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Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.

In this article, finite element method (FEM) based tool stress analysis and breakage prediction are advanced for meso-scale shoulder milling of hardened stainless steel. Rather than using simplified line loads or uniformly distributed loads, the tool stress analyses are conducted with realistic chip load predicted by the previously developed 3D coupled Eulerian-Lagrangian (CEL) FEM model. A reliable distribution of contact forces obtained with a continuous tool-chip contact region is transferred to a static FEM model for tool stress analysis and breakage prediction at each angular location of an end mill. The simulation results are validated through dedicated cutting experiments with short cutting distances under various cutting conditions. The developed FEM models are found to be capable of making accurate predictions of tool breakage. Some key features of feed rate-related tool breakage are identified. It is found that the maximum tensile stress in the tool does not coincide with the maximum resultant cutting force in terms of cutter rotation. The tensile stress in the tool is not positively correlated with the resultant cutting force. In down milling, the maximum tensile stress is found to occur just after the tool starts to contact with the workpiece. It is also observed that the tensile stresses of the tool are much smaller in up milling configuration compared to down milling processes.

In liquid crystal (LC) displays, deriving an optimum resistance level of an LC alignment polyimide (PI) layer is important because of the trade-off between the voltage holding and surface-discharging properties. In particular, to apply a power-saving low-frequency operation scheme to fringe-field switching (FFS) LC modes with negative dielectric LC (n-LC), delicate material engineering is required to avoid surface-charge-dependent image flickering and sticking problems, which severely degrade with lowering operation frequency. Therefore, this paper proposes a photocontrolled variable-resistivity PI layer in order to systematically investigate the voltage holding and discharging properties of the FFS n-LC modes, according to the PI resistivity (ρ) levels. By doping fullerene into the high-ρ PI as the photoexcited charge-generating nanoparticles, the ρ levels of the PI were continuously controllable with a wide tunable range (0.95 × 1015 Ω∙cm to 5.36 × 1013 Ω∙cm) through Ar laser irradiation under the same LC and LC alignment conditions. The frequency-dependent voltage holding and discharge behaviors were analyzed with photocontrolled ρ variation. Thus, the proposed experimental scheme is a feasible approach in PI engineering for a power-saving low-frequency FFS n-LC mode without the image flickering and image sticking issues.

This work presents a novel fabrication method for submicron to micro size textures on flat surfaces using the backside patterned texturing (BPT). The proposed method utilizes the pre-fabricated macro-features on the backside of work material, and thereafter the front side is face turned with a single point diamond tool to generate textured surfaces. Different from existing texturing methods, BPT produces textured surfaces from submicron to micro scale without any external gadgets such as vibration assisted machining or synchronized tool-spindle motion. The miniature feature arises on the diamond turned surface due to the induced residual stresses when the specimen is unleashed from the machine. To demonstrate the efficacy of the method, a series of machining experiments were conducted to fabricate various types of freeform surface textures like water-drop freeform, cylindrical freeform surfaces, etc. The fabrication methodology of different sizes of bumps with precisely controlled surface quality is illustrated. The texture profiles comprising the deformation height from hundreds of nanometer to few micrometers with mirror surface quality were successfully fabricated on the diamond machined surface. The experimental results suggest that the pre-fabricated pattern, workpiece thickness and machining condition play a critical role to determine the final shape and geometry of generated textures. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

In this article, a digitized stress function-based feed rate scheduling algorithm is formulated for the prevention of tool breakage while having an optimum material removal rate in mesoscale rough milling of hardened steel. Instead of setting limits to the cutting forces and material removal rates, the presented method regulates the tool&rsquo, s stresses. A 3D coupled Eulerian-Lagrangian finite element method (FEM) model is used to simulate a 3D chip flow-based stress according to the mesoscale tool&rsquo, s rotation during cutting of hardened steel. Maximum uncut chip thickness and tool engaging angle of the uncut chip is identified as the fundamental driving factors of tool breakage in down milling configuration. Furthermore, a multiple linear regression model is formed to digitize the stress with two major factors for digitized feed scheduling. The optimum feed rates for each segment along the tool path can be obtained through finite element models and a multiple linear regression model. The feed rate scheduling method is validated through cutting experiments with tool paths of linear and arc segments. In a series of experimental validations, the algorithm demonstrated the capability of reducing the machining time while eliminating cutting tool breakages.

We derived a new analytic physical model for describing the temperature-dependent dielectric permittivities ee(T) and eo(T) in anisotropic mesophase molecules of nematic liquid crystals (NLCs). These temperature-dependent dielectric properties of NLCs could be explained by a six-parameter dielectric permittivity model, where the analytic dielectric permittivity curves of ee(T), eo(T), and Δe(T) from the model using the six parameters e*, Ae, Be, (Δe)o, λe, and T* showed excellent agreement with experimental data. The six-parameter dielectric permittivity model was compared to the conventional four-parameter refractive index model. To systematically investigate the temperature-dependent properties of the refractive indices (ne(T) and no(T)) and dielectric permittivities according to the molecular structure of the NLCs, four similar types of fluorinated phenyl bicyclohexane NLCs were selected. Using the presented models, the temperature-dependent behaviour of these four fluorinated NLCs was discussed, according to the molecular length of the alkyl chains and the positions of the fluorine substituents. In particular, two fitting equations for the temperature-dependent properties of threshold voltage and splay elastic LC constant could also be developed using the physical coefficients extracted from the six-parameter dielectric permittivity model, and these equations coincided well with experimental results.

This paper proposes a surface generation model based on a fast and comprehensive time domain solution which simulates regenerative vibrations as well as an interaction of cross edge and flank profiles with the undulation of machined surface. Depending on the profiles of the cutter geometry, the damping forces affect the stability of the milling processes which results in variations of the machined surface profile even under the same cutting conditions. By simulating dynamic cutter center affected by shearing and damping forces, runout, and feed motions, the time-dependent angular positions of the edge profiles can be precisely located, which are used to calculate the dynamic uncut chip thickness and the intersection area of the edge profiles against the surface undulation. As the cutting edges interacts with the machined surfaces, the positions of the edges are extracted and connected to form machined surface profiles. The simulated surface profiles are compared with the experimental ones by considering the damping effect depending on the cross edge and flank profiles.

A nano-texturing method in single point diamond turning using backside patterned workpiece is presented. The back side of the workpiece is pre-machined to first create a pattern. The front side is then diamond turned on an ultra-precision lathe. After machining down to a certain thickness, periodic bumps and valleys that mirror the back side pattern start to appear on the front diamond machined surface. The periodic wavy/bumpy surfaces have nanometer depths, and possess mirror finish. The results suggest that this technique provides an alternative method to create optical features that are conventionally developed using tool-spindle synchronized cutting motions.

Structural SiC (α-type) is believed to be widely applied in hostile environments such as high-temperature, high-corrosive applications in the semiconductor industries due to its superior thermo-physical and mechanical properties. However, the extremely high hardness and brittleness of SiC makes hole drilling difficult by the conventional mechanical drilling (CMD) technique. Laser can be used to drill SiC; but the resultant holes are often tapered and uneven, with tendency for microcracks and thermal damage to occur at the hole entry due to the high thermal shock from the laser. This paper reports on the experimental results of a sequential laser-mechanical drilling (LMD) technique for drilling α-SiC. At first, an Nd:YAG laser was used to drill a series of pilot holes on a 3 mm thick SiC plate. Then a diamond-coated carbide drill was sequentially applied to these holes to obtain desired hole diameter of 0.5 mm. A number of through holes on SiC (aspect ratio: 6) were successfully obtained using this approach. The quality of the drilled holes were assessed in terms of the entrance and exit sizes and conditions, hole taper angle, hole edge shapes, and microcracks. Finally, comparisons of the LMD performances were also made against the holes predrilled by the laser itself and holes of the similar size drilled separately with the CMD technique. The experiment results show that the proposed drilling approach can effectively drill α-SiC ceramics.

This paper describes a new method of microtexture generation in precision machining through self-excited vibrations of a diamond cutting tool. Conventionally, a cutting tool vibration or chatter is detrimental to the quality of the machined surface. In this study, an attempt is made to use the cutting tool's self-excited vibration during a cutting beneficially to generate microtextures. This approach is named as “controlled chatter machining (CCM).” Modal analysis is first performed to study the dynamic behavior of the cutting tool. Turning processes are then conducted by varying the tool holder length as a means to control vibration. The experimental results indicate that the self-excited diamond cutting tool can generate microtextures of various shapes, which depend on the cutting tool shank, cutting speed, feed, and cutting depth. The potential application of this proposed technique is to create microtextures in microchannels and microcavities to be used in mass and heat transfer applications.

The IPS mode is going to lead the large TFT LCD technology for application among other liquid crystal (LC) mode. TFT-LCD TV has begun to rapidly occupy a large portion of the TV display market due to features including thinness, light weight, compactness. Aside from specification of features, the most emphasized aspect has been the contrast ratio for better image quality. To increase contrast ratio, Back Light (B/L) optical configuration, color filter material, sub-pixel structure, rubbing condition optimization, and all that cell characteristics have been investigated. As the results from experimental, applications of a newly developed color resin, new light collimation optical sheet, and surface morphology of finger electrode were the most effective for achieving high contrast ratio.

Gene therapy, the expression of genetic materials with therapeutic activity, has been considered as an encouraging approach to heal life threatening diseases with genetic deficiency. Since direct delivery of the DNA in the absence of a carrier experiences rapid degradation by nuclease and generally exhibits poor cellular uptake, it is one of the main challenges facing today’s gene therapy to establish safe and efficient vectors for delivering therapeutic genes to specific cells. Gene delivery systems investigated so far include viral and non-viral vectors. Viral vectors are intensively used in clinical applications because they efficiently integrate their genetic information into the host chromosomes; however, toxic immunological reactions and recombination events into virulent products still remain as major drawbacks. Non-viral vectors, including liposomes and cationic polymers, display nonimmunogenicity, low acute toxicity, and flexibility to design a carrier with well-defined structures and chemical properties, while they have major problems in transfection efficiency which is limited as only few percentage of that by viral vectors. Among non-viral vectors, poly(L-lysine) (PLL) has been widely used due to the reasonable efficiency and biocompatible nature of peptide bonds in the backbones. However, transfection efficiency of PLL is not so high enough that modification of PLL, such as introduction of targeting ligands or endosomal escape moieties, has been investigated for the use of PLL in vivo. Proline is the only amino acid with a cyclic structure containing a secondary α-amino group and this structural feature gives unique stereochemical and biological properties to the peptides containing proline residue. Water soluble prolinerich peptides have been reported as naturally occurring cellpermeant peptides, which are able to break the cell membrane and deliver the attached carriers into the cells without causing lethal membrane disruption. In this communication, we report the synthesis of lysine and proline based amino acid copolymers. Characterization of the copolymer and the effect of proline content on the formation of copolymer/DNA complex, in vitro cytotoxicity and transfection efficiency were investigated to examine the possibility of the synthesized copolymers as a polymeric gene delivery carrier.

Mechanical micro-end milling helps in creating complex three dimensional structures without a restriction on work-piece material. So far, the surface quality assessment achieved in mechanical micro-end milling has mainly focused on the bottom milled surface of the machined part. In this paper, surface quality of the side-wall created by mechanical micro-end milling on stainless steel (SUS 304) and aluminum alloy (Al 6061) is studied. The surface quality thus achieved is assessed by considering three surface assessment factors namely average surface roughness, form error and burr height under various cutting conditions. The cutting parameters varied for machining were spindle speed, feed/tooth, radial depth of cut, and axial depth of cut. The optimal cutting conditions were evaluated for the three surface evaluation factors according to the material and cutting conditions. The surface roughness values were found to have a turning point at a particular spindle speed for both the materials and were affected by spindle speed, feed/tooth and axial depth of cut. The form error was observed to be lesser for higher axial depth of cut due to an increase in the relative tool stiffness. The burr formation was influenced mainly by feed/tooth with an increase in burr height due to the plowing effect at lowest feed rate and pushed uncut chip at highest feed rate.

So far, the industrial application of ultrasonic vibration assistance has been successful in continuous machining process such as turning process where ultrasonic vibration velocity is much higher than cutting velocity. Recently, the application of ultrasonic vibration assistance in milling process has been experimentally investigated for feed and cross-feed directions. This paper focuses on the effect of ultrasonic vibration assistance in spindle axial direction for improvement of machined surface. With the designed ultrasonic vibration assisted milling process with 39.7 kHz and a few micro-metre amplitudes, workpiece vibrates along spindle axial direction while different RPMs and feed rates are applied. The axial directional vibration assistance acts as additional cutting motion which further reduces the leftover surface error. Experimental results validate that surface roughness can be improved from 20% to 65% for the tested conditions. The simulation results have been proposed to validate the surface quality improvement phenomenon with the help of the axial ultrasonic vibration assistance.