Age-related efficiency loss of household refrigeration appliances: Development of an approach to measure the degradation of insulation properties

Despite the omnipresence of household refrigeration appliances, there is still a lack of knowledge about their age-related efficiency loss over time. Past studies provide basic evidence for increasing electricity consumption of cooling appliances with ageing but fail to investigate the associated technical wear. Concentrating on the degradation of the thermal insulation, we first determined the ageing process of sealed samples of polyurethane rigid foam by investigating changes in cell gas composition and thermal conductivity over time. Simultaneously, the main challenge was to develop an approach that investigates the age-related efficiency loss of the insulation without its destruction. This testing procedure is referred to as the Bonn method. The non-destructive Bonn method was applied to varying refrigerator models in a series of successive experiments to evaluate the insulation degradation over time. Subsequently, the physical relationship between the test value of the Bonn method and the heat transfer through the multi-layered compartment walls of domestic refrigeration appliances was established, ultimately characterising the degrading insulation in terms of increasing heat transfer. Our results give substantiated evidence that the efficiency loss of cooling appliances is greatly influenced by insulation degradation over time. The ageing of sealed samples of polyurethane rigid foam indicates a large initial increase of thermal conductivity by 15% within the first year, corresponding to a change in cell gas composition. These results are in line with those of the Bonn method, emphasising an increasing heat flow through the multi-layered compartment walls of domestic refrigerators with ageing. Therewith, the present study is of significance to a wide range of stakeholders and forms the basis for future research.BMWi, 03ET1544, ALGE: Alterungsmechanismen von Haushaltskältegeräte

A scanning probe microscope for magnetoresistive cantilevers utilizing a nested scanner design for large-area scans

Meier T, Foerste A, Tavassolizadeh A, et al. A scanning probe microscope for magnetoresistive cantilevers utilizing a nested scanner design for large-area scans. Beilstein Journal of Nanotechnology. 2015;6:451-461.We describe an atomic force microscope (AFM) for the characterization of self-sensing tunneling magnetoresistive (TMR) cantilevers. Furthermore, we achieve a large scan-range with a nested scanner design of two independent piezo scanners: a small high resolution scanner with a scan range of 5 x 5 x 5 mu m(3) is mounted on a large-area scanner with a scan range of 800 x 800 x 35 mu m(3). In order to characterize TMR sensors on AFM cantilevers as deflection sensors, the AFM is equipped with a laser beam deflection setup to measure the deflection of the cantilevers independently. The instrument is based on a commercial AFM controller and capable to perform large-area scanning directly without stitching of images. Images obtained on different samples such as calibration standard, optical grating, EPROM chip, self-assembled monolayers and atomic step-edges of gold demonstrate the high stability of the nested scanner design and the performance of self-sensing TMR cantilevers

Determining the heat flow through the cabinet walls of household refrigerating appliances

The increase of the thermal conductivity of PUR foam in the insulation of the cabinet is an important cause for aging processes of household refrigerating appliances. To determine the influence of the PUR foam aging on energy consumption, the development of a new measurement method is necessary because current methods influence the aging behavior of household refrigerators and are therefore not applicable in general. Based on a latent heat sink, constructed as an ice water bucket, a new measurement method is developed to determine the k · A value over time. With this method, the k · A value of four household refrigerating appliances was determined over an interval of 14 months. The k · A value increased between 3.6 % and 11.5 % during this period.BMWi, 03ET1544A, Verbundvorhaben ALGE: Alterungsmechanismen von Haushaltskältegeräte

Bunnell or cross-lock Bunnell suture for tendon repair? Defining the biomechanical role of suture pretension

Background Suture pretension during tendon repair is supposed to increase the resistance to gap formation. However, its effects on the Bunnell suture technique are unknown. The purpose of this study was to determine the biomechanical effects of suture pretension on the Bunnell and cross-lock Bunnell techniques for tendon repair. Methods Eighty porcine hindlimb tendons were randomly assigned to four different tendon repair groups: those repaired with or without suture pretension using either a simple Bunnell or cross-lock Bunnell technique. Pretension was applied as a 10 % shortening of the sutured tendon. After measuring the cross-sectional diameter at the repair site, static and cyclic biomechanical tests were conducted to evaluate the initial and 5-mm gap formation forces, elongation during cyclic loading, maximum tensile strength, and mode of failure. The suture failure mechanism was also separately assessed fluoroscopically in two tendons that were repaired with steel wire. Results Suture pretension was accompanied by a 10 to 15 % increase in the tendon diameter at the repair site. Therefore, suture pretension with the Bunnell and cross-lock Bunnell repair techniques noticeably increased the resistance to initial gap formation and 5-mm gap formation. The tension-free cross-lock Bunnell repair demonstrated more resistance to initial and 5-mm gap formation, less elongation, and higher maximum tensile strength than the tension-free Bunnell repair technique. The only difference between the tensioned cross-lock Bunnell and tensioned Bunnell techniques was a larger resistance to 5-mm gap formation with the cross-lock Bunnell technique. Use of the simple instead of cross-lock suture configuration led to failure by suture cut out, as demonstrated fluoroscopically. Conclusion Based on these results, suture pretension decreases gapping and elongation after tendon repair, and those effects are stronger when using a cross-lock, rather than a regular Bunnell suture. However, pretension causes an unfavorable increase in the tendon diameter at the repair site, which may adversely affect wound healing

Tunnel Magnetoresistance Sensors with Magnetostrictive Electrodes: Strain Sensors

Tavassolizadeh A, Rott K, Meier T, et al. Tunnel Magnetoresistance Sensors with Magnetostrictive Electrodes: Strain Sensors. SENSORS. 2016;16(11): 1902.Magnetostrictive tunnel magnetoresistance (TMR) sensors pose a bright perspective in micro-and nano-scale strain sensing technology. The behavior of TMR sensors under mechanical stress as well as their sensitivity to the applied stress depends on the magnetization configuration of magnetic tunnel junctions (MTJ)s with respect to the stress axis. Here, we propose a configuration resulting in an inverse effect on the tunnel resistance by tensile and compressive stresses. Numerical simulations, based on a modified Stoner-Wohlfarth (SW) model, are performed in order to understand the magnetization reversal of the sense layer and to find out the optimum bias magnetic field required for high strain sensitivity. At a bias field of -3.2 kA/m under a 0.2 x 10(-3) strain, gauge factors of 2294 and -311 are calculated under tensile and compressive stresses, respectively. Modeling results are investigated experimentally on a round junction with a diameter of 30 +/- 0.2 mu m using a four-point bending apparatus. The measured field and strain loops exhibit nearly the same trends as the calculated ones. Also, the gauge factors are in the same range. The junction exhibits gauge factors of 2150 +/- 30 and -260 for tensile and compressive stresses, respectively, under a -3.2 kA/m bias magnetic field. The agreement of the experimental and modeling results approves the proposed configuration for high sensitivity and ability to detect both tensile and compressive stresses by a single TMR sensor

Self-sensing atomic force microscopy cantilevers based on tunnel magnetoresistance sensors

Tavassolizadeh A, Meier T, Rott K, et al. Self-sensing atomic force microscopy cantilevers based on tunnel magnetoresistance sensors. Applied Physics Letters. 2013;102(15): 153104.Here, we introduce self-sensing cantilevers for atomic force microscopy (AFM) based on tunnel magnetoresistance (TMR) sensors. These TMR sensors are integrated into the AFM cantilevers and consist of a magnetically stable layer and a sensing magnetostrictive CoFeB layer separated by a MgO tunneling barrier and can be as small as 10 mu m x 10 mu m. Their TMR values and resistance-area products are about 121% and 61 k Omega mu m(2), respectively. A comparison of AFM data simultaneously obtained with a self-sensing cantilever with a 37 mu m x 37 mu m large TMR sensor and the conventional optical beam deflection method revealed the same data quality. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801315