It was found that the main issue in these systems lies in mastering the materials interactions during firing, especially at the silver-based resistor terminations. The interaction of silver, resistor and dielectric tends to give rise to highly resistive zones at the terminations, affecting reliability. This can be circumvented by post-firing the resistor terminations at a moderate temperature.

Consequences of phenotypic plasticity vs. interspecific differences in leaf and root traits for acquisition of aboveground and belowground resources.

Trade-offs between acquisition capacities for aboveground and belowground resources were investigated by studying the phenotypic plasticity of leaf and root traits in response to different irradiance levels at low nutrient supply. Two congeneric grasses with contrasting light requirements, Dactylis glomerata and D. polygama, were used. The aim was to analyze phenotypic covariation in components of leaf area and root length in response to above- and belowground resource limitation and the consequences of this variation for resource acquisition and plant growth. At intermediate shading (30 and 20% of full sunlight) the plants were able to maintain their total root length, despite a strongly increased total leaf area and a reduced biomass allocation to roots. This was associated with an unaltered or slightly increased nutrient uptake and growth. At 5.5% relative irradiance, growth was severely reduced, especially in the shade-tolerant D. polygama. The results show that constraints on acquisition capacities for aboveground and belowground resources, caused by biomass allocation, may be alleviated by plasticity in other traits such as tissue-mass density and thickness of roots and leaves. The results also suggest different adaptive constraints for phenotypic plasticity and for genetically determined interspecific variation. Phenotypic plasticity tends to maximize resource acquisition and growth rate in the short term, whereas the higher tissue-mass density and the longer leaf life-span of shade-tolerant species indicate reduced loss rates as a more advantageous species-specific adaptation to shade in the long term.

Students and drug abuse, 1974 and 1980.

The paper presents a secondary analysis of students treated in hospital emergency rooms. The comparison of data provided in 1974 and 1980 data reveals a significant increase in the abuse of alcohol with drugs, methaqualone and marijuana. Fifty percent of these students ingested drugs to commit suicide or as a suicide gesture. These data suggest that school health professionals and counselors be alerted to student drug overdoses to assist students confronted with suicidal ideation.

Sex differences in substance abuse: 1976-1979.

Drug abuse patterns are examined for 326,611 males and females who abused drugs and were treated at a hospital emergency room or died. The data reveal a marked percentage difference between male and female mentions of heroin, PCP, and diazepam. A significantly larger proportion of females than males indicated their motivation for drug abuse to be a suicide attempt or gesture. Diazepam, alcohol-in-combination, and aspirin are the most frequently mentioned drugs by those whose motivation is suicidal. However, the percentage difference between males and females for these three drugs is less than 6%. The data were collected during the years 1976-1979.

Effects of client characteristics on admission activity in United States federally funded drug treatment facilities.

Utilizing data from the National Institute on Drug Abuse's CODAP, this paper examines the effect of client characteristics and drug of abuse on certain admission activity variables. It generally sustains the hypotheses (1) that the demographic characteristics of clients (i.e., sex, age, race-ethnicity, and education) admitted to treatment have no effect on admission activity and (2) that type of primary drug problem (opiates, marijuana, alcohol, barbiturates, and amphetamines) at admission is strongly associated with admission activity. The implications of these data for nondiscriminatory admissions policies; for effective utilization of intake, treatment, and follow-up resources; and for training treatment agents are discussed.

Impact of sex on length of time spent in treatment and treatment success.

This paper examines the null hypothesis that there is no difference between male and female drug abuse clients in length of time spent in treatment and successful outcome of that treatment. The hypothesis is tested by means of multiple classification analysis on a random sample of 20,264 clients who were discharged from federally-funded drug abuse programs in 1975. Upon controlling for the effects of demographic client characteristics, treatment history, and drug usage, the authors conclude that the null hypothesis is sustained. Small differences are apparent, especially with respect to the length of time spent by clients in drug-free modality and day-care environment and in detoxification modality and hospital environment, and with respect to treatment success for clients in drug-free environment and hospital environment. However, these differences are not sufficient to warrant a rejection of the null hypothesis.