Sintering Parameters Research Articles

Investigation of laser sintering process parameters for anode foils in aluminium electrolytic capacitors considering temperature distribution

Abstract The anode foil is a critical component of aluminium electrolytic capacitors, with its performance directly impacting the overall quality of the capacitors. Currently, sintered anode foil with excellent bending resistance and high specific capacitance is considered an ideal material for capacitor manufacturing; however, research on its optimal sintering parameters remains insufficient. In this study, a three-dimensional temperature field model is developed within the Comsol Multiphysics (6.0) environment, accounting for the temperature dependence of aluminium. By varying laser power and scanning speed, the temperature distribution along the laser scanning trajectory is determined, facilitating the identification of optimal process parameters for laser sintering anode foils in electrolytic capacitors. Subsequent laser sintering experiments validate the accuracy of these parameters. The findings indicate that the peak temperature of the molten pool rises with increased laser power and decreased scanning speed. The optimal process parameters for laser sintering anode foils in electrolytic capacitors are a powder layer thickness of 50 µm, a laser power of 140 W, and a scanning speed of 0.05 m/s. The specific capacitance of laser-sintered anode foil, formed at voltages of 375 V and 520 V, ranges from 0.847 to 1.157 μF/cm² and 0.717 to 0.935 μF/cm², respectively, when the particle size is between 3 and 4 μm. A specific capacitance of 0.733 μF/cm² can be achieved, which meets the performance requirements for aluminium electrolytic capacitors.

3D printing of cordierite glass-ceramics

Compared to traditional ceramic materials, glass-ceramics can also exhibit superior properties and are now widely used in cutting-edge national defense industries technology, aerospace, electronics, construction, and other fields. In this study, it is for the first time reported that novel photopolymerization 3D printable slurries using cordierite glass ceramic powder were prepared. The effects of slurry properties and printing process parameters on the warpage of the printed samples were studied. Cordierite glass-ceramic parts fabricated using photopolymerization 3D printing and heat treatment processes were studied to evaluate the effects of sintering parameters on the properties of the samples. The obtained additively manufactured cordierite glass-ceramics exhibit excellent microstructural and mechanical properties, with a maximum bending strength of 181 MPa, an increase of 95.66 % compared to that of the previously reported 3D printed cordierite ceramics. The underlying mechanisms of the glass and ceramic phase characteristics influence the density and the mechanical properties during and after the courses of sintering were also investigated and unveiled in detail. This study provides a viable method for manufacturing complex-shaped cordierite glass-ceramic components using 3D printing.

Enhancing microstructure homogeneity and electrical conductivity in flash-sintered 8YSZ: Impact of electric-current ramp control and thermal insulation

Our study aimed to comprehend how variations in flash sintering parameters affect the evolution of microstructure and electrical properties of yttria-stabilised zirconia. For this purpose, cylindrical samples were sintered via a classic flash method (voltage-current control), with controlled increase in the current ramp, employing thermal insulation, and combining current ramp control with thermal insulation. The results demonstrated that the combination of current ramp control and thermal insulation yielded higher densification compared to other variations. Microstructural analysis revealed finer grain sizes and lower grain-boundary tensions on employing current ramp control, particularly when combined with thermal insulation. For all flash-sintering methods, greater electrical conductivity of grain boundaries was observed compared to conventional sintering, suggesting an enhancement in ionic conduction. This study underscores the potential of customised FS techniques in optimising sintering processes and enhancing material properties.

Production and Characterization of Fine-Grained Multielement AlCoxCrFeNi (x = 1, 0.75, 0.5) Alloys for High-Temperature Applications.

In the present work, three different AlCoxCrFeNi (x = 1, 0.75, 0.5) alloys were produced through the mechanical milling of powders and spark plasma sintering. These alloys were characterized in terms of their microstructural, mechanical, and oxidation behaviors. Mechanical milling and spark plasma sintering were chosen to achieve a fine and homogeneous microstructure. Pore-free samples were produced by properly setting the sintering parameters. The unavoidable uptake of oxygen from the powders when exposed to air after milling was advantageously used as a source of oxides, which acted as reinforcing particles in the alloy. Oxidation behavior, studied through TGA tests, showed that decreasing the Co content promotes better oxidation protection due to the formation of a dense, compact Al2O3 layer. The alloy containing the lowest amount of Co is considered a good candidate for high-temperature structural applications.

Effect of Sintering Parameters and Liquid Phase Content on the Properties of Fe-Rich Based Impregnated Diamond Bit Matrix

Effect of Sintering Parameters and Liquid Phase Content on the Properties of Fe-Rich Based Impregnated Diamond Bit Matrix

Experimental investigation of material extrusion additive manufacturing of copper with high powder loading rate

Beam-based additive manufacturing has challenges in fabricating pure copper due to high reflectivity. Material extrusion additive manufacturing (MEAM) avoids these issues with low costs, making it suitable to fabricate complex structures in pure copper. In this work, the effects of several printing parameters on the geometry of the samples are studied from single line, thin wall to cube using homemade copper feedstocks with high powder loading rate (65 vol%). The results show that the single line is most affected by layer thickness, while the thin wall has stricter requirements for printing temperature and printing speed. The geometric accuracy of the cube demonstrates large parametric tolerance due to the overlap and support between the lines. Central composite face-centered (CCF) design and central composite design (CCD) are used to analyze and optimize process parameters for densification. The measured porosity at the optimized printing (printing temperature = 177 °C, air gap = −0.05 mm, layer thickness = 0.3 mm) and sintering parameters (sintering temperature = 1045 °C, holding time = 4 h) is in good agreement with the predicted values. Air gap and sintering temperature have the greatest influence on the porosity, and other parameters within a certain range of on-demand adjustments do not have a significant effect. The ultimate tensile strength and elongation of the sintered samples under the optimal parameters are 153.9 ± 6.1 MPa and 31.36 ± 3.24%, respectively. No printing defects are found at the fracture and the elongation is good, showing the effectiveness of the parameter optimization.

DETERMINATION OF OPTIMAL PARAMETERS OF SINTERING OF ROLLING SCALE

This article presents the results of studies conducted to determine the optimal parameters of sintering of rolled scale in a mixture with iron-containing steelmaking waste. The conducted studies of sintering of rolled scale have shown that the gas permeability of the charge depends on a number of physical properties, the size and shape of the grains, the amount of return, the height of the charge layer, and the moisture content of the charge. The initial gas permeability strongly depends on the degree of humidification of the agglomeration charge, especially when sintering small classes of materials under study. The maximum gas permeability of the charge is ensured with an optimal granulometric composition of the charge, which is characterized by an average equivalent diameter: the higher this indicator, the higher the gas permeability of the charge. The value of the surface tension reaches its maximum at a certain moisture content of the charge, which is the optimal moisture for this charge. By increasing the amount of return during agglomeration, the vertical sintering rate increases and the quality of the agglomerate improves. These factors lead to an increase in the productivity of sintering plants. According to the results of the study, the optimal parameters of the sintering sintering process of rolled scale in a mixture with iron-containing steelmaking waste were worked out. It is established that the optimal amount of return is the use of 20 % of the mass of the sintering charge at the optimal height of the sintered layer of 300 mm. As a result of the conducted research, the following optimal indicators were achieved: mechanical strength – 70.2 %, sintering rate – 28.7 mm/min, specific productivity – 1.75 t/m2·hour. Keywords: Rolling scale, agglomeration, optimal parameters, sintering, return, layer height.

Synthesizing high-density U₃O₈ powder from UO₂F₂ solution via AUC precipitation

Uranium trioxide octaoxide compound - U3O8 is a crucial nuclear material in nuclear technology. It is used as nuclear fuel for research reactors. To achieve this goal, an important characteristic that U3O8 powder must possess is a density ranging from 88-98% of the theoretical density (TD). This paper reports the results of an investigation of the ammonium uranyl carbonate (AUC) precipitation from uranyl fluoride (UO2F2) solution and optimization of sintering parameters for synthesizing high-density U3O8 powder, meeting the specified standards for manufacturing dispersed nuclear fuel for research reactors. The AUC precipitation was conducted using uranyl fluoride (UO2F2) solutions with uranium concentrations ranging from 80 to 120 gL-1 and ammonium carbonate ((NH4)2CO3) concentrations as precipitant were maintained between 200 and 400 gL-1, while the (NH4)2CO3 to U (C/U) molar ratios were kept equal to or greater than 6. The investigated parameters for sintering of the high-density U3O8 nuclear material derived from AUC (ex-AUC U3O8) are the sintering temperature and time. The experimental studies are designed by using the Response Surface Methodology (RSM) based on a Central Composite Design (CCD). As a result, a regression equation describing the dependency of U3O8 powder density on sintering temperature and time has been established. Based on this equation, the sintering for synthesizing high-density U3O8 powder has been optimized. The regression equation aids in controlling the parameters of the U3O8 powder sintering.

Improvement of BGO ceramic scintillators through hot‐pressing sintering methodology

AbstractIn the present work, the sintering of bismuth germanate through hot pressing and the improvement of scintillator performance were investigated. The linear shrinkage, crystalline structure, microstructure, transparency degree, and radioluminescence (RL) were studied as functions of the sintering pressure. X‐ray diffraction revealed that the samples were predominantly composed of the Bi4Ge3O12 phase, accompanied by small amounts of Bi12GeO20, with concentrations varying according to the sintering parameters. These concentrations were quantified through Rietveld refinement, which also indicated a tendency for the cell parameters to shrink as the sintering pressure increased. The microstructure of the ceramic pellets produced under varying hot‐pressing parameters was investigated using scanning electron microscopy (SEM), revealing that while the grain size was preserved, the porosity and grain boundary thickness were reduced by the hot pressing, forming a quasicontinuum in some areas of the sample. The RL of all the samples exhibited a green color, with a maximum at 540 nm, ascribed to the transitions from the 3P0,1,21P1 excited states to the fundamental 1S0 state of the Bi3+ ions. For samples sintered under a pressure of .18 MPa, the enhancement in the optical transmittance, accompanied by a 61.36% increase in light output at the maximum wavelength, was observed.

Fine-grained BCZT piezoelectric ceramics by combining high-energy mechanochemical synthesis and hot-press sintering

Different stoichiometries of lead-free BaZr0.2Ti0.8O3–Ba0.7Ca0.3TiO3 (BCZT) prepared by mechanosynthesis and sintered by either conventional sintering (CS) or hot pressing (HP) techniques were studied to establish the dependence of piezoelectric and dielectric properties on sintering parameters and microstructure. All synthesized stoichiometries showed a pseudocubic perovskite phase with homogeneously distributed A- and B-cations in the structure. The BCZT retained the pseudocubic symmetry after sintering and an average grain size <1.8 µm was obtained in all cases. HP sintering hindered the secondary phase segregation observed in the CS ceramics and increased the relative density. Piezoelectric coefficients (d33) ranging from 5.1 to 21 pC/N and from 10.0 to 88.0 pC/N were obtained for CS and HP ceramics, respectively, despite the pseudocubic symmetry and the fine grain size. The higher d33 values for the HP ceramics are a consequence of the higher density, better chemical homogeneity and lower sintering temperature and time required for the mechanosynthesized BCZT powders with high sintering activity.

Investigating Nanoindentation and Wear Properties of Spark Plasma Sintered Ti48Al48Cr2Nb2 Alloy

This work studied spark plasma sintered Ti48Al48Cr2Nb2 alloy for their nanoindentation and wear properties. Results showed an improvement in nanoindentation properties of the compacts with an increase in holding time from 5min to 7.5min for the heating rate of 50°C/min and 100°C/min. X-ray diffraction (XRD) analysis showed the intensity of diffraction peaks of two major crystalline phases of Ti3Al2.25Nb0.75 and γ-TiAl. Scanning electron microscopy (SEM) images of sintered compacts possessed varied microstructures due to different sintering parameters. The nearly lamellar structure showed a good balance of yield stress and fracture toughness. Wear tracks SEM analysis revealed the worn surface morphologies and the sample sintered for 7.5 min at the eating rate of 50°C/min sustained the least damage evident by the shallow grooves and narrow wear track. Profilometer measurements revealed the wear track depth and width, confirming the degree of wear for the samples sintered at varied conditions.

Evaluating the sintering behaviors of ceramic oxide powders processed via binder jet additive manufacturing

AbstractBinder jet additive manufacturing is well suited for fabricating large (order of cm) and geometrically complex ceramic preforms. However, the main challenge in producing ceramic oxide parts via binder jetting is the high‐temperature postprocess tasked with eliminating internal porosity to achieve full densities. In this work, we demonstrate the ability to produce oxide ceramic parts with desirable densities by sintering binder jetted preforms. We investigate the sintering behavior of binder jetted preforms composed of three oxide powders with distinct morphologies: ball‐milled alumina, gas‐atomized silica, and sintered‐agglomerated zirconia. We fabricate the preform samples using a commercial binder jetting system and a conventional die‐pressing technique to understand the effect of starting densities. Furthermore, we parametrize the heating profiles to understand the effect of sintering temperature, sintering duration, and heating rate on each powder's densification behavior, microstructure, and phase composition. Results show the relatively low starting densities within the binder jetted preforms caused the onset sintering temperature to be higher than what is documented in conventional sintering studies. As expected, we observed sintered densities increase with respect to sintering temperature and duration. These findings were utilized to downselect sintering parameters capable of achieving high densities (&gt;96%). Herein, this study validates the sintering of binder jetted preforms as a suitable way to manufacture ceramic parts, regardless of powder morphologies, thereby increasing the robustness of the supply chain involved in additive manufacturing of ceramic oxides.

Effect of Laser Sintering Parameters on the Microstructure, Mechanical Properties and Corrosion Behavior of Titanium Grade 5 Alloy

Effect of Laser Sintering Parameters on the Microstructure, Mechanical Properties and Corrosion Behavior of Titanium Grade 5 Alloy

Influence of nozzle temperatures on the microstructures and physical properties of 316L stainless steel parts additively manufactured by material extrusion

PurposeMaterial extrusion (ME) is a low-cost additive manufacturing (AM) technique that is capable of producing metallic components using desktop 3D printers through a three-step printing, debinding and sintering process to obtain fully dense metallic parts. However, research on ME AM, specifically fused filament fabrication (FFF) of 316L SS, has mainly focused on improving densification and mechanical properties during the post-printing stage; sintering parameters. Therefore, this study aims to investigate the effect of varying processing parameters during the initial printing stage, specifically nozzle temperatures, Tn (190°C–300°C) on the relative density, porosity, microstructures and microhardness of FFF 3D printed 316L SS.Design/methodology/approachCube samples (25 x 25 x 25 mm) are printed via a low-cost Artillery Sidewinder X1 3D printer using a 316L SS filament comprising of metal-polymer binder mix by varying nozzle temperatures from 190 to 300°C. All samples are subjected to thermal debinding and sintering processes. The relative density of the sintered parts is determined based on the Archimedes Principle. Microscopy and analytical methods are conducted to evaluate the microstructures and phase compositions. Vickers microhardness (HV) measurements are used to assess the mechanical property. Finally, the correlation between relative density, microstructures and hardness is also reported.FindingsThe results from this study suggest a suitable temperature range of 195°C–205°C for the successful printing of 316L SS green parts with high dimensional accuracy. On the other hand, Tn = 200°C yields the highest relative density (97.6%) and highest hardness (292HV) in the sintered part, owing to the lowest porosity content (<3%) and the combination of the finest average grain size (∼47 µm) and the presence of Cr23C6 precipitates. However, increasing Tn = 205°C results in increased porosity percentage and grain coarsening, thereby reducing the HV values. Overall, these outcomes suggest that the microstructures and properties of sintered 316L SS parts fabricated by FFF AM could be significantly influenced even by adjusting the processing parameters during the initial printing stage only.Originality/valueThis paper addresses the gap by investigating the impact of initial FFF 3D printing parameters, particularly nozzle temperature, on the microstructures and physical characteristics of sintered FFF 316L SS parts. This study provides an understanding of the correlation between nozzle temperature and various factors such as dimensional integrity, densification level, microstructure and hardness of the fabricated parts.

Sintering parameter investigation for bimetallic stainless steel 316L/inconel 718 composite printed by dual-nozzle fused deposition modeling

PurposeFused deposition modeling (FDM) nowadays offers promising future applications for fabricating not only thermoplastic-based polymers but also composite PLA/Metal alloy materials, this capability bridges the need for metallic components in complex manufacturing processes. The research is to explore the manufacturability of multi-metal parts by printing green bodies of PLA/multi-metal objects, carrying these objects to the debinding process and varying the sintering parameters.Design/methodology/approachThree different sample types of SS316L part, Inconel 718 part and bimetallic composite of SS316L/IN718 were effectively printed. After the debinding process, the printed parts (green bodies), were isothermally sintered in non-vacuum chamber to investigate the fusion behavior at four different temperatures in the range of 1270 °C−1530 °C for 12 h and slowly cooled in the furnace. All samples was assessed including geometrical assessment to measure the shrinkage, characterization (XRD) to identify the crystallinity of the compound and microstructural evolution (Optical microscopy and SEM) to explore the porosity and morphology on the surface. The hardness of each sample types was measured and compared. The sintering parameter was optimized according to the microstructural evaluation on the interface of SS316L/IN718 composite.FindingsThe investigation indicated that the de-binding of all the samples was effectively succeeded through less weight until 16% when the PLA of green bodies was successfully evaporated. The morphology result shows evidence of an effective sintering process to have the grain boundaries in all samples, while multi-metal parts clearly displayed the interface. Furthermore, the result of XRD shows the tendency of lower crystallinity in SS316L parts, whilst IN718 has a high crystallinity. The optimal sintering temperature for SS316L/IN718 parts is 1500 °C. The hardness test concludes that the higher sintering temperature gives a higher hardness result.Originality/valueThis study highlights the successful sintering of a bimetallic stainless steel 316 L/Inconel 718 composite, fabricated via dual-nozzle fused deposition modeling, in a non-vacuum environment at 1500 °C. The resulting material displayed maximum hardness values of 872 HV for SS316L and 755.5 HV for IN718, with both materials exhibiting excellent fusion without any cracks.

Integrating High-Performance Flexible Wires with Strain Sensors for Wearable Human Motion Detection.

Flexible electronics have revolutionized the field by overcoming the rigid limitations of traditional devices, offering superior flexibility and adaptability. Conductive ink performance is crucial, directly impacting the stability of flexible electronics. While metal filler-based inks exhibit excellent conductivity, they often lack mechanical stability. To address this challenge, we present a novel conductive ink utilizing a ternary composite filler system: liquid metal and two micron-sized silver morphologies (particles and flakes). We systematically investigated the influence of filler type, mass ratio, and sintering process parameters on the composite ink's conductivity and mechanical stability. Our results demonstrate that flexible wires fabricated with the liquid metal/micron silver particle/micron silver flake composite filler exhibit remarkable conductivity and exceptional bending stability. Interestingly, increasing the liquid metal content results in a trade-off, compromising conductivity while enhancing mechanical performance. After enduring 5000 bending cycles, the resistance change in wires formulated with a 4:1 mass ratio of micron silver particles to flakes is only half that of wires with a 1:1 ratio. This study further investigates the mechanism governing resistance variations during flexible wire bending. Additionally, we observed a positive correlation between sintering temperature and pressure with the conductivity of flexible wires. The significance of the sintering parameters on conductivity follows a descending order: sintering temperature, sintering pressure, and sintering time. Finally, we demonstrate the practical application of this technology by integrating the composite ink-based flexible wires with conductive polymer-based strain sensors. This combination successfully achieved the detection of human movements, including finger and wrist bending.

Holistic view on cation interdiffusion during processing and operation of garnet all-solid-state batteries

All-solid-state batteries based on the active cathode material LiCoO2 (LCO), a garnet-type Li7La3Zr2O12 (LLZO) electrolyte and a Li-metal anode are attracting a lot of attention as a robust and safe alternative to conventional lithium-ion batteries. The challenges in the practical realization of such cells are related to high-temperature sintering, which compacts the ceramic powder but also leads to undesirable material interactions such as cation interdiffusion and secondary phase formation. Even if high initial capacities can be achieved, the all-inorganic cells suffer from a strong capacity drop due to various degradation phenomena during processing and operation, which are not yet fully understood. In this study, the thermodynamic and kinetic aspects of co-sintering as well as the structural evolution of materials and interfaces during processing and operation of co-sintered LCO-LLZO cathodes are investigated in detail. A thermodynamic model for the interdiffusion of cations is derived and the effects of the diffusion of Al- and Co-ions, which occurs during the processing and cycling of the cells, are investigated. In LLZO, the diffusion of 0.13 Co per formula unit (pfu) has a negligible effect on ionic and electronic conductivity and electrochemical stability. In contrast, the substitution of 0.01 pfu Al and the induced disorder in the layer structure of LCO increases the polarization during cycling. All-inorganic cells fabricated with optimized sintering parameters to minimize interdiffusion between LCO and LLZO show good initial performance but similar degradation during cycling, as the used processing parameters result in a more porous microstructure leading to the development of cracks along the LLZO/LCO interface. The results obtained highlight the inherent instabilities of all-ceramic cathodes with unprotected LCO/LLZO interfaces, which require precise tuning of materials and processing parameters to achieve both high mechanical stability and low interdiffusion.

A novel manufacturing process for intermetallic thin-walled components: Forming and hot pressing of thin-walled laminates prepared by laying/winding metal foil strips

To address the challenge of manufacturing NiAl intermetallic thin-walled components, an integrated manufacturing process using the preform prepared by laying/winding of metal foil strips was proposed. This method entails the initial fabrication of a stacked foil preform via the laying/winding of metal foil strips, followed by pre-sintering to prepare a laminated preform. Subsequently, precise shaping is achieved through hot bulging of the laminated preform, while the NiAl intermetallic phase is obtained through reaction synthesis of Ni and Al foils under hot pressing. The microstructure of Ni/Al/Ni laminated preforms under varied sintering parameters and different laminating methods were analyzed. Additionally, the deformation behaviors of these preforms at both room and high temperatures were investigated via tensile tests and bulge tests. Results show: (1) The micro-cracks and Ni-Ni interface delamination occurred in the brittle Ni2Al3 layer of the laminated preform. The deformation capability of laminated plates decreased as the thickness of the brittle Ni2Al3 layer increased with sintering time. (2) Tiny gaps between foils due to laying path and foil size deviations, causing reduced deformability and fractures when the tensile direction was perpendicular to these gaps. (3) These gaps and micro-cracks can be progressively eliminated through hot pressing sintering processes. The findings verify the preliminary feasibility of the proposed novel manufacturing process and provide a novel solution for manufacturing NiAl intermetallic thin-walled components.

Predicting hardness of graphene-added Si3N4 using machine learning: A data-driven approach

This study presents a data-driven framework based on machine learning (ML) using extreme gradient boosting (XGBoost) for predicting the hardness of silicon nitride (Si3N4) ceramics reinforced with graphene. The XGBoost model takes into account various factors such as graphene type and content, characteristics of the raw Si3N4 powder, the parameters of the sintering process (sintering technique, temperature, pressure, holding time), and the characteristics of the sintered samples, i.e., the density, αβ content and Vickers hardness. The parameters that influence the Si3N4 hardness most strongly are identified, with sintering pressure, sintering time and density being the most influential. The addition of graphene content up to a certain threshold (1 wt%) has a positive impact on hardness. However, beyond that it leads to a lower density and a lower mechanical performance. Sintering parameters, particularly the sintering pressure, temperature, holding time and technique, strongly affect the density, final grain size, αβ Si3N4 composition and subsequently the hardness. The study highlights the importance of density and the densification process in achieving high hardness in Si3N4 ceramics. The developed ML model provides a valuable tool for predicting the hardness of Si3N4+graphene ceramics composites and offers insights into selecting suitable graphene type, content, and processing parameters. While the study primarily focuses on Si3N4+graphene composites, this novel approach holds promise for the in-silico design and analysis of diverse ceramic materials.

Hyperdoping: doping TiO2 beyond thermodynamic limits using Flash sintering

For the first time, it has been demonstrated that TiO2 optical properties can be improved by doping Fe using flash sintering. Hyperdoped TiO2 (doped with 10 at% Fe) fabricated using flash sintering, exhibited remarkable improvements in optical characteristics. Hyperdoping (exceeding the equilibrium solubility limit), induces charge trap centers or inhibits charge recombination in semiconductors. By employing XPS and UV-Vis spectroscopy, surface species alterations including oxygen vacancies, Ti4+, Ti3+, and optical characteristics were investigated. With varying holding periods, flash sintering unveiled substantial enhancements in optical absorbance and absorption regions by generating Ti3+ and oxygen vacancies. Notably, a 10-second hold time during flash sintering displayed the highest redshift among the investigated samples, indicating superior optical improvement. Oxygen vacancies and Ti3+ increased with decreased holding time during flash, suggesting the influence of flash sintering parameters on forming a metastable phase, signifying the pivotal role of flash sintering's time parameter in modulating material properties.