Structural and electronic properties of silver/silicon interfaces and implications for solar cell performance

We present the results of an experimental and atomistic modelling investigation of the Sili- con/Silver (Si/Ag) interfaces found in industrial solar cells. We use small ab initio calculations to parameterize a new interatomic potential for the Si/Ag interaction. This interatomic potential is then validated against larger ab initio calculations as well as the results of previous experimental and theoretical studies of Si/Ag systems. The interatomic potential allows us to perform a large- scale search of the conformational space of Si/Ag interfaces identified from transmission electron microscopy (TEM) studies. The most favourable geometries thus identified are then used as the input for more accurate ab initio calculations. We demonstrate that the two interfaces which we identify experimentally have significantly different geometric and electronic structures. We also demonstrate how these different structures result in significantly different Schottky barriers at the interfaces

Influence of beam cross-section, loading arrangement and aggregate type on shear strength

This paper describes 14 tests on simply supported and continuous reinforced concrete beams which were designed to investigate the influences of aggregate type, loading arrangement and shear reinforcement ratio on shear strength. The beams were rectangular in cross-section with a/d , 3.5 (where a is the shear span and d the effective depth). Marine dredged gravel was used in ten beams (two without shear reinforcement) and limestone in the remaining four (two without shear reinforcement). The cracks typically passed around the gravel aggregate but through the limestone aggregate. Aggregate fracture was found only to reduce the shear strength of the beams without shear reinforcement. The factors of safety for shear failure implicit in Eurocode 2, BS 8110 and CSA A.23.3-04 are examined with data from this project and elsewhere. Eurocode 2 is shown to provide the lowest factor of safety for most beams. This is particularly significant for rectangular sections where Eurocode 2 can give lower factor of safety for shear than flexural failure which is undesirable. An amendment is proposed to address this

Shear enhancement near supports in RC beams

Shear strength is enhanced within the short shear span of reinforced concrete beams that are loaded on their upper side within a distance of around 2-2·5d of supports (where d is the beam effective depth). Eurocode 2 and fib Model Code 2010 (MC2010) account for this by reducing the design shear force, unlike the previous UK code BS 8110, which increases the shear resistance provided by concrete. Eurocode 2 and MC2010 also allow shear enhancement to be modelled using strut-and-tie models. Very few test data are available to assess the comparative merits of these approaches for the design of beams with multiple point loads within 2d of supports. Consequently, 12 beams were tested to investigate the influence of loading arrangement on shear resistance. Comparisons are made between the strengths of the tested beams and the predictions of BS 8110, Eurocode 2, MC2010, strut-and-tie modelling and nonlinear finite-element analysis. Significantly, the BS 8110 approach of enhancing shear resistance is found to give better strength predictions than the load reduction methods of Eurocode 2 and MC2010. Accuracy of the non-linear finite-element analysis and strut-and-tie model is broadly comparable, but the former requires calibration, unlike the latter

Shear enhancement in RC beams with multiple point loads

© 2014 Elsevier Ltd.Eurocode 2 (EC2) allows short shear spans in reinforced concrete beams with loads near supports to be designed using either its shear provisions or strut and tie models (STM). In the case of beams with loads applied on their upper side within a distance av≤. 2. d from the edge of supports, EC2 allows the contribution of these loads to the design shear force to be reduced by the multiple β=. av/2. d≥. 0.25 (where d is the beam effective depth). Conversely, the previous UK code BS8110 increases the shear resistance provided by the concrete within 2. d of supports to vc/. β (in which vc is the concrete shear resistance in MPa) up to a maximum shear resistance of 0.8fcu or 5. MPa. The two methods are only equivalent for beams without shear reinforcement and single point loads within 2. d of supports. However, there is a lack of test data to assess the comparative merits of the two methods and STM for beams with multiple point loads within 2d of supports. Therefore, twelve beams were tested with up to two point loads within 2. d of supports. Significantly, BS8110 is found to give better estimates of shear resistance than the EC2 shear provisions. STM gives good predictions of shear resistance if strut strengths are calculated in accordance with the recommendations of the modified compression field theory (MCFT) but some unsafe predictions are obtained when using EC2 strut strengths

Influence of aggregate fracture on shear transfer through cracks in reinforced concrete

Design methods for shear in reinforced concrete structures typically rely upon shear transfer through cracks, which depends upon the crack opening and sliding displacements and the roughness of the crack surfaces. The effectiveness of shear transfer through aggregate interlock is commonly believed to be reduced if the coarse aggregate fractures at cracks, as is frequently the case in high-strength and lightweight aggregate concretes. This paper describes two sets of push-off tests that were carried out to investigate the effect of aggregate fracture on shear transfer through cracks. Marine dredged gravel was used in one set of specimens and limestone in the other. The cracks typically passed around the gravel aggregate but through the limestone aggregate. The experimental results are compared with the predictions of various existing analytical models including those in design codes MC90, Eurocode 2 and ACI-318. The paper also examines the contribution of aggregate interlock to the shear strength of a parallel set of reinforced concrete beams, tested by the authors, which used the same types of aggregate as the push-off specimens

The effect of shear and lap arrangement on reinforcement lap strength

The paper is concerned with the design of tension laps in reinforced concrete structures. The most recent design recommendations for laps are found in fib Model Code 2010 which is likely to influence the next revision of EN-1992. This is of concern to UK industry since laps designed to MC2010 can be significantly longer than laps designed to EN-1992 which UK designers already consider excessive compared with previous UK code requirements. Unlike the previous UK code, BS8110, EN-1992 requires adjacent laps to be offset by 0.3 of the lap length which complicates reinforcement detailing. The paper describes an experimental programme which was undertaken to assess the influence on lap performance of increasing lap length beyond that required for bar yield, shear and staggering of laps. The influence of shear was assessed by comparing the performance of laps of the same length positioned in zones of uniform and varying bending moment. Reinforcement strains were monitored and detailed measurements of crack development and crack widths were obtained with digital image correlation. Results show that very long laps are inefficient with the central half contributing little to force transfer between bars. Shear was found to have no significant influence on lap strength while lapping only 50% of bars at a section increased forces in the lapped bars leading to premature bond failure. Test results are compared with EN-1992 predictions, which are shown to be conservative for the tested laps