Enhancement of complex permittivity and attenuation properties of activated carbon derived from oil palm fruit fiber for microwave application

This study aimed to synthesize activated carbon (AC) from empty oil palm fruit bunch (OPEFB) fiber to enhance its complex permittivity properties by modifying the particle size and surface area via physical activation. Sample characterizations of complex permittivity were conducted using open-ended coaxial technique and a vector network analyzer. The absorption properties were analyzed using the finite element method (FEM) simulations of the transmission coefficients and the distribution of electric fields via the microstrip models. Simulations and measurements were all carried out within the range 8–12 GHz. The initial surface area of the unactivated sample was 4.02, after 700, 750, and 800 °C activation, 730.40, 814.00, and 927.01 m2/g was obtained respectively. With increased surface area, the dielectric constant and loss factor values increased and attained maximum values of 6.13 and 0.83 at 8 GHz, respectively from initial values of 3.63 and 0.52, as the surface area increased from 4.02 to 927.01 m2/g. The improved absorption properties displayed by the AC in the simulations demonstrated their capacity to attenuate X-band microwaves

Prognostic model to predict postoperative acute kidney injury in patients undergoing major gastrointestinal surgery based on a national prospective observational cohort study.

Background: Acute illness, existing co-morbidities and surgical stress response can all contribute to postoperative acute kidney injury (AKI) in patients undergoing major gastrointestinal surgery. The aim of this study was prospectively to develop a pragmatic prognostic model to stratify patients according to risk of developing AKI after major gastrointestinal surgery. Methods: This prospective multicentre cohort study included consecutive adults undergoing elective or emergency gastrointestinal resection, liver resection or stoma reversal in 2-week blocks over a continuous 3-month period. The primary outcome was the rate of AKI within 7 days of surgery. Bootstrap stability was used to select clinically plausible risk factors into the model. Internal model validation was carried out by bootstrap validation. Results: A total of 4544 patients were included across 173 centres in the UK and Ireland. The overall rate of AKI was 14·2 per cent (646 of 4544) and the 30-day mortality rate was 1·8 per cent (84 of 4544). Stage 1 AKI was significantly associated with 30-day mortality (unadjusted odds ratio 7·61, 95 per cent c.i. 4·49 to 12·90; P < 0·001), with increasing odds of death with each AKI stage. Six variables were selected for inclusion in the prognostic model: age, sex, ASA grade, preoperative estimated glomerular filtration rate, planned open surgery and preoperative use of either an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker. Internal validation demonstrated good model discrimination (c-statistic 0·65). Discussion: Following major gastrointestinal surgery, AKI occurred in one in seven patients. This preoperative prognostic model identified patients at high risk of postoperative AKI. Validation in an independent data set is required to ensure generalizability

Fabrication and effect of applied field stresses on stability of ZnO ceramics with CaMnO₃ as additive for varistor application

The major challenge in the continuing development of varistor has been to reduce the energy loss which is due to temperature and applied field stresses and to attain a good stability. The aims of this study are; to study the effect of CaMnO3 (CMO) contents on nonlinearity coefficient (α) enhancement of ZnO based varistor ceramics; and secondly to study the nonlinear stability of this ceramic against DC electrical field and thermal stresses as function of CMO contents. The first aim has been carried out by synthesis of ZnO + xCMO ceramics by citrate gel coating technique, where x ranging from 3 – 7 mol% and sintered at various sintering times from 1 – 3 hrs. The prepared samples were characterized by using TG/DTA, EDX, SEM, XRD, and I-V measurements. The second aim was carried out by choosing the samples with best electrical properties subjecting them simultaneously to stresses of specific temperature and DC voltage over a certain period of time to study the degree of degradation. XRD analysis shows that the main phase in the material systems was ZnO while CMO as the secondary phase increase gradually with the increase of mol%, x. The TG/DTA analysis shows that, good quality of ZnO-CMO can be achieved at calcination temperature above 650 °C. Average density of the ceramics in all systems has the same trend where it decreases with the increase of doping level and sintering time. The maximum grain size was found to be 23.4 μm at the highest sintering time at 3 hrs at 7 mol% CMO doping level, as Mn is a strong grain enhancer. The SEM and EDX results verified that Ca and Mn were distributed within the grain as well as in the grain boundaries and triple point junctions. The value of α was found to increase with amount of CMO up to 5 mol% concentration. An optimum sintering time of 3 hrs for 5 mol% doping level gave the best α, attaining a value of 42.3 which represent a high protective function of ZnO varistor. The DC stress has caused a decrease in varistor voltage, α, and also an increase in leakage current. After exposure to DC and heat stresses for 18 hours, the varistor ceramics experienced a slight drop in nonlinearity. Thus, the varistor ceramic samples exhibited a high level of leakage current compared to samples before degradation which indicated they have been degraded or deteriorated. The degradation percentage was calculated in terms of changes in varistor parameters before and after stress. It showed that sample with 3 mol% CMO possess lowest degradation percentage. From this result, it was found that samples fabricated at 3 hrs sintering time have the best microstructure and electrical properties

Dielectric/thermal characteristics of recycled borosilicate glass-filled polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) composites filled with recycled borosilicate (rBRS) glass were fabricated for printed circuit board (PCB) substrate application. The rBRS powder prepared via a ball milling technique was dispersed in the PTFE matrix in different volume fractions (5%–25%) to produce the composites through a dry powder mixing technique. The effects of filler content on the composites' structural, dielectric, and thermal properties were examined. Scanning electron microscopy showed that rBRS filler was more dispersed in the composites at lower filler contents. The composites showed excellent microwave properties with relative permittivity of 2.31 and a loss tangent of 0.0018 at the highest filler content of 25%. The composites also revealed a good thermal property with a mean CTE of 60.54 ppm/°C at 25% filler content. Experimental data were compared with theoretical predictions. The results showed that experimental values of relative permittivity agreed with calculated values from regression analysis. It was also found that the logarithmic general mixing model gave an excellent estimate of the loss tangent of the composites