Gradient Materials Research Articles

Diversity in Burned Pinyon–Juniper Woodlands Across Fire and Soil Parent Material Gradients

Co-varying disturbance and environmental gradients can shape vegetation dynamics and increase the diversity of plant communities and their features. Pinyon–juniper woodlands are widespread in semi-arid climates of western North America, encompassing extensive environmental gradients, and a knowledge gap is how the diversity in features of these communities changes across co-varying gradients in fire history and soil. In pinyon–juniper communities spanning soil parent materials (basalt, limestone) and recent fire histories (0–4 prescribed fires or managed wildfires and 5–43 years since fire) in Grand Canyon-Parashant National Monument (Arizona, USA), we examined variation at 25 sites in three categories of plant community features including fuels, tree structure, and understory vegetation. Based on ordinations, canonical correlation analysis, and permutation tests, plant community features varied primarily with the number of fires, soil coarseness and chemistry, and additionally with tree structure for understory vegetation. Fire and soil variables accounted for 33% of the variance in fuels and tree structure, and together with tree structure, 56% of the variance in understories. The cover of the non-native annual Bromus tectorum was higher where fires had occurred more recently. In turn, B. tectorum was positively associated with the percentage of dead trees and negatively associated with native forb species richness. Based on a dendroecological analysis of 127 Pinus monophylla and Juniperus osteosperma trees, only 18% of trees presently around our study sites originated before the 1870s (Euro-American settlement) and <2% originated before the 1820s. Increasing contemporary fire activity facilitated by the National Park Service since the 1980s corresponded with increasing tree mortality and open-structured stands, apparently more closely resembling pre-settlement conditions. Using physical geography, such as soil parent material, as a landscape template shows promise for (i) incorporating diversity in long-term community change serving as a baseline for vegetation management, (ii) customizing applying treatments to unique conditions on different soil types, and (iii) benchmarking monitoring metrics of vegetation management effectiveness to levels scaled to biophysical variation across the landscape.

Analytical solution of the electro-magneto-elasto-hydrothermal coupling problem of rotating functionally graded piezoelectric/piezomagnetic hollow spheres under a hydrothermal environment

Functionally graded piezoelectric/piezomagnetic (FGPEPM) materials in a hygrothermal environment exhibit complex multi-physical field coupling effects when subjected to various loads such as mechanical, electric, magnetic, temperature, and moisture concentration. In this paper, a theoretical analysis of the electro-magneto-elasto-hygrothermal (EMEHT) coupling effects for a rotating FGPEPM hollow sphere is presented. By assuming that the properties of hygrothermo, electro-magneto-elastic, hygrothermal expansion, and density of the FGPEPM sphere obey different power laws along the thickness direction, the distributions of temperature, moisture, magnetic potential, radial displacement, electric potential, and stresses within the rotating FGPEPM sphere are determined. The present theoretical solutions are verified with existing analytical solutions for some related simplified problems. The FGPEPM hollow sphere exhibits complex multi-physical field coupling effects, containing positive/inverse piezoelectric, positive/inverse piezomagnetic, positive/inverse magnetoelectric, thermal/moist stress, pyroelectric/pyromagnetic, and hydroelectric/hydromagnetic effects under a hygrothermal environment. The influence of gradient parameters and other material properties on these EMEHT coupling effects is also investigated for FGPEPM hollow spheres under various types of loads. This theoretical research provides some reference for the optimal design for this type of functionally gradient multifield coupling material.

Study of energy absorption and recovery characteristics of shape memory material zero Poisson’s ratio gradient structures

The integration of smart materials with metamaterials offers significant research value in additive manufacturing. This paper proposes a novel approach combining shape memory polymers (SMP) with zero Poisson’s ratio (ZPR) metamaterials to create better space-filling, lightweight, and high-energy absorbing materials. A cellular structure with three deformation processes was designed by adjusting the rib arm lengths of the ZPR lattice AuxHex, forming a 3D gradient structure. The cellular units were also prepared using the dual-nozzle fusion filament printing technique, and the energy absorption and shape recovery properties of various hierarchical gradient structures, compared to the uniform structure, were simulated and analyzed using the viscoelastic intrinsic model. The results indicate that all gradient structures exhibit superior energy absorption capabilities compared to uniform materials, with the energy absorption performance of the optimal gradient material being 44.32% higher than that of the nongradient material, and they also enhance the shape recovery performance to some extent. Furthermore, it was observed that when a lattice layer, which avoids internal rib contact throughout the compression process, is positioned in the middle layer, it harmonizes the deformation of the upper and lower layers, thereby improving overall stability. This configuration enhances both energy absorption and partial shape recovery performance, offering a novel approach for the design of smart material assemblies with graded metamaterial structures. This study provides a new perspective and methodology for the future design of supergradient smart material collections.

Cross-scale backscattered-electron imaging and its application in revealing the microstructure-property relations

Scanning electron microscopy (SEM) has been widely utilized in the field of materials science due to its significant advantages, such as large depth of field, wide field of view, and excellent stereoscopic imaging. However, at high magnification, the limited imaging range in SEM cannot cover all the possible inhomogeneous microstructures. In this research, we propose a novel approach for generating high-resolution SEM images across multiple scales, enabling a single image to capture physical dimensions at the centimeter level while preserving submicron-level details. We adopted the SEM imaging on the AlCoCrFeNi2.1 eutectic high entropy alloy as an example. SEM videos and image stitching are combined to fulfill this goal, and the video-extracted low-definition images are clarified by a well-trained denoising model. Furthermore, we segment the macroscopic image of the eutectic high entropy alloy, and the area of various microstructures is distinguished. By combining the segmentation results and hardness experiments, we found that the hardness is positively correlated with the content of the body-centered cubic phase and negatively correlated with the lamella width. The whole procedure is also applied to a thermoelectric gradient material (PbSe-SrSe). Our work provides a feasible solution to generate macroscopic images based on SEM for further analysis of the correlations between the microstructures and spatial distribution, and can be widely applied to other types of microscopes.

Symplectic contact analysis of a finite-sized horizontally graded magneto-electro-elastic plane

Material characterization using high-throughput testing (HTT) techniques has shown great promise in expediting the advancement of high-performance materials. A key step in HTT is the adoption of functionally graded specimens, whose material inhomogeneity, however, imposes a great challenge for analytically evaluating its responsive behaviour under external loads. This work establishes the first symplectic framework for contact analysis of a finite-sized magneto-electro-elastic plane with an exponential material gradient along the horizontal direction. The governing equations are first represented in matrix form in a state space, with the full state vector defined and the operator matrix derived. The operator matrix is found to be distinct from the Hamiltonian operator matrix in the case of a homogeneous medium. With all the eigen-solutions obtained, the Hamiltonian mixed energy variational principle is then introduced to derive the coefficients in the symplectic expansion. The developed analytical solution not only exhibits asymmetry properties but also converges rapidly. The analytical results are validated through comparison with the finite element simulations. The analytical platform established here would work as an effective theoretical basis for the subsequent development of quantitative HTT techniques.

Comparative Analysis of Soil Fertility in Sandy Soils along a Toposequence Transect in Sandai, West Kalimantan

Addressing food crises and land degradation potential requires multisteps agricultural development, including soil fertility assessment. This study evaluates sandy soil fertility status along a toposequence transect in Sandai District, Ketapang Regency, West Kalimantan. Seven observation points (TP1, TP2, TP3, TK1, TK2, TK3, and TK4) were established, with soil samples collected from depths of 0-30 cm and 30-60 cm. Soil fertility assessment was conducted using three criteria: Five Major Soil Chemical Properties (FMSCP), Basic Cation Saturation Ratio (BCSR), and Sufficiency Level of Available Nutrients (SLAN). The FMSCP method exhibited low to very low fertility statuses, while the BCSR and SLAN methods revealed significant variations in soil fertility, ranging from deficient to excessive. Both the BCSR and SLAN methods demonstrated strong relationships with soil parent material and slope gradient, as evaluated through a multivariate approach. The BCSR method indicated deficient to balanced status at all profile points, whereas dominant balanced to excessive statuses were observed at all fertility points. The SLAN national criteria predominantly indicated deficient status for calcium (Ca), magnesium (Mg), and potassium (K), while the international criteria identified K deficiency only. This study served as forums to discuss fertility assessment in tropical soils. Also, recommends the potential for implementing the FMSCP criteria-based soil fertility assessment method for tropical Indonesian sandy soils and consider the involvement of balancing ratios in a more comprehensive soil fertility evaluation approach.

Longitudinal thermoelastic guided waves in functionally graded hollow cylinders with Green–Naghdi thermoelastic theory

Abstract At present, functional gradient material (FGM) pipelines are widely used in high-temperature environments, and the need for online inspection of such pipelines is becoming more and more urgent. Ultrasonic guided wave is undoubtedly one of the most promising methods for detection, but research on the propagation characteristics of thermoelastic guided wave in high-temperature FGM pipelines is still limited. In this paper, based on Green–Naghdi thermoelastic theory, a theoretical model of longitudinal thermoelastic guided wave in a hollow cylinder of FGM considering temperature effect is established by using the Voigt model, and the governing equations of thermoelastic guided wave are solved by the Legendre series method. The dispersion, displacement, and temperature distribution curves of guided waves in a hollow cylinder of FGM are plotted. The convergence of the method and the influence of the coupling of thermodynamic equations on the dispersion characteristics of guided waves are discussed. The influence of circumferential order, ratio of radius to thickness, gradient index, and temperature change on the dispersion characteristics of guided waves is analyzed. This study provides a theoretical basis for nondestructive testing and evaluation of high-temperature FGM pipelines.

Study on Microstructure and Thermal Cracking Sensitivity of Deposited Ti6Al4V/Inconel 718 Composites Made by Two-Wire Arc Additive Manufacturing by Current.

Ti6Al4V/Inconel 718 composites were prepared using arc additive manufacturing technology at different deposition currents. The properties of the composites directly influence the performance of the gradient materials, while heat input further affects the composites' properties. The results indicate that at a deposition current of 35 A, Ti elements diffuse into the Inconel 718 alloy. Increasing the current leads to the formation of brittle intermetallic compounds such as TiNi, Cr2Ti, and Fe2Ti in the deposited layer. At deposition currents below 50 A, no cracks appear, but cracks develop at a current of 50 A. Additionally, the microhardness of the deposited layer increases with higher deposition currents. Compared to the 35 A condition, microhardness rises by 31.51% at a current of 50 A. This research can expand the application field of the arc additive manufacturing of direct deposition Ti6Al4V/Inconel 718 composites.

Thermoelastic flexural behaviour of sandwich plates stacked with porous FGM layer under the 3D state heat conduction

This work intends to analyze numerically the thermoelastic bending behaviour of sandwich plates stacked with FGM (functionally graded material) layer containing porosities. For this purpose, a quasi-3D (three-dimensional) shear deformable theory based IGA (isogeometric analysis) method is established, and the 3D steady-state conduction of heat is considered as the thermal environment to reflect the real circumstance. An equation to dictate the nonlinear temperature distribution induced by the 3D state heat conduction of the porous functionally graded sandwich plate is presented accordingly. Two kinds of the porous sandwich plates having various lamination schemes are taken into consideration. Benchmark problems on the thermoelastic bending responses of the sandwich plates with FGM layer have been solved, and the results are compared to the reference solutions to demonstrate the performance capability of the proposed IGA method. Further parametric examinations illustrate the impacts of the composition pattern, material gradient, porosity degree, plate geometric parameter as well as the stacking sequence, i.e., the location of the FGM lamina on the 3D heat conduction induced thermoelastic flexural response behaviour of the porous sandwich plates. The achieved new outcomes for the thermoelastic bending response behaviour of the sandwich plates having porous FGM core or skins are regarded to be used as the future reference.

Multimaterial inkjet printing of mechanochromic materials

AbstractInkjet printing technology achieves the precise deposition of liquid-phase materials via the digitally controlled formation of picoliter-sized droplets. Beyond graphical printing, inkjet printing has been employed for the deposition of separated drops on surfaces or the formation of continuous layers, which allows to construct materials gradients or periodic features that provide enhanced functionalities. Here, we explore the use of multinozzle, drop-on-demand piezoelectric inkjet technology for the manufacturing of mechanochromic materials, i.e., materials that change their color or fluorescence in response to mechanical deformation. To accomplish this, suitable polyurethane polymers of differing hardness grades were tested with a range of organic solvents to formulate low-viscosity, inkjet-printable solutions. Following their rheological characterization, two solutions comprising “soft” and “hard” polyurethanes were selected for in-depth study. The solutions were imbibed with a mechanochromic additive to yield fluorescent inks, which were either dropcast onto polymeric substrates or printed to form checkerboard patterns of alternating hardness using a laboratory-built, multimaterial inkjet platform. Fluorescence imaging and spectroscopy were used to identify different hardness grades in the dropcast and printed materials, as well as to monitor the responses of these gradient materials to mechanical deformation. The insights gained in this study are expected to facilitate the development of inkjet-printable, mechanochromic polymer materials for a wide range of applications.

Impact resistance of biomimetic gradient sinusoidal composites by 3D printing: Tunable structural stiffness and damage tolerance

Taking inspiration from the remarkable impact resistance of the dactyl club of Odontodactylus scyllarus and utilizing the material extrusion-based 3D printing process for continuous fiber reinforced composites (CFRCs), the biomimetic gradient sinusoidal CFRCs (BGS-CFRCs) was designed and manufactured. This material combines the bidirectional sinusoidal structure with a gradient layering configuration, mimicking the natural design found in the dactyl club. Experimental tests revealed that BGS-CFRCs achieved a Charpy impact strength of up to 63.24 kJ/m2, surpassing flat-layered polylactic acid (PLA) and continuous carbon fiber reinforced PLA (CCF/PLA) specimens by 143 % and 80 %, respectively. Moreover, BGS-CFRCs exhibited tunable structural stiffness and damage tolerance. This can be attributed to the innovative in-plane fiber architecture and out-of-plane material gradient, revealing the synergistic effects of composite materials, bidirectional sinusoidal structure, and gradient layering configuration. Overall, this study combines multi-degree-of-freedom 3D printing of CFRCs with biomimetic structural design, providing new dimensions of design space. This breakthrough surpasses the limitations of traditional additive manufacturing techniques and structural design of composites, opening new possibilities for developing next-generation high-performance structural materials.

Calculation method for brittle fracture of functional gradient materials

Combined the phase field model with the wavelet dummy node-virtual crack closure technique (WDN-VCCT) used for stress intensity factors (SIFs) calculation. Calculated the node displacement using an improved brittle fracture phase field method, established the relationship between node displacement and node force using WDN-VCCT, and calculated the SIFs at the crack tip. The correctness and accuracy of the proposed method were verified through functional gradient material (FGM) tensile experiment. The influence of crack inclination angle, crack position and gradient index on mechanical response, and SIFs values at the crack tip was discussed. This study provides important computational tools for estimating the service life of FGMs and important references for structural optimization design methods.

A sizing model for a tube in tube sensible thermal energy storage heat exchanger with solid storage material

Sensible thermal energy storage (STES) systems are generally the most affordable and least complex type of thermal energy storage systems available. The main question when modeling STES systems is the outlet state of the heat transfer fluid for a given inlet condition. Traditional heat exchanger design methods such as the logarithmic mean temperature difference method do not apply to TES systems since these systems do not operate in steady state. Other design methods are only applicable to specific cases such as packed bed systems with a thermocline. The present paper proposes a novel framework for STES system design with solid, stationary storage material. The framework is validated based on the case of a tube in tube STES system. The novel design model characterizes the local conductive heat transfer in the Laplace domain. This local behavior is integrated over the surface of the heat exchanger using three methods: a height split, a height lumped and a fully lumped approach. Each method corresponds to a different assumption regarding the temperature gradient in the storage material and will therefore be applicable to different cases. By numerically inverting the Laplace transform, a prediction is obtained of the outlet temperature of the HTF in the time domain. The solution is compared to the result of a computational fluid dynamics (CFD) reference model for five selected cases, with the CFD requiring calculation times in the order of days on a dedicated server while the novel model requires seconds on a laptop. This difference in calculation time is the result of the need of the CFD model to discretize the domain in space and apply a time stepping algorithm. The transfer function approach requires neither discretization nor time stepping but it does require a numerical inverse Laplace transform. The transfer function approach has a good fit with the CFD predictions. Furthermore, the quality of the fit and the behavior of the system is predicted based on the Biot number, the thermocline width ratio and the cross conduction number where the latter two numbers are derived in the present paper. The novel approach can provide a method to obtain sizing models for STES systems with a solid storage material.

Pin fin heat bridge-based CPV-TEG hybrid system performance enhancement and optimization design

The concentrated photovoltaic-thermoelectric generator (CPV-TEG) hybrid system can enhance solar energy utilization and demonstrate superior performance. However, the performance of the CPV-TEG hybrid system is unsatisfactory due to the limited downward conduction of unused photovoltaic (PV) heat, which results in suboptimal PV and TEG performance. To address this issue, this paper proposes the addition of pin fin heat bridges between the PV and TEG components to reduce thermal resistance, and the introduction of functionally gradient thermoelectric materials to further enhance TEG performance. To achieve optimal performance, an intelligent optimization method is proposed for designing the optimal arrangement of pin fin heat bridges and the distribution of functionally graded thermoelectric materials. This method utilizes a multi-objective Archimedean optimization algorithm (MAOA) in conjunction with the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) decision-making process. Initially, various distributions of functionally graded thermoelectric materials are compared. Subsequently, three typical models for pin fin heat bridge arrangements are constructed and subjected to preliminary analysis. Finally, the joint optimization method, combining the MAOA algorithm and TOPSIS decision-making, is implemented. This method considers the distribution of functionally graded thermoelectric materials and the arrangement of pin fin heat bridges as optimization variables, with TEG module output power (PTEG) and CPV module output power (PPV) serving as objective functions. Following optimization, the CPV-TEG hybrid system achieves an optimal balance between PTEG and PPV. Specifically, the optimal design results in increases of 22.89% and 19.58% in PTEG and PPV, respectively, compared to the initial solution.

Different compositions of TC4/TC11 functional gradient materials interface by wire arc additive manufacturing

Different compositions of TC4/TC11 functional gradient materials interface by wire arc additive manufacturing

Biomimetic, Interface-Free Stiffness-Gradient PDMS-Co-Polyimide-Based Soft Materials for Stretchable Electronics and Soft Robotics.

Soft materials play a pivotal role in the efficacy of stretchable electronics and soft robotics, and the interface between the soft devices and rigid counterparts is especially crucial to the overall performance. Herein, we develop polyimide-polydimethylsiloxane (PI-PDMS) copolymers that, in various ratios, combine on a molecular level to give a series of chemically similar materials with an extremely wide Young's modulus range starting from soft 2 MPa and transitioning to rigid polymers with up to 1500 MPa. Of particular significance is the copolymers' capacity to prepare seamless stiffness gradients, as evidenced by strain distribution analyses of gradient materials, due to them being unified on a molecular level. The copolymers and gradient materials were successfully used as substrates for stretchable thin-film conductors and tested as dielectric elastomer actuators, demonstrating their potential application as enabling components in stretchable electronics and soft robots.

GEOMETRICLY NONLINEAR BENDING OF FUNCTIONAL-GRADIENT SHALLOW SHELLS ON AN ELASTIC FOUNDASHION

The paper considers the problem of geometrically nonlinear bending of shallow elements of structures made of functional gradient materials (FGM) under the influence of various transverse loads. The shallow shells under consideration can have arbitrary plan shapes and be in contact with an elastic base of the Winkler-Pasternak type. The mathematical formulation is made within the framework of classical geometric-nonlinear theory. To linearize the nonlinear system of differential equations of equilibrium, the sequential load method in combination with Newton's method is used. The effective mechanical properties of FGM vary along the thickness and are calculated according to the power law. The use of the theory of R-functions made it possible to build the necessary systems of coordinate functions for shells with arbitrary geometry and support conditions. The proposed approach is implemented in software, tested, and applied to solve the problems of bending shallow shells of complex plan shapes with holes. The influence of the elasticity coefficients of the base, the gradient index in the distribution of metal and ceramic particles, as well as other parameters on the deflections of structural elements, was studied.

UV-induced coumarin-based spiropyran gradient photodimerization and photoisomerization to construct near-infrared responsive gradient hydrogel actuators in one-step

In recent years, the fabrication process of near-infrared (NIR) light-driven gradient hydrogels remains complex. Furthermore, NIR light-driven hydrogels based on the photothermal conversion mechanism have continued to face challenges in achieving spatio-temporal control of the internal structure due to the diffusive nature of heat. Therefore, it is sensible to develop NIR light-driven gradient hydrogels based on the near-infrared upconversion luminescence mechanism through a simple method. In this study, a novel photoresponsive coumarin-based spiropyran monomer (SPCMA) was designed and synthesized, followed by the fabrication of a series of double gradient hydrogels (MHMs) with rapid NIR light-driven capability and reusability through fast in situ photopolymerization of the precursor comprising SPCMA, upconversion nanoparticles (MUCNPs), and hydroxyethyl acrylate (HEA). MHMs exhibit excellent mechanical properties with fracture stress of 4.4 MPa and fracture elongation of 1282 %. Compared with hydrogel using N, N′-Methylenebis(2-propenamide) as a crosslinker, MHMs possess outstanding NIR light-driven capability and reversible cycling characteristics. MHMs rapidly transform from a three-dimensional spiral tubular to a sheet-like morphology within 30 s of NIR light irradiation and achieve a larger actuating angle of 450° within 120 s. Additionally, MHMs maintain similar driveability to the original samples after 10 cycles of NIR light-driven and UV light-recovery processes. More importantly, cargoes with seven times the mass of the "gripper" are firmly grasped and easily lifted by the "gripper" prepared using MHMs within 60 s of NIR light irradiation, demonstrating its powerful gripping ability. This work offers a new strategy for the development of biomimetic smart gradient materials.

Advanced Crack Detection in Bidirectional Gradient Material FGM Beams: A Neural Network Approach with Adam Optimization

Advanced Crack Detection in Bidirectional Gradient Material FGM Beams: A Neural Network Approach with Adam Optimization

Implicit modular coupled heat transfer analysis for functionally graded materials using the SVC-FMC method

Functionally graded materials have found extensive applications in the aerospace industry and solar energy systems due to their excellent performance in enhancing heat transfer/insulation. Developing corresponding theoretical frameworks for the coupled radiation and conduction heat transfer (CRCHT) is crucial for unlocking their full potential and expanding their applicability in various high-performance and critical applications. Considering the limitations of existing frameworks in addressing CRCHT problems in practical applications, such as the explicit iteration procedure and the linearization assumption, a fully implicit framework as the source value caching function Monte Carlo (SVC-FMC) method is proposed based on the null collision reverse Monte Carlo and the Green's Function Markov Superposition Monte Carlo, which are suitable for solving the radiative transfer equation and the energy equation within non-uniform media. The accuracy of the proposed method is validated with different heat transfer systems. It was observed that the maximum root mean square error during the verification process did not exceed 0.08, thereby demonstrating the efficacy of SVC-FMC in analyzing CRCHT processes involving gradient materials. Results reveal that the material properties that transcend the functional distribution form will augment the strengthening effect of heat transfer/insulation. The scattering characteristics significantly influence the strengthening effect caused by refractive index distribution.