Intrinsic Nature Research Articles

PH-Sensitive Fluorescence Emission of Boron/Nitrogen Co-Doped Carbon Quantum Dots

Carbon quantum dots (CQDs) with their strong photoluminescence (PL) activity, high biocompatibility, robust stability, low cytotoxicity, and flexible surface structures have been employed in many fields including chemical sensing, biosensing, photocatalyst, energy storage, and biomedical applications. Of note, CQDs present an intrinsic pH-sensitive PL nature indicating their intense potential for pH-mediated sensing and imaging. Despite the numerous studies performed in the last two decades, the pH-sensitive PL mechanism of CQDs is still under debate and must be clarified to overcome the limitations in practical applications. Therefore, in this report, we performed a systematical study to determine the pH-sensitive PL nature of boron/nitrogen co-doped CQDs (B/N CQDs). In the first part, B/N CQDs with a strong blue emission were fabricated via a hydrothermal synthesis procedure. B/N-CQDs showed a strong blue PL emission with high quantum yield and excitation-dependent nature. Under the low pH conditions (pH 3), B/N-CQDs exhibited a robust green fluorescence emission with a significant red-shift (48 nm) and the loss of the excitation-dependent nature. The change in PL nature originated from the protonation of surface groups, a decrease in negative surface charge (from −20.6 to −1.23 eV), and finally, aggregation of the nanostructure (the size of CQDs from 4.8 to 7.5 nm). However, in the case of alkaline conditions, the deprotonation surface groups significantly enhanced the surface charge and led to the emergence of a negative ‘protective shell’ with a zeta potential of −71.3 eV. In a high pH medium (pH 13), PL spectra showed the loss of excitation-dependent features and a red-shift (35 nm) in emission peak maxima with lower intensity. This report provides significant progress in the clarification of the pH-sensitive PL mechanism of CQDs. We envision that the proposed CQDs would provide unique opportunities in the fabrication of novel pH sensor systems and fluorescence imaging where a wide range of pH sensitivity is required.

Optimal Preference Satisfaction for Conflict-Free Joint Decisions

When presented with multiple choices, we all have a preference; we may suffer loss because of conflicts arising from identical selections made by other people if we insist on satisfying only our preferences. Such a scenario is applicable when a choice cannot be divided into multiple pieces owing to the intrinsic nature of the resource. Earlier studies examined how to conduct fair joint decision-making while avoiding decision conflicts in terms of game theory when multiple players have their own deterministic preference profiles. However, probabilistic preferences appear naturally in relation to the stochastic decision-making of humans, and therefore, we theoretically derive conflict-free joint decision-making that satisfies the probabilistic preferences of all individual players. To this end, we mathematically prove conditions wherein the deviation of the resultant chance of obtaining each choice from the individual preference profile (loss) becomes zero; i.e., the satisfaction of all players is appreciated while avoiding conflicts. Further, even in scenarios where zero-loss conflict-free joint decision-making is unachievable, we present approaches to derive joint decision-making that can accomplish the theoretical minimum loss while ensuring conflict-free choices. Numerical demonstrations are presented with several benchmarks.

Improved Global-Local Method for Ultrasonic Guided Wave Scattering Predictions in Composite Waveguides and Defects

Abstract As structures increase in complexity, in the use of high-performing materials and designs, their health assessment becomes increasingly challenging. Ultrasonic guided waves (UGWs) have shown to be very promising in the inspection of large (i.e. aerospace components) attenuating (i.e. composite materials) structures and have been successfully employed for damage detection in a variety of fields. The intrinsic complex nature of UGWs, due to their dispersive behavior, combined with the structural complexity of the applications, though, makes the interpretation of UGW inspections very challenging. Numerical simulations of UGW propagation become crucial to this end and have been addressed with fully numerical, semi-analytical and hybrid approaches. The capability of predicting UGW scattering can inform experimental testing in optimizing the sensitivity of UGW inspections to specific waveguides and defects, and in interpreting the acquired data for the non-destructive identification and quantification of damages. In this work, an improved computational tool for UGW scattering predictions is presented. The approach relies on the Global-Local method and leverages the efficiency of the semi-analytical finite element (SAFE) method and the parallelized implementation of the coupled solution. 2D applications of the Global-Local approach for UGW scattering predictions in composite structures over a wide range of frequencies will be presented, together with the demonstration of the improved computational performance. The computational efficiency promises feasible and reliable UGWs predictions in multi-layered complex assemblies and different damage scenarios, and enables virtual UGWs inspections and future integration in NDE testing.

Multiferroic and ferroelectric phases revealed in 2D Ti3C2Tx MXene film for high performance resistive data storage devices

Multiferroic materials, showing simultaneous ferroelectric and ferromagnetic orders, are considered to be promising candidates for future data storage technology however, the multiferroic phenomenon in two-dimensional (2D) materials is rarely observed. We report a simple approach to observe frequency-dependent ferroelectricity and multiferroicity in 2D Ti3C2Tx MXene film at room-temperature. To study the frequency and poling effect on ferroelectricity, we performed electric polarization vs. electric field (P-E) measurement at different frequencies, measured under zero and non-zero static magnetic fields. The results not only indicate a clear frequency dependence of electric domains owing to varying time relaxation during reversal dynamic but also showed magnetic field control of electric polarization thus, confirmed the presence of strong magneto-electric (ME) coupling at room-temperature. The existence of ME coupling was attributed to the coupling between disordered electric dipoles with local spin moments as well reduced dielectric loss after heat-treatment. Moreover, the ferroelectric Ti3C2Tx MXene film was employed as an active layer within the resistive data storage device that showed a stable switching behavior along with improved on/off ratio in comparison to non-ferroelectric Ti3C2Tx active layer. The unique multiferroic behavior along with ferroelectric-tuned data storage devices reported here, will help understand the intrinsic nature of 2D materials and will advance the 2D ferroelectric data storage industry.

Children’s evaluations of scarce (and abundant) resources: When does the “why” matter?

Determining value is key to reasoning about resources. Scarcity has been proposed as an easily accessible cue children can use to judge value, but prior studies have yielded inconsistent results, raising the question of whether children may be sensitive to the reasons underlying why items are scarce. We tested whether children (N = 145, ages 4–9 years) used object scarcity as a cue to value and, if so, whether they considered the reasons underlying scarcity (or abundance, as a comparison). A preference for scarcity increased with age, and children were more likely to select scarce objects when the reasons for their availability were popularity and intrinsic nature. Together, our findings suggest that researchers should consider how resource availability is framed when exploring children’s evaluations of resources.

Baculoviral COVID-19 Delta DNA vaccine cross-protects against SARS-CoV2 variants in K18-ACE2 transgenic mice.

After severe acute respiratory syndrome coronavirus-2 (SARS-CoV2) made the world tremble with a global pandemic, SARS-CoV2 vaccines were developed. However, due to the coronavirus's intrinsic nature, new variants emerged, such as Delta and Omicron, refractory to the vaccines derived using the original Wuhan strain. We developed an HERV-enveloped recombinant baculoviral DNA vaccine against SARS-CoV2 (AcHERV-COVID19S). A non-replicating recombinant baculovirus that delivers the SARS-CoV2 spike gene showed a protective effect against the homologous challenge in a K18-hACE2 Tg mice model; however, it offered only a 50% survival rate against the SARS-CoV2 Delta variant. Therefore, we further developed the AcHERV-COVID19 Delta vaccine (AcHERV-COVID19D). The AcHERV-COVID19D induced higher neutralizing antibodies against the Delta variant than the prototype or Omicron variant. On the other hand, cellular immunity was similarly high for all three SARS-CoV2 viruses. Cross-protection experiments revealed that mice vaccinated with the AcHERV-COVID19D showed 100% survival upon challenge with Delta and Omicron variants and 71.4% survival against prototype SARS-CoV2. These results support the potential of the viral vector vaccine, AcHERV-COVID19D, in preventing the spread of coronavirus variants such as Omicron and SARS-CoV2 variants.

High throughput single-molecule technology.

Single-molecule fluorescence resonance energy transfer (smFRET) has been an indispensable tool to probe the structure of biomolecules on the nanometer scale. The intrinsic low-throughput nature of single molecule observation has, however, prevented the technique to be used to study a large pool of samples. Here, we present a highly parallel smFRET measurement scheme by combining the conventional smFRET measurement with a next generation sequencing method. A library of DNA molecules, each of which carries one of the thousands of different sequences of interest, was prepared and measured in a one pot recipe. By using this high-throughput approach, we experimentally determine the sequence-dependent end-to-end distance of single stranded DNA in various native aqueous buffer conditions.

Distinct relaxation mechanism at room temperature in metallic glass

How glasses relax at room temperature is still a great challenge for both experimental and simulation studies due to the extremely long relaxation time-scale. Here, by employing a modified molecular dynamics simulation technique, we extend the quantitative measurement of relaxation process of metallic glasses to room temperature. Both energy relaxation and dynamics, at low temperatures, follow a stretched exponential decay with a characteristic stretching exponent β = 3/7, which is distinct from that of supercooled liquid. Such aging dynamics originates from the release of energy, an intrinsic nature of out-of-equilibrium system, and manifests itself as the elimination of defects through localized atomic strains. This finding is also supported by long-time stress-relaxation experiments of various metallic glasses, confirming its validity and universality. Here, we show that the distinct relaxation mechanism can be regarded as a direct indicator of glass transition from a dynamic perspective.

Fractal codimension of nilpotent contact points in two-dimensional slow-fast systems

In this paper we introduce the notion of fractal codimension of a nilpotent contact point p, for λ=λ0, in smooth planar slow–fast systems Xϵ,λ when the contact order nλ0(p) of p is even, the singularity order sλ0(p) of p is odd and p has finite slow divergence, i.e., sλ0(p)≤2(nλ0(p)−1). The fractal codimension of p is a generalization of the “traditional” codimension of a slow-fast Hopf point of Liénard type, introduced in (Dumortier and Roussarie (2009) [7]), and it is intrinsically defined, i.e., it can be directly computed without the need to first bring the system into its normal form. The intrinsic nature of the notion of fractal codimension stems from the Minkowski dimension of fractal sequences of points, defined near p using the so-called entry-exit relation, and slow divergence integral. We apply our method to a slow-fast Hopf point and read its degeneracy (i.e., the first nonzero Lyapunov quantity) as well as the number of limit cycles near such a Hopf point directly from its fractal codimension. We demonstrate our results numerically on some interesting examples by using a simple formula for computation of the fractal codimension.

Using Metamorphic Testing to Improve the Quality of Tags in OpenStreetMap

We present a metamorphic testing approach to validate the information included in OpenStreetMap, a collaborative effort to produce a free map of the world. We focus on the quality of the tags storing the information about the elements of the map. We identified metamorphic relations with the potential to detect different types of tagging errors. In particular, we carefully designed mechanisms to automatically generate <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">follow-up inputs</i> , a fundamental component in the successful application of a metamorphic testing approach. The intrinsic nature of automatically analysing tags implies that we will detect real errors but some false positives as well. In order to obtain a good trade-off between real errors and false positives, we introduce <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">thresholds</i> . Our MRs will raise an error associated with a certain value if, depending on the nature of the MR, we have a certain number of elements (not) fulfilling a given condition. In order to evaluate the goodness and versatility of our framework, we chose four cities in different continents with the goal of analysing very heterogeneous contributors adding information in different languages. The application of this framework to the analysis of the chosen cities revealed errors in all of them and in all the considered categories. In addition, around 66% of the errors found by our MRs in the analysed areas have not been previously reported by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Osmose</i> , the de facto standard OSM error checker.

Design and Optimization of a Boost Interface for Magnetostrictive Energy Harvesting

Magnetostrictive alloys are very promising for Vibration Energy Harvesting applications to supply power to Wireless Sensor Network (WSN) and Internet of Things (IoT) devices, especially because of their intrinsic robustness. Typically, vibration energy sources are random in nature, usually providing exploitable voltages much lower than the electronic standards 1.6, 3.3 and 5 V. Therefore, a Power Electronic Interface (PEI) is needed to improve the conversion to DC output voltage from AC input over a wide range of frequencies and amplitudes. Very few or no conversion techniques are available for magnetostrictive devices, although several have been presented over the years for other smart materials, such as piezoelectrics. For example, hybrid buck–boost converters for piezoelectrics use one or more external inductors with a high-frequency switching technique. However, because of the intrinsic nature of harvesters based on magnetostrictive materials, such energy conversion techniques are proved to be neither efficient nor applicable. An improved AC–DC boost converter seems very promising for our purpose instead. The key feature is represented by the direct exploitation of the active harvester coil as a storage element of the boost circuit, without using other passive inductors as in other switching methods. Experimental tests of such a converter, driven with a real-time operating Arduino controller to detect the polarity of the input voltage, are presented with the aim to assess the potentiality of the scheme with both sinusoidal and impulse-like inputs. Simulations have been performed with LTspice, and the performance and efficiency have been compared with other energy conversion techniques.

Feasibility analysis of a wave powered autonomous underwater vehicle

An autonomous underwater vehicle (AUV) powered by renewable energy would enable it to extend endurance and forward presence with no logistics tail for fuel. A wave powered concept AUV is proposed and analyzed for this purpose. The AUV is in a torpedo shape and employs a fully hull-encapsulated mass-spring-damper system as its power take-off (PTO) unit. While in the recharging mode, the AUV surfaces and converts wave energy for electricity. Considering the AUV of a common size, our analysis indicates that resonance of the AUV and/or its PTO, to be excited by ocean waves, becomes impossible in typical ocean conditions. Then by making the AUV almost fully submerged to eliminate its intrinsic resonant nature and by applying some realistic constraints, the AUV’s power generation capability without resonance is characterized. Our simulation results show that the AUV is still capable of generating power at a decent level in a wide range of the wave frequency upon optimization. Some guiding principles have been established for AUV design of this type.

Interplay of dopant and polarons in trifunctional semimagnetic semiconductor for supercapacitor applications: Local structure and electronic structure investigations

Diluted magnetic semiconductors (DMS), fractionally doped by magnetic ions, are peculiar, and a single semiconductor effectively contributes to charge and spin properties for various applications, including data and energy storage. Here, a low temperature assisted co-precipitation method produced a series of Co doped ZnO nanoparticles. After structural characterizations, zinc vacancy defects were realized via the hyperfine signal with g ∼ 1.96 (spin resonance) and the emissions (photoluminescence) at 410 and 465 nm. The synergistic effect among the Co dopant, native, and polaron defects makes the system exhibit enhanced polarization and electrochemical reactions. Magnetization hysteresis curves demonstrate the intrinsic magnetic nature of the materials that are essential for realizing useful polaronic states. As an anode material, 20 at. % Co doped electrode has a higher specific capacitance and superior cycling stability with charge retention above 98 %, which is much higher than the other ZnO-based electrodes and is associated with the polaron-assisted rapid redox reactions. These polarons also stimulate a ferroelectric polarization of 0.28 μC cm−2 via local lattice distortion. All these findings confirm that the Co doped semi-magnetic ZnO system with multifunctionality is adequate for employment in device fabrication as a low-cost, highly stable material.

Lithiophilic hyperbranched Cu nanostructure for stable Li metal anodes

AbstractPorous copper (Cu) current collectors are regarded as a promising host for stabilizing lithium (Li) metal anodes but suffer from uncontrollable Li metal deposition due to the intrinsic lithiophobic nature of Cu. This study proposes a vertically aligned Cu host with hyperbranched CuxO nanostructure to provide lithiophilic nucleation sites for homogeneous Li metal deposition. Specifically, the vertically aligned Cu nanostructure dramatically reduces the local current density and brings homogeneous Li‐ion flux. The lithiophilic hyperbranched CuxO nanostructure with a low nucleation barrier could induce homogeneous Li nucleation and growth. As a result, the Cu@CuxO nanostructured host exhibits a low nucleation overpotential of 44.3 mV and achieves highly electrochemical reversibility with high Coulombic efficiency of 98.33% in a half‐cell. The Cu@CuxO nanostructured electrode is capable of working under different current densities varying from 0.5 to 5 mA/cm2 in a symmetric cell. The assembled full cell coupling of the Li/Cu@CuxO composite anode with the LiFePO4 cathode manifests stable long‐term cycling life at 1 C. This study elaborates on the synergistic effect of electrode structure design and interfacial chemistry modification to regulate the Li deposition/dissolution behavior, thus exhibiting remarkable electrochemical performances for next‐generation Li‐metal batteries.

Seed amplification assay for the detection of pathologic alpha-synuclein aggregates in cerebrospinal fluid.

Misfolded alpha-synuclein (αSyn) aggregates are a hallmark event in Parkinson's disease (PD) and other synucleinopathies. Recently, αSyn seed amplification assays (αSyn-SAAs) have shown promise as a test for biochemical diagnosis of synucleinopathies. αSyn-SAAs use the intrinsic self-replicative nature of misfolded αSyn aggregates (seeds) to multiply them in vitro. In these assays, αSyn seeds circulating in biological fluids are amplified by a cyclical process that includes aggregate fragmentation into smaller self-propagating seeds, followed by elongation at the expense of recombinant αSyn (rec-αSyn). Amplification of the seeds allows detection by fluorescent dyes specific for amyloids, such as thioflavin T. Several αSyn-SAA reports have been published in the past under the names 'protein misfolding cyclic amplification' (αSyn-PMCA) and 'real-time quaking-induced conversion'. Here, we describe a protocol for αSyn-SAA, originally reported as αSyn-PMCA, which allows detection of αSyn aggregates in cerebrospinal fluid samples from patients affected by PD, dementia with Lewy bodies or multiple-system atrophy (MSA). Moreover, this αSyn-SAA can differentiate αSyn aggregates from patients with PD versus those from patients with MSA, even in retrospective samples from patients with pure autonomic failure who later developed PD or MSA. We also describe modifications to the original protocol introduced to develop an optimized version of the assay. The optimized version shortens the assay length, decreases the amount of rec-αSyn required and reduces the number of inconclusive results. The protocol has a hands-on time of ~2 h per 96-well plate and can be performed by personnel trained to perform basic experiments with specimens of human origin.

Insight into the Heavy Atom Effect Induced by Environmental Heavy Atoms for Gadolinium-Labeled Hematoporphyrin Monomethyl Ether

Environmental heavy atoms can further enhance the room temperature phosphorescence (RTP) emissions of gadolinium-labeled hematoporphyrin monomethyl ether (Gd-HMME) by way of the external heavy atom effect (HAE). However, the macroscopic phosphorescence intensity covered the intrinsic effect of the environmental heavy atoms. In this study, a method of separating the external HAE from the total is performed, and a quantity to describe the intrinsic nature of external HAE is defined. The environmental Gd3+ concentration evolution of the phosphorescent transition rate (kP) is obtained by correlated absorption, emission, and time-resolved spectroscopy. The kP increases linearly with environmental Gd3+ concentration, while the intercept kP0 coincides with that of the internal HAE. The slope κ could be calculated, and it is a quantity free of the Gd3+ concentration and only relies on the type of environmental heavy atoms. In addition, the environmental Lu3+ exhibits similar functionality to Gd3+ in external HAE. Environmental Pd2+ quenches the phosphorescence intensity macroscopically, while it enhances the HAE intrinsically. Our method provides an alternative insight into the intrinsic nature of environmental heavy atoms.

Rapid microwave fabrication of new nanocomposites based on Tb-Co-O nanostructures and their application as photocatalysts under UV/Visible light for removal of organic pollutants in water

Herein, a novel and stable Tb-Co-O nanocomposite photocatalyst is fabricated through a one-pot microwave route for 2 min (600 W, 20 s On, 60 s Off), which is introduced as UV/Visible light active catalysts in wastewater treatment. Employing various combined parameters of Tb:Co ratio, pH adjustment agents, chemical and natural templates, the resulting nanostructures displayed the intrinsic structure nature, narrow size distribution, good optical properties, and excellent photocatalytic efficiency. The formation of Tb-Co-O nanostructures and their features were verified via X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and ultraviolet–visible diffuse reflection spectroscopy (DRS) technologies. Detailed physic-chemical measurements exhibited that all as-prepared nano-photocatalysts possesses both cubic (TbO1.81) and orthorhombic (TbCoO3) crystal structures. Furthermore, optical characterization by DRS developed light-sensitive channelization with band-gap energies at approximately 2.95 and 3.20 eV for Tb-Co-O nanocomposites. Finally, the photocatalytic studies of the resulting nanocomposites were compared by determining the elimination of Erythrosine (EY), Acid Violet 7 (AV7), and Acid Black 1(AB1) under UV and Visble light illumination. As a results, the TbCoO3/TbO1.81 nanocomposties with molar ratio of 1:3 (Tb:Co), valerian distillate as natural directing agent and ethylenediamine (en) as alkaline template yielded the optimum degradation percentage of 88 % for EY dye under the reaction condition of 10 ppm dye concentration in the presence UV light toward other pathways. Also, we proposed a mechanistic insight of photodegradation based on the radical scavengers, which revealed that h+ did the important role, and OH• and •O2− represented an irrelevant role in the degradation of EY. The current study offer an effective way for development of a high-efficiency Tb-Co-O photocatalyst in eliminating dyes from contaminant water.

Assessment of the benefits of 3D printing of advanced thermosetting composites using process simulation and numerical optimisation

3D printing of continuous fibre reinforced thermosetting matrix composites is set to revolutionise composite manufacturing practice. The potential of curing additively is anticipated to bring significant improvement in terms of increasing process speed, producing geometries that are inaccessible with current processing routes and eliminating detrimental exothermic effects during the process. This study presents a comparison between the curing stage of the 3D printing and standard batch processing for carbon fibre/epoxy components of varying thickness and size. An optimisation methodology links simulation of the cure using Finite Element solver Abaqus with a Genetic Algorithm capable of dealing with multi-objective problems. Optimal cure cycles to minimise both process time and temperature overshoot in 3D printing and batch processing are identified and the optimal trade-offs compared. The results highlight that temperature overshoot reduction up to 85 % is possible and that the intrinsic additive nature of the 3D printing allows eliminating the dependence of temperature overshoot on thicknesses and producing components with thicknesses that are very difficult to manufacture conventionally. A simplified procedure for the estimation of 3D printing process duration is proposed based on the results of finite element simulation. This is used for exploration of the limits of the process with respect to part size and for a generic comparison of process applicability against batch processing. The analysis shows that 3D printing is highly advantageous for small components, is efficient for mid-size components and can – on the basis of its scalability – offer a feasible route for producing large and very large components.

MoS2-PVDF/PDMS based flexible hybrid piezo-triboelectric nanogenerator for harvesting mechanical energy

A green energy generating device, which can harvest the energy from ambient sources present in our surroundings to fulfill the energy needs of future technologies without polluting our surrounding are becoming more in demand. To extract electrical energy from mechanical vibrations, nanogenerators based on piezoelectric and triboelectric phenomena are being explored recently. When a piezoelectric material is used as one of the two components in the triboelectric nanogenerator, these two effects can be coupled. It is possible to further improve the output efficiency of the nanogenerator by integrating these two effects to form a hybrid nanogenerator. In this work, we have fabricated the different piezoelectric energy harvesters based on MoS2-PVDF materials by varying the weight percentage (0%, 2%, 5% and 7 wt%) of MoS2. The piezoelectric output of the PVDF was found to be increased due to the incorporation of MoS2. In comparison to bare PVDF film, the piezoelectric nanogenerator based on 7 wt% of MoS2 as filler in PVDF shows nearly 2-fold increase in output voltage from 9.4 V to 18.0 V. Further, a piezo-tribo based hybrid nanogenerator (HNG) is fabricated by integrating the MoS2-PVDF film having highest piezoelectric output with PDMS thin film as two layers required for the HNG device. It may be highlighted that the introduction of MoS2 in PVDF matrix not only enhanced the piezoelectric output which may be attributed to the intrinsic piezoelectric nature of MoS2 and enhanced β-phase crystallization, the triboelectric charge generation also gets enhanced. The enhanced hybrid output performance may be attributed to its increased dielectric property and surface roughness. The MoS2-PVDF/PDMS based HNG provides an output voltage of 35.3 V, which is 1.6 times greater than the HNG based on bare PVDF/PDMS layers. The generated output power successfully lit up 21 LED bulbs by the application of a small mechanical force generated by finger tapping and could charge a 10 μF capacitor to ∼9 V in ∼400 s. The present findings suggest that by hybridizing different device mechanism in a single device the energy harvesting performance of resulting HNG can be enhanced, making it possible to drive smart wearable electronic devices.

Spherical indentation of polycrystalline cubic boron nitride (PcBN): Contact damage evolution with increasing load and microstructural effects

The extremely severe conditions at which cutting tools are exposed in hard machining operations require tool materials with outstanding mechanical properties. Excellent combinations of high hardness and fracture toughness are aimed to enhance wear and contact damage resistance; and thus, reliability of the tool. Here, spherical indentation is used to induce controlled damage, under monotonic loads, and to assess the resulting damage scenario of polycrystalline cubic boron nitride (PcBN). In doing so, two grades with different content of cBN particles as well as chemical nature of the binder are investigated. Systematic and detailed inspection of residual imprints, by means of optical and scanning electron microscopy, is conducted to document critical loads for damage emergence and evolution through different stages, as a function of increasing load and microstructural assemblage. It is found that H-PcBN grade, with higher content of cBN particles and a metallic binder, exhibits higher resistance to crack nucleation as well as a more gradual transition between distinct damage scenarios. This enhanced response is linked to the concomitant increase of hardness and fracture toughness observed for that grade, as compared to the L-PcBN one, consisting of a significantly lower amount of superhard particles bound by a binder of ceramic nature. In-depth inspection of interaction between fissures and microstructure indicates the existence of four different crack paths for both PcBN grades - transgranular through cBN particles, across the binder phase, and intergranular along either cBN/cBN or cBN/binder interfaces. Relative frequency of these crack paths as well as operative toughening mechanisms define the effective crack growth resistance of each PcBN grade. In this regard, a higher fracture toughness is assessed for H-PcBN, as a result of crack bridging by metallic ductile ligaments together with the effective barrier-like role played by substructural features within cBN particles. Meanwhile, L-PcBN grade shows a rather low crack growth resistance, in direct correlation with the intrinsic brittle nature of the contiguous ceramic binder through which crack extension predominantly takes place. These relative differences in resistance to crack nucleation and growth become key for understanding the enhanced contact damage behaviour determined for the grade with higher content of cBN particles in this study.