Subsequent Annealing Research Articles

Deformation and Annealing Behavior of Cr Coating Prepared by Pack-Cementation on the Surface of Austenitic Stainless Steel.

In this paper, a Cr coating was prepared by induction heating and pack-cementation chromizing on AISI 304 austenitic stainless steel. Then, the cold-rolling deformation and annealing treatment were introduced to refine the coarse matrix grains caused by pack-chromizing and improve the overall performance of 304 austenitic stainless steel. The phase composition, element distribution, and microstructure of the coating were carefully characterized. The microhardness, wear resistance, and corrosion resistance of the coating were tested. The results show that the Cr coating with a thickness of 100 μm is mainly composed of a (Cr,Fe)23C6, (Cr,Fe)7C3, and α-Fe-Cr solid solution. After the cold-rolling deformation and subsequent annealing treatment, the grains are significantly refined and the Cr coating is divided into two layers, consisting of carbon-chromium compounds such as Cr23C6, Cr7C3, Cr2C, and Cr3C2 in the surface layer and a Fe-Cr solid solution in the subsurface layer. The cold-rolling deformation and annealing treatment significantly improved the microhardness and wear resistance of the coated sample, and the corrosion resistance was also better than that of the uncoated sample.

Phase formation, structure and properties of quaternary MAX phase thin films in the Cr-V-C-Al system: A combinatorial study

Tailoring of individual atomic layers via alloying to synthesize quaternary MAX phases allows for expanding their chemical diversity and fine-tuning of their properties. In this study, Cr-V/C/Al multilayered precursors were deposited via combinatorial magnetron sputtering, creating nanostructured architectures with periodic stacking of nanolayers and varying Cr:V ratios. Their phase transformation and underlying reaction mechanisms during subsequent thermal annealing in argon towards the prospective quaternary solid solution (CrV)n+1AlCn MAX phases formation was systematically studied. Crystallization of (CrV)2AlC starts at approximately 500°C, while growth of higher-ordered (CrV)4AlC3 is observed from 960°C. Notably, intergrowth of (CrV)2AlC and (CrV)4AlC3 structures with coherent interface suggests that (CrV)2AlC acts as template for nucleation of (CrV)4AlC3. Thermal stability and high-temperature oxidation tests found that (CrV)2AlC exhibits higher thermal stability compared to (CrV)4AlC3 and films with Cr72.7V27.3 displayed good oxidation resistance at 1000°C, forming a protective bilayer oxide scale consisting of (Cr,Al)2O3 and Al2O3.

Repairing surface defects of carbon fibers: Electrostatic spraying nano-carbons on stabilized PAN-fibers and subsequent thermal annealing

The introducing of nanomaterials on the carbon fibers (CFs) surface is an effective method for repairing surface defects. Due to the chemical inertness of CFs surface, the bonding force between reinforcing material and CFs is weak. In this paper, nano-carbons (graphene oxide and graphene quantum dots) were assembled to defects on surface of stabilized fibers (SFs) through electrostatic spraying assisted by airflow and then thermally annealed to enhance bonding force between nano-carbons and fibers. The distribution of nano-carbons on fibers surface was explored and it was found that the nano-carbons were directionally covered on or filled in defects on fibers surface to reduce specific surface area and pore volume. The chemical structures on fiber surface were discussed by X-ray photoelectron spectroscopy and atomistic ReaxFF molecular dynamics simulation, and it suggested that covalent crosslinking occurred between CFs surface and nano-carbons. And further, the nano-scratch on fibers surface revealed a significant increase in the bonding force between nano-carbons and fiber after thermal annealing. The maximum tensile strength and Young´s modulus of CFs with nano-carbons increased 24.36% and 38.63%, respectively. Subsequently, the finite element simulation ABAQUS was adopted to demonstrate that the addition of nano-carbons alleviated stress concentration at defects on CFs surface. At the same time, the smaller stress and stress diffusion were found in CFs reinforced by graphene oxide than that with graphene quantum dots.

Sustainable conversion of vine shoots and pig manure into high-performance anode materials for sodium-ion batteries

Sodium-ion batteries (SIBs) are considered promising candidates for future grid energy storage, with hard carbons emerging as key commercial anode materials. This study presents a novel approach to synthesize N-doped hard carbons via co-hydrothermal treatment of vine shoots and pig manure and subsequent thermal annealing of the resulting hydrochar. This method enhances the development of micro- and ultra-microporosity in the synthesized hard carbons, with nitrogen, and to a lesser extent phosphorus and sulfur, introduced as doping elements. Furthermore, the incorporation of hydrochloric acid during the hydrothermal step promotes biomass hydrolysis, leading to increased mesoporosity and the formation of microsphere clusters. In the realm of electrochemical performance, an investigation into various ester- and ether-based electrolytes has revealed NaPF6 in diglyme as the best formulation, thanks to its thinner and more stable solid electrolyte interface (SEI). Using this electrolyte, the best-performing electrode showed an initial Coulombic efficiency (ICE) of 73 %, with reversible capacities of 239, 180, 86, and 57 mAh g−1 at 0.1, 1, 5, and 10 A g−1, respectively. In addition, the electrode exhibited a remarkable capacity retention of 88 % after 250 cycles as well as a compatible behavior when paired with a NVPF-based cathode.

In-situ investigation of helium effect on diverse precipitate/matrix interface areas in irradiated 5052 aluminum alloy

Due to its low neutron cross-section and high irradiation swelling resistance, 5052 aluminum alloy is extensively utilized in the core structure and irradiation shielding of research nuclear reactors. Indeed, irradiation-induced defects may accumulate preferentially in the precipitates of aluminum alloys during service, leading to prior impairment of these precipitates, thereby inducing the degradation of mechanical properties. The interaction between irradiation defects and precipitates under irradiation, however, remains inadequately understood. In this work, the microstructure of 5052 aluminum alloy irradiated in-situ with He+ at different temperatures and subsequent annealing was investigated by transmission electron microscopy. After 1.0 dpa irradiation at different temperatures, the evolution of He bubbles within the precipitate was greater than in the matrix, and manifested a pronounced growth threshold between 373 K and 473 K. During post-irradiation annealing, compared to the matrix, also the dynamic evolution of bubbles was observed on the inner side of the precipitate interface, where bubbles coalesced and rapidly dissolved. The dynamic behavior of He bubbles within different precipitates manifested a certain discrepancy, and sink strength of two typical Al-containing precipitates was estimated. The difference in bubble evolution can be attributed to the different sink efficiency and vacancy concentration gradient around the precipitate.

Controlled Fabrication of Metallic MoO2 Nanosheets towards High-Performance p-Type 2D Transistors.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are extensively employed as channel materials in advanced electronic devices. The electrical contacts between electrodes and 2D semiconductors play a crucial role in the development of high-performance transistors. While numerous strategies for electrode interface engineering have been proposed to enhance the performance of n-type 2D transistors, upgrading p-type ones in a similar manner remains a challenge. In this work, significant improvements in a p-type WSe2 transistor are demonstrated by utilizing metallic MoO2 nanosheets as the electrode contact, which are controllably fabricated through physical vapor deposition and subsequent annealing. The MoO2 nanosheets exhibit an exceptional electrical conductivity of 8.4 × 104 S m‒1 and a breakdown current density of 3.3 × 106 A cm‒2. The work function of MoO2 nanosheets is determined to be ≈5.1eV, making them suitable for contacting p-type 2D semiconductors. Employing MoO2 nanosheets as the electrode contact in WSe2 transistors results in a notable increase in the field-effect mobility to 92.0 cm2 V‒1 s‒1, which is one order of magnitude higher than the counterpart devices with conventional electrodes. This study not only introduces an intriguing 2D metal oxide to improve the electrical contact in p-type 2D transistors, but also offers an effective approach to fabricating all-2D devices.

Machine Learning-Aided Analysis of the Rolling and Recrystallization Textures of Pure Iron with Different Cold Reduction Ratios and Cold-Rolling Directions.

We performed a machine learning-aided analysis of the rolling and recrystallization textures in pure iron with different cold reduction ratios and cold-rolling directions. Five types of specimens with different cold reduction ratios and cold-rolling directions were prepared. The effect of two-way cold-rolling on the rolling texture was small at cold reduction ratios different from 60%. The cold reduction ratio in each stage hardly affected the texture evolution during cold-rolling and subsequent short-term annealing. In the case of long-term annealing, although abnormal grain growth occurred, the crystal orientation of the grains varied. Moreover, the direction of cold-rolling in each stage also hardly affected the texture evolution during cold-rolling and subsequent short-term annealing. During long-term annealing, sheets with the same cold-rolling direction in the as-received state and in the first stage showed the texture evolution of conventional one-way cold-rolled pure iron. Additionally, we conducted a machine learning-aided analysis of rolling and recrystallization textures. Using cold-rolling and annealing conditions as the input data and the degree of Goss orientation development as the output data, we constructed high-accuracy regression models using artificial neural networks and XGBoost. We also revealed that the annealing temperature is the dominant factor in the nucleation of Goss grains.

Effect of Bi2MoO6 Morphology on Adsorption and Visible-Light-Driven Degradation of 2,4-Dichlorophenoxyacetic Acid.

The development of highly efficient and stable visible-light-driven photocatalysts for the removal of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) from water is still a challenge. In this work, Bi2MoO6 (BMO) materials with different morphology were successfully prepared via a simple hydrothermal method by altering the solvent. The morphology of the BMO material is mainly influenced by the solvent used in the synthesis (H2O, ethanol, and ethylene glycol or their mixtures) and to a lesser extent by subsequent thermal annealing. BMO with aggregated spheres and nanoplate-like structures hydrothermally synthesized in ethylene glycol (EG) and subsequently calcined at 400 °C (BMO-400 (EG)) showed the highest adsorption capacity and photocatalytic activity compared to other synthesized morphologies. Complete degradation of 2,4-D on BMO upon irradiation with a blue light-emitting diode (LED, λmax = 467 nm) was reached within 150 min, resulting in 2,4-dichlorophenol (2,4-DCP) as the main degradation product. Holes (h+) and superoxide radicals (⋅O2-) are assumed to be the reactive species observed for the rapid conversion of 2,4-D to 2,4-DCP. The addition of H2O2 to the reaction mixture not only accelerates the degradation of 2,4-DCP but also significantly reduces the total organic carbon (TOC) content, indicating that hydroxyl radicals are crucial for the rapid mineralization of 2,4-D. Under optimal conditions, the TOC value was reduced by 84.5% within 180 min using BMO-400 (EG) and H2O2. The improved degradation performance of BMO-400 (EG) can be attributed to its particular morphology leading to lower charge transfer resistance, higher electron-hole separation, and larger specific surface area.

Hydrogen exposure-enhanced superconductivity transition in FeSe/SrTiO3 monolayer

Interface-enhanced superconductivity in FeSe/SrTiO3 (FeSe/STO) monolayers provides the record for the highest transition temperature (Tc) in iron-based compounds. Long-term post annealing is the commonly adopted recipe to induce the superconductivity transition in the not-superconductive as-grown FeSe/STO monolayer. Here, we developed a kinetic method, i.e., hydrogen exposure followed by gentle annealing, to enhance the superconductivity of the FeSe/STO monolayer. Our approach is more efficient than the long-term post annealing. Scanning tunneling microscopy (STM) characterization demonstrated the so far largest superconducting gap of ∼22 mV, indicating an enhanced superconductivity. We believe that the hydrogen-induced lattice Fe diffusion facilitates to remove the interfacial excess Se atoms fatal to the superconductivity, resulting in the formation of a second layer FeSe. The subsequent annealing helps to annihilate the generated Fe vacancies and, thus, enhance the superconductivity in the FeSe/STO monolayer.

New Insights into the Ingot Breakdown Mechanism of Near-β Titanium Alloy: An Orientation-Driven Perspective

The ingot breakdown behavior of a typical near-β titanium alloy, Ti-55511, was investigated by various multi-pass upsetting processes. Particular emphasis was placed on the breakdown mechanism of the ultra-large β grains. The results showed that the upsetting far above the β-transus yielded uniform and refined macrostructure with relatively coarse grain size. In contrast, subtransus deformation within the (α + β) dual-phase field caused severe strain localization and macroscale shear bands. It was found that the static recrystallization during the post-deformation annealing was determined by the preferential grain orientations, which were closely related to the processing conditions. During β-working, the stable <001>-oriented grains were predominant and fragmentized mainly via a so-called “low-angle grain boundary merging” mechanism, even under a fairly low deformation. However, the vast <001> grain area was unbeneficial for microstructural conversion since it provided minor nucleation sites for the subsequent annealing. In contrast, the α/β-working produced the majority <111>-orientated grains, which were strongly inclined to strain localization. Highly misoriented deformation/shear bands were massively produced within the <111> grains, providing abundant nucleation sites for static recrystallization and, hence, were favorable for microstructural refinement. Furthermore, the intrinsic causes for deformation nonuniformity were discussed in detail, as well as the competition between microstructural homogeneity and refinement.

Integrated design of multifunctional lightweight magnetic cellulose-based aerogel with 1D/2D/3D hierarchical network for efficient microwave absorption

Developing electromagnetic wave absorption materials that are both multifunctional and high-performance, while using rational dimensional design and component strategies, remains extremely challenging. Herein, multidimensional, gradient porous, electrically, and magnetically conductive carbon nanocellulose aerogels composed of nanocellulose and highly oriented Ni chains are prepared through freeze-drying and subsequent low-temperature annealing. Interestingly, the Ni/CNCA composite achieves exceptional microwave absorption with a minimum reflection loss (RLmin) value of −62.0 dB and a maximum effective absorption bandwidth (EAB) of 6 GHz, covering the entire Ku-band. The superior electromagnetic wave absorption stems from 1D/2D/3D double-supported interpenetrating network with satisfactory impedance matching, dielectric/magnetic losses, interface/dipole polarization, and multiple/reflection scattering. Furthermore, the Ni/CNCA exhibits a low density, excellent mechanical adaptability, hydrophobicity, and corrosion resistance. This work introduces innovative approaches for developing lightweight, multifunctional microwave absorbers for practical applications.

Isolated single-photon emitters with low Huang–Rhys factor in hexagonal boron nitride at room temperature

The development of stable room-temperature bright single-photon emitters using atomic defects in hexagonal boron nitride flakes (h-BN) provides significant promise for quantum technologies. However, an outstanding challenge in h-BN is the creation and detection of isolated, stable single-photon emitters with high emission rates and with very low Huang–Rhys (HR) factor. Here, we discuss the quantum photonic properties of a single, isolated, stable quantum emitter that emits single photons with a high emission rate and a low HR value of 0.6 ± 0.2 at room temperature. A scanning confocal image confirms the presence of a deserted, single-quantum emitter with a prominent zero-phonon line at ∼578 nm with a well-separated phonon sideband at 626 nm. The second-order intensity-intensity correlation measurement shows an anti-bunching dip of ∼0.25 with an emission lifetime of 2.46 ± 0.1 ns, reinforcing distinct features of the single-photon emitter. The importance of low-energy electron beam irradiation and subsequent annealing is emphasized to achieve stable, reproducible single-photon emitters.

Improvement in Performance and Stability of PbS QD/IGZO Phototransistors Through the Introduction of Ga2O3 Film for Broadband Sensor Applications.

The development of broadband photosensors has become crucial in various fields. Indium-gallium-zinc oxide (IGZO, In:Ga:Zn = 1:1:1) phototransistors with PbS quantum dots (QDs) have shown promising features for such sensors, such as reasonable mobility, low leakage current, good photosensitivity, and low-cost fabrication. However, the instability of PbS QD/IGZO phototransistors under an air atmosphere and prolonged storage remain serious concerns. In this article, two concepts to improve the reliability of PbS QD/IGZO phototransistors were implemented. P-type doping in the PbS QD layer through oxidation allows increasing the built-in potential between IGZO and PbS QDs, leading to enhancement in photoinduced electron-hole pair creation. Second, agglomeration and fusion of a PbS QDs layer were controlled via thermal annealing, which facilitated the transport of photocreated carriers. The p-type doping and interconnection of a PbS QD layer can be achieved by deposition and subsequent thermal annealing of gallium oxide (Ga2O3) on PbS QD/IGZO stacks. The resulting Ga2O3/PbS QD/IGZO phototransistors exhibited high-performance switching characteristics under dark conditions. Notably, they showed a remarkable photoresponsivity of 196.69 ± 4.05 A/W and a detectivity of (5.47 ± 1.4) × 1012 Jones even at a long-wavelength illumination of 1550 nm. While the unpassivated PbS/IGZO phototransistor suffered serious degradation in optical performance after 2 weeks of storage, the Ga2O3/PbS QD/IGZO phototransistor demonstrated enhanced stability, maintaining high performance for over 5 weeks. These findings suggest that Ga2O3/PbS QD/IGZO phototransistors offer a feasible approach for the fabrication of large-scale active matrix broadband photosensor arrays, potentially revolutionizing optical sensing in various cutting-edge applications.

Improving Hydrogen Release From Oxygen‐Functionalized LOHC Molecules by Ru Addition to Pt/C Catalysts

AbstractBimetallic PtRu/C catalysts have been prepared by depositing Ru onto a commercial Pt/C catalyst and subsequent thermal annealing. Different Pt : Ru ratios of 1 : 1, 2 : 1, 4 : 1, 8 : 1, and 16 : 1 have been investigated with annealing temperatures of 350 and 500 °C. Using X‐ray diffraction (XRD), synchrotron powder X‐ray diffraction (SPXRD) combined with pair distribution function (PDF) analysis and scanning transmission electron microscopy with energy dispersive X‐ray spectrum imaging (STEM‐EDXS), we show that diffusion of Ru into the fcc crystal structure of the Pt nanoparticles takes place during thermal annealing. Partial segregation and the formation of Pt nanoclusters on the bimetallic surface was observed depending on the Pt : Ru ratio. The annealing process does not only cause a high Pt dispersion but also affects the electronic structure of the Pt surface by broadening the d‐band thus facilitating the product's desorption from the surface of the bimetallic particle. These beneficial effects have been exemplified for the catalytic dehydrogenation of dicyclohexylmethanol, a promising hydrogen carrier compound with 7.2 wt% hydrogen capacity. The most active catalyst material among the tested supported alloys and monometallic reference materials was Pt4Ru1/C after annealing at 500 °C, which increased the hydrogen release productivity by 65 % compared to the unmodified Pt/C catalyst.

Revealing the high strength and high thermal stability of a nano-lamellar Cu-0.1 at.% Zr alloy

Nano-lamellar structured Cu-0.1 at.% Zr alloys are prepared by the combination of equal channel angular pressing and subsequent cryogenic rolling. The advanced processing route enables a systematic reduction of the lamellae boundary spacing down to 45 nm and a concurrent increase of yield strength up to 626 MPa. Subsequent annealing at temperatures up to 673 K induced recovery processes and uniform coarsening through triple junction motion, avoiding a catastrophic degradation of the microstructure and performance and preserving the yield strength at a level comparable to ultrafine-grained copper. Simultaneously, the tensile uniform elongation experienced a substantial recovery up to 11.9 %. The high thermal stability is mainly attributed to the low mobility of grain boundary and the relaxation of non-equilibrium grain boundaries, both of which are facilitated by the microalloying of Cu with Zr. Based on the theory of dislocation pile-up at boundaries, the Hall-Petch equation is adopted and extended to the lamellar structured Cu-0.1 at.% Zr alloy, revealing a lower-critical bound of 90 nm for lamellae boundary spacing, corresponding to an effective boundary spacing of about 150 nm.

Multinuclear solid state NMR spectroscopy of ternary rare-earth silicides RET 2Si2 and germanides LaT 2Ge2 (RE = Sc, Y, La, Lu; T = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au)

Abstract A series of ternary rare earth – transition metal – tetrelides RET 2 Tt 2 (RE = Sc, Y, La, Lu; T = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au; Tt = Si, Ge) was synthesized by arc melting of the elements and subsequent annealing. The samples were characterized by powder X-ray diffraction and in addition, the structures of REOs2Si2 (RE = Y, La, Lu), LaAu2Si2, LaAg2Ge2 and LaAu2Ge2 were refined from single crystal X-ray diffractometer data. The tetrelides crystallize with the ThCr2Si2 type (I4/mmm) except the platinum compounds which adopt the klassengleiche superstructure of the CaBe2Ge2 type (P4/nmm). The transition metal atoms have tetrahedral tetrel coordination and the tetrahedra condense to layers via common edges. The stacking of these layers leads to Tt−Tt bonds in the ThCr2Si2 type phases and heteroatomic T−Tt bonds in the CaBe2Ge2 type phases. The rare earth atoms fill larger cages within these three-dimensional networks (coordination number 16 with RE@T 8 Tt 8) with site symmetries 4/mmm (ThCr2Si2 type) and 4mm (CaBe2Ge2 type). Systematic multinuclear solid state NMR spectroscopic investigations allowed observing the effect of the involved rare-earth metal, transition metal and tetrel group element, respectively. In particular, 29Si isotropic resonance shifts can be predicted from element-specific increments and interatomic Si–Si bonding interactions manifest themselves in axially symmetric magnetic shielding anisotropies.

Effects of annealing time on microstructural characteristics and mechanical properties of cryo-rolled CoCrFeMnNi high entropy alloys

High-entropy CoCrFeMnNi alloy was subjected to cryo-rolling with a reduction ratio of 25 % and was then annealed at 1050 °C for 5, 25, and 45 min. The results of the recrystallization textures of the various annealed samples showed the presence of similar texture components, indicating that cryo-rolling had only a limited effect on the formation of the subsequent annealing texture. The volume fractions of the different texture components similarly showed no obvious dependence on the annealing temperature. The uniformity of the recrystallization texture suggests the absence of strong preferential nucleation and growth during annealing. The cryo-rolled samples processed with different holding times exhibited a remarkable ambient temperature strength-ductility combination.

Microstructure Evolution of Alloy 800H during Cold Rolling and Subsequent Annealing

Microstructure Evolution of Alloy 800H during Cold Rolling and Subsequent Annealing

Template-assisted growth of Ga-based nanoparticle clusters on Si: effect of post-annealing process on the Ga ion beam exposed 2D arrays fabricated by focused ion beam nanolithography

We demonstrate template-assisted growth of gallium-based nanoparticle clusters on silicon substrate using a focused ion beam (FIB) nanolithography technique. The nanolithography counterpart of the technique steers a focussed 30 kV accelerated gallium ion beam on the surface of Si to create template patterns of two-dimensional dot arrays. Growth of the nanoparticles is governed by two vital steps namely implantation of gallium into the substrate via gallium beam exposure and formation of the stable nanoparticles on the surface of the substrate by subsequent annealing at elevated temperature in ammonia atmosphere. The growth primarily depends on the dose of implanted gallium which is in the order of 107 atoms per spot and it is also critically influenced by the temperature and duration of the post-annealing treatment. By controlling the growth parameters, it is possible to obtain one particle per spot and particle densities as high as 109 particles per square centimetre could be achieved in this case. The demonstrated growth process, utilizing the advantages of FIB nanolithography, is categorized under the guided organization approach as it combines both the classical top-down and bottom-up approaches. Patterned growth of the particles could be utilized as templates or nucleation sites for the growth of an organized array of nanostructures or quantum dot structures.

Silver nanoparticle functionalized heterojunction NiO/SnO2 nanotubes for comprehensive sensitization of acetone sensor

Acetone detection with a good sensitivity and a low operating temperature is crucial owing to the serious harm that acetone can cause to the environment and humans and its importance as a marker of diabetes in human exhalation. One-dimensional (1D) semiconductor metal oxides with easily accessible active sites are potential candidates for use in gas sensors; however, the facile synthesis of 1D nanotubes (NTs) remains a challenge. Herein, Ag–NiO/SnO2 NTs are synthesized via an electrospinning process, followed by spin-coating permeation and subsequent annealing and chemical reduction; furthermore, the gas-sensing properties of the Ag–NiO/SnO2 NTs are investigated. The Ag–NiO/SnO2 NT-based sensor exhibits a rapid response to acetone (12 s) with excellent sensitivity and selectivity at a low operating temperature (190 °C). The lower acetone detection limit of the Ag–NiO/SnO2 NT-based sensor is 50 ppb. The 1D porous NTs efficiently promote gas diffusion during reversible resistance signal cycling and provide numerous active sites. Furthermore, the conjunction of NiO nanograins with SnO2 nanograins forms a heterogeneous construction, enhancing the gas sensitivity of the sensing layers. In addition, Ag nanoparticles homogeneously incorporated onto the surface of the 1D NTs enhance substantially the acetone detection capability based on noble-metal sensitization.