Ar Atmosphere Research Articles

Critical Behavior and Magnetocaloric Effect in Co2CrAl Heusler Alloy

The analysis of structural, electrical transport, critical magnetic behavior, and magnetocaloric properties of Co2CrAl Heusler alloys has been conducted here. Co2CrAl full Heusler alloy has been prepared by the conventional arc melting method in Ar atmosphere and post‐annealing in a vacuum‐sealed quartz tube. X‐ray diffraction (XRD) pattern reveals the crystallization of the alloy to the Fmm space group with L21 crystal symmetry. The correspondence between the magnetic and electrical transport properties of the Co2CrAl alloy has been observed. This alloy exhibits paramagnetic to ferromagnetic transition (Curie temperature) just above room temperature at around 340 K. Metallic to semiconducting transition occurs near the magnetic Curie temperature. Critical magnetic behavior has been analyzed by employing the Arrott plot, Kouvel–Fisher methods, and critical isotherm analysis. These conclude that the exchange interaction in Co2CrAl alloy follows between mean‐field theory having spin–spin long‐range interaction and the 3D Heisenberg model. The magnetocaloric effect has been explored from the isothermal magnetization data by employing Maxwell's equations. The highest magnetic entropy change is found to be ≈0.99 J (kg K)−1 for a magnetic field change of 30 kOe. A large working temperature window of 57 K is the advantage of the present alloy.

Synthesis of ZrB2–ZrC hybrid powders via boro-carbothermal reduction of ZrO2 by B4C and carbon black

ZrB2–ZrC hybrid powders were synthesized by a novel two-step reduction on basis of ZrO2 + B4C + C→ ZrC + ZrB2 + CO reaction in Ar atmosphere, using ZrO2, B4C, and carbon black powders as starting materials. Thermodynamics of relevant reactions were evaluated. Effects of excess additions of B4C and C on phase constituents were investigated. Morphology and chemistry of the powder products were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and transmission electron microscopy (TEM). The results showed that ZrB2–ZrC hybrid powders with no obvious impurity content could be obtained after heating at 1350 °C for 1 h followed by further reaction at 1700 °C for 1 h with 16 wt% B4C + 8 wt% C excess addition. Relative contents of the ZrB2: ZrC phase in the product powders could be conveniently regulated by varying the B4C and C content in the starting compositions. The resultant powders had good oxidation resistance with an oxidation activation energy value of 433 kJ/mol. Good sinterability of the powder products was demonstrated by hot pressing at 1950 °C for 60min under 30 MPa pressure, which resulted in fully dense ZrB2–ZrC composite ceramics with Vickers hardness value larger than 18.3 ± 0.6 GPa.

Constructing ordered macropores in hollow Co/C polyhedral nanocages shell toward superior microwave absorbing performance

Rational design of porous carbon architecture is essential for superior microwave absorbing performance. Herein, we report a new type of hollow porous Co/C polyhedral nanocages with ordered macropores of ∼60 nm (HP-Co/C) as microwave absorber, which were readily manufactured by epitaxial growth of ZIF-67/SiO2 nanolayers on the surfaces of polyhedral ZIF-8 nanoparticle, and followed by simple calcination in Ar atmosphere and subsequent removal of SiO2 with HF. The ordered macropores can effectively tune the electromagnetic parameters of HP-Co/C, affording the obtained HP-Co/C composites strong attenuation capability and excellent impedance matching characteristics for electromagnetic wave (EMW) absorption. As a result, the reflection loss (RL) and effective absorption bandwidth (EAB) of HP-Co/C prepared under pyrolysis temperature of 600 °C can reach up to −66.5 dB and 8.96 GHz, respectively, at filler fraction of only 15 wt%. Together, this study offers a new design philosophy to make lightweight and broadband microwave absorbent and can be extended to other types of microwave absorbers, significantly enriching the categories of the efficient microwave absorbing materials.

Depth-Resolved Study of the SiO<sub>2</sub>- SiC Interface Using Low-Energy Muon Spin Rotation Spectroscopy

In this work, the interface between 4H-SiC and thermally grown SiO2 is studied using low energy muon spin rotation (LE-μSR) spectroscopy. Samples oxidized at 1300 °C were annealed in NO or Ar ambience and the effect of the ambience and the annealing temperature on the near interface region is studied in a depth resolved manner. NO-annealing is expected to passivate the defects, resulting in reduction of interface traps, which is confirmed by electrical characterization. Introduction of N during annealing, to the SiC matrix, results in a thin, carrier rich region close to the interface leading to an increase in the diamagnetic asymmetry. Annealing in an inert environment (Ar) seems to have much lesser impact on the electrical signal, however the μSR shows a reduced paramagnetic asymmetry, indicating a narrow region of low mobility at the interface.

Dual-Functional Enantiomeric Compounds as Hole-Transporting Materials and Interfacial Layers in Perovskite Solar Cells.

In this paper, we describe the application of the enantiomeric compounds YLC-1-YLC-4, each featuring a bulky spiro[fluorene-9,9'-phenanthren]-10'-one moiety, as both hole-transporting materials (HTMs) and interfacial layers in both n-i-p and p-i-n perovskite solar cells (PSCs). These HTMs contain an enantiomeric mixture and a variety of core units linked to triarylamine donors to extend the degree of π-conjugation. The n-i-p PSCs incorporating YLC-1(a) exhibited a power conversion efficiency (PCE) of 19.15% under AM 1.5G conditions (100 mW cm-2); this value was comparable with that obtained using spiro-OMeTAD as the HTM (18.25%). We obtained efficient and stable p-i-n PSCs having the dopant-free structure indium tin oxide (ITO)/NiOx/interfacial layer (YLC)/perovskite/PC61BM/BCP/Ag. The presence of the spiro-based compounds YLC-1 and YLC-2 efficiently passivated the interfacial and grain boundary defects of the perovskite and enhanced the sizes of its grains, more so than did YLC-3 and YLC-4. These spiro-based YLC derivatives packed densely and functioned as Lewis bases to coordinate Pb and Ni ions in the perovskite and NiOx layers, respectively. Together, the effects of smaller grain boundaries and defect passivation of the perovskite enhanced the optoelectronic properties of the PSCs. The photoinduced charge carrier extraction in the linearly increasing voltage (photo-CELIV) curves of NiOx/YLC-1(a) showed the faster carrier transport 3.3 × 10-3 cm2 V-1 s-1, which improved the carrier mobility, supporting the notion of defect passivation of the perovskite. The best-performing NiOx/YLC-1(a) device provided a short-circuit current density (JSC) of 22.88 mA cm-2, an open-circuit voltage (VOC) of 1.10 V, and a fill factor (FF) of 80.93%, corresponding to an overall PCE of 20.37%. In addition, the PCEs of the NiOx/YLC-1(a) and NiOx/YLC-4(b) PSC devices underwent decays of only 98.1 and 97.0% of their original values after 41 days under an Ar atmosphere. Thus, these YLC derivatives passivated the NiOx surface and optimized the film quality of perovskites, thereby leading to superior PCEs of their respective PSCs.

The effects of annealing atmosphere and intrinsic component on high temperature evolution behaviors of SiC fibers

In this work, the second (F1) and third (F2) generation SiC fibers were treated at different temperature under vacuum and Ar atmosphere to analyze the influence of fiber composition and atmosphere on the high temperature evolution of SiC fibers. The results reveal that SiC decomposition and graphene formation could occur in SiC fibers over 1600 °C under vacuum or 2000 °C under Ar atmosphere. The mechanical properties of both F1 and F2 fibers are remarkably degraded accompanied by the SiC decomposition. The formed graphene size is related to the SiC grain size before decomposition, while the increment of temperature will promote SiC grain growth and SiC decomposition simultaneously. Under Ar atmosphere, the Si sublimation rate is slow so that SiC grains can grow fully, therefore, the converted graphene crystallite size is much larger than that under vacuum. SiC decomposition rate is related to the excess carbon and pores in the fibers. The inhibition effect of carbon micro-coating around SiC grains formed by excess carbon and promotion effect of pores formed by SiCxOy decomposition co-exist in SiC fibers. This work reveals that effective inhibition of Si sublimation is of significance for the stability of SiC fibers under ultra-high temperature.

Tailored design of well-defined hierarchical nitrogen-doped carbon via salt-confined strategy for selective Cd(Ⅱ) adsorption

Efficient cadmium (Cd) removal is vital to ensure freshwater safety, yet it still lacks high-efficiency treatment technologies, especially for the technique with simple-operation and eco-friendliness. Herein, three different carbon-based nanomaterials of hierarchical nitrogen-doped carbon (h-NC-750, h-NC-800 and h-NC-850) were successfully synthesized, via carbonizing the sealed ZIF-8 particles in NaCl crystals at various carbonization temperatures of 750, 800 and 850 °C in Ar atmosphere. Amongst them, the acquired dual-carbon of h-NC-800 harvesting a total nitrogen content of 15.5% rendered the maximum Cd(Ⅱ) adsorption capacity of 356.4 mg g−1, which was 12.5 and 48.3% higher than that of h-NC-750 and h-NC-850 counterparts. The density functional theory (DFT) simulations in combination with characterizations unveiled that the exceptional Cd(Ⅱ) removal of h-NC-800 was derived from the harmonized edge- to graphitic-nitrogen ratio that strengthened the binding energies between Cd(Ⅱ) and h-NC. Impressively, thus-optimized adsorbents afforded the favorable and stabilized removal efficiency (>92%) toward Cd(Ⅱ) in realistic water and five-consecutive regeneration cyclicality. This work is anticipated to offer an innovative insight into the molecular-level architecture design of h-NC with well-defined nitrogen doping and controllable edge- to graphitic-nitrogen ratio, targeting high performance in aqueous Cd(Ⅱ) removal.

Controlled Growth Interface of Charge Transfer Salts of Nickel-7,7,8,8-Tetracyanoquinodimethane on Surface of Graphdiyne

Controlled Growth Interface of Charge Transfer Salts of Nickel-7,7,8,8-Tetracyanoquinodimethane on Surface of Graphdiyne

Ultralight and hydrophobic MXene/chitosan-derived hybrid carbon aerogel with hierarchical pore structure for durable electromagnetic interference shielding and thermal insulation

Lightweight, robust and durable high-performance electromagnetic interference (EMI) shielding materials with excellent thermal insulation properties are highly desired for modern integrated electronic systems used in military and aerospace applications. Although MXenes have exhibited high shielding performance, it remains a great challenge to build high-performance EMI shielding MXene-based aerogels with minimal MXene content and high durability. Here, hierarchically porous hybrid carbon aerogels have been constructed by assembling chitosan (CS) hydrogels and MXenes via freeze-drying followed by a suitable heat treatment strategy. Due to the special MXene–TiO2–C heterostructures and high conductivity resulting from heating at 800 °C under an Ar atmosphere, an ultralight hybrid carbon aerogel (11.9 mg cm−3) with 0.1322 vol% MXene exhibits a high EMI shielding effectiveness (EMI SE) of ∼61.4 dB and a specific SE of 5155.46 dB cm3 g−1 at a thickness of ∼3 mm. Furthermore, the hybrid carbon aerogel also provides significant thermal insulation (∼25.88 mW m−1·K−1 at 25 °C and ∼62.65 mW m−1·K−1 at 800 °C under Ar) as well as fire resistance. Superior EMI shielding and thermal insulation performance can be retained even after storage in a hygrothermal environment for 30 days, which guarantees long-term application of the hybrid carbon aerogel in humid or high-temperature environments.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO 3

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO ₃

Construction of Synergistic Co and Cu Diatomic Sites for Enhanced Higher Alcohol Synthesis

Construction of Synergistic Co and Cu Diatomic Sites for Enhanced Higher Alcohol Synthesis

Facile synthesis of La2O3/Co@N-doped carbon nanotubes via Prussian blue analogues toward strong microwave absorption

Prussian blue, a member of the metal-organic frameworks (MOFs), has attracted great interest in electromagnetic wave absorption due to its unique properties, such as multiple morphologies and tunable structures. In this paper, La2O3/Co@N-doped carbon nanotubes (La2O3/Co@NCNTs) composites were synthesized by thermal annealing La[Co(CN)6]·5H2O Prussian blue analogue (LaCo-PBA) precursors under an Argon (Ar) atmosphere. The studies of electromagnetic wave absorption show that La2O3/Co@NCNTs demonstrated the minimum reflection loss (RL) of −53.40 dB with the effective absorption bandwidth (EAB) of 4.34 GHz at the thickness of 2.45 mm. These excellent microwave absorption properties are attributed to multiple loss mechanisms, including conduction loss, polarization loss, magnetic resonance loss, and eddy current loss. This study offers a new possibility for the design of multifaceted lightweight carbon-based microwave absorbing materials.

Fe-doped MOF-derived N-rich porous carbon nanoframe for H2 S cataluminescence sensing.

Metal-doped porous carbon matrix composites are considered as outstanding H2 S cataluminescence sensing materials for their good sulfur tolerance and high cataluminescence activity. In this work, an Fe-doped MOF-derived N-rich porous carbon nanoframe was successfully fabricated using the pyrolysis of Fe-doped ZIF-8 in an Ar atmosphere at a temperature of 900°C, and used for H2 S cataluminescence sensing. Along with zinc volatilization, the obtained porous carbon nanoframe not only had high specific surface area and abundant voids, but also had well dispersed Fe species doped in the skeleton. Compared with Fe2 O3 /ZnO composites derived from the same precursor but different pyrolysis terms, this as-prepared Fe-doped N-rich porous carbon presented a three times increase in the cataluminescence intensity towards H2 S, attributed to the porous carbon skeleton that is indispensable for dispersing catalytic active sites and providing more absorptive surface and voids. Comparably, this proposed sensor demonstrated high sensitivity and good selectivity, with the detection range of 1.57-19.58 μg·ml-1 and detection limit of 0.13 μg·ml-1 towards H2 S. This work may provide a new pathway for preparing catalysts for cataluminescence sensing with better metal distribution, higher specific surface area, and richer pores than ever before.

Rapid and solvent-free mechanochemical synthesis of Na iron hexacyanoferrate for high-performance Na-ion batteries

Sodium iron hexacyanoferrate (NaFeHCF) containing Fe as the only transition metal element has attained extensive interest for Na-ion batteries due to the low raw material cost and high resource abundance; however, the traditional solution-based synthesis methods, including coprecipitation and hydrothermal reaction, are usually time-consuming and difficult to control the interstitial water and Na content in the lattice, which puts a challenge on the high-efficiency and controllable preparation of NaFeHCF. Herein, we demonstrate a rapid and solvent-free mechanochemical route under the Ar atmosphere to synthesize Na1.59Fe[Fe(CN)6]0.95□0.051.92H2O cathode, which delivers an initial discharge capacity of ∼130 mA h g−1 at 0.2 C and 118 mA h g−1 at a high rate of 10 C. Besides, ex situ XRD confirms that NaFeHCF cathode experienced a phase transition from the initial cubic phase to Na-free cubic structure during the first charging process and followed by a reversible phase change between the monoclinic and Na-free cubic structures in the subsequent cycles. More impressively, the optimized NaFeHCF/NaTi2(PO4)3 full cell reveals a decent capacity retention of 80% over 1000 cycles at 1 C, exhibiting a promising prospect in the practical application. Our current work sheds light on the high-efficient scalable production of high-performance NaFeHCF cathodes for Na-ion batteries via mechanochemical approach.

Morphology and defects design in g-C3N4 for efficient and simultaneous visible-light photocatalytic hydrogen production and selective oxidation of benzyl alcohol

Small surface area, deficient reaction sites, and poor visible-light harvest ability of the original graphitic carbon nitride (g-C3N4) severely restrict its photocatalytic H2 production activity. Here, an ultrathin porous and N vacancies rich g-C3N4 (VN-UP-CN) was fabricated by thermal oxidation exfoliation and high-temperature calcination under the Ar atmosphere. The ultrathin porous morphology increases the surface area and reaction sites of original g-C3N4, moreover, the produced N vacancies greatly broaden the light harvest ability of ultrathin porous g-C3N4 (UP–CN). Therefore, VN-UP-CN displays the maximal H2 production rate of 2856.7 μmol g−1 h−1 in triethanolamine solution under visible-light, and adding 0.5 M of K2HPO4 can further improve its H2 production rate to 4043.9 μmol g−1 h−1. Importantly, VN-UP-CN also shows good performance in simultaneous photocatalytic H2 production and benzyl alcohol oxidation to benzaldehyde with the activities of 196.08 and 198.28 μmol g−1 h−1, respectively, which avoids the waste of sacrificial agent and photogenerated holes. This work affords an achievable way to design the efficient g-C3N4 photocatalyst by morphology and defect regulation, which can effectively utilize both photogenerated electrons and holes for H2 and value-added chemical production.

Sucrose-based sol-gel synthesis of microporous titanium carbide as target material for the production of radioisotopes

An environment-friendly route for the synthesis of titanium carbide by sol-gel processing is reported. The initial solution is prepared via a simple one-pot approach, using titanium isopropoxide and sucrose as molecular precursors under acidic conditions, therefore promoting intimate interactions between the networks of sucrose and of the titanium alkoxide-derived inorganic polymer at the molecular level. Furthermore, the usage of sucrose represents an established and green solution, and provides a viable alternative to the use of toxic compounds. The resulting gels are dried at 120 °C and then undergo carbothermal reduction in Ar atmosphere at different temperatures ranging from 700 °C to 1750 °C. A systematic study of the obtained material is carried out by using FT-IR spectroscopy, X-ray diffraction, Scanning Electron Microscopy, Raman spectroscopy, N 2 physisorption and He pycnometry. The characterization techniques show that the sucrose-derived titanium carbide exhibits a tailored microporous structure and larger specific surface area values compared with those of materials produced using the hazardous phenolic resin-based system. This method thus allows the synthesis of titanium carbide with exquisite textural properties and attractive porous architectures, meeting the requirements for the fabrication of target materials in the field of nuclear medicine, with particular interest in the production and release of scandium radioisotopes. • TiC is proposed as target material for the production of Sc isotopes. • Microporous TiC samples were synthesized via sol-gel chemistry. • Sucrose was successfully employed as an eco-friendly carbon source. • Stoichiometric TiC phase is obtained after carbothermal reduction at 1750 °C. • The material shows larger surface area than that of hazardous-compound-based system.

Two-Layer Rt-QFN: A New Coreless Substrate Based on Lead Frame Technology

Lead frames have been widely used in the semiconductor package assembly industry; a lot of demand is still maintained in fields requiring high reliability, such as automobiles, although many fields are being replaced by laminated substrates according to the recent electronic package product trend that requires high I/O pin count. The purpose of this paper is to introduce two-layer Rt-QFN, one of the lead frame-based coreless substrates. (Rt-QFN is a trademark of Haesung DS, which means premold type lead frame substrate.) two-layer Rt-QFN can secure more advanced design freedom compared with the lead frame and thus has I/O pin count coverage intermediate between the lead frame and laminated substrate. In addition, Rt-QFN can exhibit excellent heat dissipation performance by replacing via holes of the laminated substrate with Cu bumps formed by etching. CAE analysis showed that the thermal resistance of the two-layer Rt-QFN substrate was about 23% lower than that of the laminate substrate. The excellent heat dissipation property of two-layer Rt-QFN allows it to replace the existing expensive ceramic substrate and can achieve cost savings. In addition, the sputtering technique, including the LIS (Linear Ion Source) module, was introduced as a method to sufficiently secure the interfacial adhesion between the resin/Cu interface, which is a key factor in producing a two-layer substrate. As a method to enhance the interfacial adhesion between the resin/Cu interface, the collimated mode of LIS was used in the Ar atmosphere inside the vacuum chamber to activate the resin surface. After plasma pretreatment on the surface of the resin, a Cu seed layer was continuously formed by sputtering. As a result, it was possible to secure the high reliability of the two-layer Rt-QFN substrate, and it was confirmed through the evaluation of interfacial adhesion of more than 1.2 kgƒ/cm during the peel-off tape test at the resin/Cu interface and further moisture absorption evaluation.

Structure and electrical properties of BaCe1−xLaxO3−δ mixed conductors

In this study, BaCe1−xLaxO3−δ (x = 0.05, 0.10, 0.20, and 0.30) oxides synthesized using citrate–nitrate combustion method were calcined at 1100 °C for 5 h and subsequently sintered at 1550 °C for 5 h. Crystal structure, conductivity, and chemical properties were investigated via X-ray diffraction (XRD), alternating current (AC) impedance spectroscopy, and X-ray photoelectron spectroscopy (XPS). XRD analysis shows that all samples have orthorhombic perovskite-type structure, and lattice volume increases with increasing La content. XPS results show that Ba and La valence states are bivalent, and trivalent, respectively. Mixed Ce4+ and Ce3+ valence states can be identified in the samples. Furthermore, there are two types of O binding states in perovskite structure: (i) O–H groups/oxygen vacancies and (ii) lattice oxygen. AC impedance spectra reveal that all samples are proton–oxide ion–electron hole conductors at 400–800 °C in wet O2 atmosphere and proton–electron conductors at 400–800 °C in wet Ar atmosphere. Among these samples, BaCe0.90La0.10O3−δ and BaCe0.95La0.05O3−δ display the highest conductivities at temperatures below and above 500 °C, respectively. Furthermore, BaCe0.95La0.05O3−δ exhibits the highest total conductivity of 1.06 × 10−2 S cm−1 at 800 °C under PO2 ≈ 1 atm and PH2O = 0.006 atm.

Inexpensive and eco-friendly nanostructured birnessite-type δ-MnO2: A design strategy from oxygen defect engineering and K+ pre-intercalation

Nanostructured birnessite-type δ-MnO2 (KMO-V) with intercalated K+ and oxygen vacancies is synthesized by a hydrothermal method at 160 °C followed by annealing under 300 °C in Ar atmosphere with non-toxic and inexpensive γ-MnO2 (Unit price: 0.96–1.6 US$ Kg−1) as raw material. The interplanar spacing in (001) diffraction direction of KMO-V is near 0.719 nm. The considerable initial discharge potential of about 1.8 V, specific energy of 389.88 Wh kg−1, and cyclic stability of 91.9% after 1500 cycles at 1.0 A g−1 of KMO-V are obtained. A hybrid intercalation mechanism of H+ and Zn2+ is revealed, KMO-V has the best diffusion kinetics performance of Zn2+ among the samples according to the calculation based on density functional theory (DFT). The by-product Zn4(OH)6SO4·4H2O and Mn2+ are thought to be the primary factors of capacity decay, the synergy of K+ pre-intercalation and oxygen vacancies plays a critical role of enhancing structural stability and reaction kinetics in KMO-V. Thus, this work provides an eco-friendly research method of inexpensive electrode material in aqueous Zn-ion batteries (AZIBs) for large-scale energy storage.

Controllable Synthesis of Mo3C2 Encapsulated by N-Doped Carbon Microspheres to Achieve Highly Efficient Microwave Absorption at Full Wavebands: From Lemon-like to Fig-like Morphologies.

Mo3C2@N-doped carbon microspheres (Mo3C2@NC) have been discovered to be a family of superior microwave absorbing materials. Herein, Mo3C2@NC was synthesized through a simple high-temperature carbonization process by evaporating a graphite anode and Mo wire in Ar and N2 atmospheres with an N-doping content of 6.4 at. %. Attributing to the self-assembly mechanism, the number of Mo wires inserted into the graphite anode determined the morphologies of Mo3C2@NC, which were the unique lemon-like (1- and 2-Mo3C2@NC) and fig-like (3-, 4-, and 5-Mo3C2@NC) microstructures. 1- and 2-Mo3C2@NC exhibited powerful reflection losses (RLs) of -45.60, -45.59, and -47.11 dB at the S, C and X bands, respectively, which corresponded to thinner thicknesses. 3-, 4-, and 5-Mo3C2@NC showed outstanding absorption performance at the C, X, and Ku bands, respectively, with each value of a minimum RL less than -43.00 dB. In particular, the strongest RL (-43.56 dB) for 5-Mo3C2@NC corresponded to an ultrathin thickness of 1.3 mm. In addition, the maximum effective absorption bandwidth was 6.3 GHz for 4-Mo3C2@NC. After analysis, all Mo3C2@NC samples showed well-matched impedance due to the enhanced dielectric loss caused by the unique carbon structure and moderate magnetic loss derived from the weak magnetic property of Mo3C2. More importantly, the unique lemon-like and fig-like microstructures created sufficient interfaces and differentiated multiple reflection paths, which greatly contributed to the strong microwave absorptions at full wavebands. In full consideration of the simple preparation method and tunable absorption properties, Mo3C2@NC composites can be regarded as excellent electromagnetic wave absorption materials.