Variability In Degradation Rates Research Articles

A Competing Risk Model of Reliability Analysis for NAND-Based SSDs in Space Application

This paper develops a competing risk model to simultaneously analyze censored catastrophic failures and nonlinear degradation data of the NAND-based solid-state drives for space application. Two dominant failure modes are the hard failure of the controller due to single-event latch-up (SEL) and the soft failure of the NAND Flash manifesting as random write current degradation. As hard failure probability increases with radiation intensity and particle number, we establish the inverse power law-Weibull model for SEL cross section to model the accelerated censored data. The hard failure model is presented based on the invariance principle of total environmental particles’ energy. On the other hand, soft degradation is described by the nonlinear Wiener-process-based accelerated degradation test model. Specifically, the temporal variability concerning the inherent variability of the degradation process over time and the unit-to-unit variability in degradation rates are both taken into account. Then, we derive the reliability functions and other quantities of interest under normal conditions with the assumption of independence of failure modes. Furthermore, to estimate the unknown parameters in the competing risk model, the transformed extreme value regression analysis other than the least square fitting method is adapted to issue the problem of data uncertainty of hard failures, whereas the maximum likelihood estimation method is developed for soft failures. Finally, a detailed simulation example is given to illustrate the procedure of the proposed reliability model with a sensitivity analysis.

Study of the spatial variation of the biodegradation rate of the herbicide bentazone with soil depth using contrasting incubation methods

Vertical and horizontal spatial variability in the biodegradation of the herbicide bentazone was compared in sandy-loam soil from an agricultural field using sieved soil and intact soil cores. An initial experiment compared degradation at five depths between 0 and 80 cm using sieved soil. Degradation was shown to follow the first-order kinetics, and time to 50% degradation (DT 50), declined progressively with soil depth from 56 d at 0–10 cm to 520 d at 70–80 cm. DT 50 was significantly correlated with organic matter, pH and dehydrogenase activity. In a subsequent experiment, degradation rate was compared after 127 d in sieved soil and intact cores from 0 to 10 and 50 to 60 cm depth from 10 locations across a 160 × 90 m portion of the field. Method of incubation significantly affected mean dissipation rate, although there were relatively small differences in the amount of pesticide remaining in intact cores and sieved soil, accounting for between 4.6% and 10.6% of that added. Spatial variability in degradation rate was higher in soil from 0 to 10 cm depth relative to that from 50 and 60 cm depth in both sieved soil and intact core assessments. Patterns of spatial variability measured using cores and sieved soil were similar at 50–60 cm, but not at 0–10 cm depth. This could reflect loss of environmental context following processing of sieved soil. In particular, moisture content, which was controlled in sieved soil, was found to be variable in cores, and was significantly correlated with degradation rate in intact topsoil cores from 0 to 10 cm depth.

In-field spatial variability in the degradation of the phenyl-urea herbicide isoproturon is the result of interactions between degradative Sphingomonas spp. and soil pH.

Substantial spatial variability in the degradation rate of the phenyl-urea herbicide isoproturon (IPU) [3-(4-isopropylphenyl)-1,1-dimethylurea] has been shown to occur within agricultural fields, with implications for the longevity of the compound in the soil, and its movement to ground- and surface water. The microbial mechanisms underlying such spatial variability in degradation rate were investigated at Deep Slade field in Warwickshire, United Kingdom. Most-probable-number analysis showed that rapid degradation of IPU was associated with proliferation of IPU-degrading organisms. Slow degradation of IPU was linked to either a delay in the proliferation of IPU-degrading organisms or apparent cometabolic degradation. Using enrichment techniques, an IPU-degrading bacterial culture (designated strain F35) was isolated from fast-degrading soil, and partial 16S rRNA sequencing placed it within the Sphingomonas group. Denaturing gradient gel electrophoresis (DGGE) of PCR-amplified bacterial community 16S rRNA revealed two bands that increased in intensity in soil during growth-linked metabolism of IPU, and sequencing of the excised bands showed high sequence homology to the Sphingomonas group. However, while F35 was not closely related to either DGGE band, one of the DGGE bands showed 100% partial 16S rRNA sequence homology to an IPU-degrading Sphingomonas sp. (strain SRS2) isolated from Deep Slade field in an earlier study. Experiments with strains SRS2 and F35 in soil and liquid culture showed that the isolates had a narrow pH optimum (7 to 7.5) for metabolism of IPU. The pH requirements of IPU-degrading strains of Sphingomonas spp. could largely account for the spatial variation of IPU degradation rates across the field.

Degradation of herbicides in shallow Danish aquifers: an integrated laboratory and field study.

Degradation of pesticides in aquifers has been evaluated based on a number of co-ordinated field and laboratory studies carried out in Danish aquifers. These studies included investigations of vertical and horizontal variability in degradation rates from the vadose zone to an aquifer, the effects of aerobic versus anaerobic conditions, and the importance of concentration on degradation kinetics for a selected range of herbicides. The studies were based on different experimental approaches ranging from simple batch experiments to column studies to field injection experiments and, where appropriate, results were compared. Some herbicides were degraded under aerobic conditions (some phenoxy acids, DNOC and glyphosate) and others under aerobic conditions (other phenoxy acids, DNOC; there was some indication of atrazine transformation). Certain pesticides were not degraded in any investigations (dichlobenil, the dichlobenil metabolite 2,6-dichlorobenzamide (BAM), bentazone, isoproturon, metamitron and metsulfuron-methyl). The spatial variability was substantial, since hardly any of the investigated pesticides were degraded in all comparable samples. This means that it is very difficult to claim that a given pesticide is readily degradable in aquifers. However, the experimental approaches used (with incubations lasting more than a year) may not be sensitive enough to verify the low degradation rates that may be significant as a result of the long retention time of groundwaters.