Phase Of Cu Research Articles

Experimental study of the Al\u2013Cu\u2013Zn ternary phase diagram

The phase diagram of the Al–Cu–Zn ternary system was re-investigated experimentally. The current study was designed to contribute to a better description of those parts of the phase diagram, which are disputed in the current scientific literature. Mutual relations in the family of ternary intermetallic phases τ with cubic, rhombohedral and modulated structure at temperatures 400 °C and 550 °C were described. The phase relation between the γ and γ′ phases was studied at different temperatures. A two-phase field between γ and γ′ was observed below 400 °C, while the transition appears to be second order at higher temperatures. A vertical section between γ and γ′ phases in Cu–Zn and Al–Cu and four isothermal sections at 400 °C, 550 °C, 700 °C and 820 °C, respectively, were constructed.

Effect of Bonding Time on Microstructure and Shear Property of Cu/In-50Ag/Cu TLP Solder Joints

Here, we study the effect of bonding time on the intermetallic compound (IMC) formation and shear property of Cu/In-50Ag/Cu TLP solder joints based on low-temperature transition liquid phase (TLP) bonding of In-Ag composite powder. The bonding process was carried out at a temperature of 260°C using a wafer bonding machine. The results show that the solder joints are composed of an (Ag,Cu)2In phase and a Cu2In phase in the interfacial diffusion reaction zone and an Ag2In phase and In rich phase in the in␣situ reaction zone. It was observed that the (Ag,Cu)2In phase is formed firstly in the diffusion reaction zone when the bonding time is 0.5 min, then transforms completely into the Cu2In phase when the bonding time reaches 30 min. The shear strength increased and then decreased with increasing bonding time. The observed shear fracture mode of solder joints is brittle.

The effect of iron on the microstructure and mechanical properties of a cast Cu–12Sn-1.5Ni (wt. %) alloy

Effect of Fe content on microstructure and mechanical properties in a gravity-cast Cu–12Sn-1.5Ni (wt. %) alloy has been investigated. Addition of 0.053, 0.63 and 1.44 wt % Fe caused grain refinement, vanishment of continuous interdendritic δ phase, and formation of nano-sized iron precipitates within the dendritic matrix. There was a simultaneous increase in the yield and tensile strength, uniform and total elongation and hardness of Cu–12Sn-1.5Ni-(0.05–1.5)Fe alloys relative to their Fe-free counterpart. Fracture surface analysis revealed an intergranular fracture along the interdendritic δ phase in Cu–12Sn-1.5Ni alloy. But in Cu–12Sn-1.5Ni-(0.05–1.5)Fe alloys, after initiation at interdendritic area, crack propagated into the dendrite matrix in a dimple rupture mode. The evolution mechanism of mechanical properties upon alloying content was discussed in terms of the microstructure characterization, analysis of strain-hardening rate, fractographic examination, as well as the strengthening model. This work sheds light on the alloying effect of minor Fe below 1.0 wt % on the microstructure-mechanical optimization in tin bronze alloys.

Secondary-Phase-Assisted Grain Boundary Migration in CuInSe_{2}.

Significant structural evolution occurs during the deposition of CuInSe_{2} solar materials when the Cu content increases. We use insitu heating in a scanning transmission electron microscope to directly observe how grain boundaries migrate during heating, causing nondefected grains to consume highly defected grains. Cu substitutes for In in the near grain boundary regions, turning them into a Cu-Se phase topotactic with the CuInSe_{2} grain interiors. Together with density functional theory and molecular dynamics calculations, we reveal how this Cu-Se phase makes the grain boundaries highly mobile.

A DFT Study and Microkinetic Simulation in Propylene Oxidation on the “29” CuxO/Cu(111) Surface

The propylene epoxidation reaction in a rich variety of other industrially important catalytic processes was used for synthesis of many value-added products. As an attractive and environmentally friendly process for direct epoxidation of propylene by Cu-based catalysts, the active oxidation states of Cu (Cu0, Cu+, or Cu0/Cu+) in the epoxidation reaction were widely studied. Herein, “29” CuxO/Cu(111) was chosen to act as a precursor to bulk oxidation, as it can be observed in special oxide phases of Cu (Cux+). In our work, the crucial competitive reactions of dehydrogenation versus the epoxidation using the lattice oxygen (Olatt*) and adsorbed molecular oxygen as the oxidants were investigated on “29” CuxO/Cu(111) surface by virtue of the periodic density functional theory computational method. Because of the higher energy barriers of the key steps for the lattice oxygen (Olatt*) as the oxidant, there are two active species in the whole reaction processes of propylene oxidation with O2 as oxidant: the molecular O2* species and the atomic O* species. It was found that the molecular oxygen is difficult to dissociate on the surface directly, while the atomic O* species can be produced via the C3H6* + Olatt* + OO* = C3H5* + Olatt* + OOH* = C3H5Olatt* + O* + OH* mechanism or by the mechanism of C3H6* + OO* = dioxametallacycle = oxametallacycle + O*. It was found that the O* mechanism is preferred for the PO formation as compared to that of O2* on pure “29” CuxO/Cu(111) surface. Promising results in terms of selectivity were achieved for PO formation, which can reach 49% with the apparent activation energy of only 0.62 eV by microkinetic simulation. Therefore, “29” CuxO/Cu(111) surface involving the Cux+ active phase is expected to show an efficient PO selectivity, which is much higher that of Cu+ of Cu2O(111) and similar to that of metallic Cu0. It is hoped that the present study could provide a theoretical guide for the further development of catalysts for propylene epoxidation.

Модификация структурно-фазового состояния и электрических свойств медьсодержащих фуллеритовых пленок при термическом отжиге в вакууме

Методами растровой и атомно-силовой микроскопии, рентгеновской дифракции, рентгеноспектрального микроанализа и дифракции отраженных электронов исследованы изменения структуры, элементного и фазового состава пленок фуллерит–медь с атомной долей металла 0.5, 1, 2 и 4%, осажденных на окисленные монокристаллические подложки кремния и подвергнутых термическому воздействию в вакууме при разных температурах (470, 520, 570 и 620 К). Установлено, что в результате термического отжига при Т = 520 К (t = 1 ч) формируется нанокристаллическая структура со средним размером структурных элементов 33, 42, 50 и 65 нм для пленок фуллерит–медь с атомной долей металла 0.5, 1, 2 и 4% соответственно. Повышение температуры и времени отжига приводит к увеличению размера структурных элементов и образованию новой фазы Cux C60, относящейся к моноклинной пр. гр. P2/m. Методом электросиловой микроскопии и четырехзондовым методом исследованы изменения локальных электрических свойств и удельного электросопротивления медьсодержащих фуллеритовых пленок при отжиге в вакууме.

Investigation of Cu particles size and bonding time on the microstructure and shear property of Cu/In-45Cu/Cu solder joints

The effect of Cu particles size and bonding time on the microstructure and shear property of Cu/In-45Cu/Cu solder joint was studied, and the shedding mechanism of intermetallic compounds (IMCs) in the solder joint during transient liquid phase (TLP) bonding process was investigated. The results showed that the microstructure of Cu/In-45Cu/Cu solder joint was composed of Cu11In9 phase, residual In phase and Cu particles, and the microstructure of solder joints prepared by small Cu particles was dense. The content of IMCs was increased with increasing bonding time, and the Cu/In-45Cu/Cu solder joint was composed of Cu11In9 phase and Cu particles under bonding time 30 min. The Cu2In phase was formed in the solder joint at 60 min, and many cracks appeared at the interface of Cu11In9 and Cu2In phases. The shear strength of Cu/In-45/Cu solder joints with brittle fracture was increased firstly and then decreased with increasing bonding time, and the maximum shear strength of the solder joint was 16.35 MPa at 30 min.

Judging the phase transition pressure of the unknown parent phase if the resulting state is known.

In high-pressure phase transition experiments, the crystal structure of the intermediate phase in some phase transitions is difficult to successfully measure due to the limitations of the experimental conditions. The absence of crystal structure data for the intermediate phase also makes it difficult to calculate the pressure point from the intermediate phase to the new phase by the traditional thermodynamic criterion in theoretical simulations. The Conch Criterion is employed by us to successfully verify the phase transition points by observing the reverse shifts of the DOS (electron density of states) curves for the new phase of Cu2S, PbS, PbSe and PbTe, which breaks through the constraints of the traditional criterion and realizes tracing the source of the phase transition in theoretical calculations.

Manifestation of intermediate phase in Cu doped Si-Te glasses

Alternating differential scanning calorimetry (ADSC) analysis has been carried out on bulk Si15Te85-xCux (1 ≤ x ≤ 10) glasses to examine the thermal properties of the glassy samples in detail. The investigations on the compositional dependence of ΔHNR (non- reversing enthalpy) manifests a trough between the composition 2 ≤ x ≤ 6 which stands as the basis of the presence of Boolchand's Intermediate phase in that range. Additionally, anomalous behavior has been observed at the compositional dependence of various thermal parameters at x = 9, which stipulates the appearance of chemical threshold at the stated composition. Annealed bulk samples have been exposed to XRD studies to discern the type of formed crystalline phases. The study has reported the manifestation of Te, Si1Te2, Si2Te3, Cu3Te2 phases.

Photodegradation of Methylene Blue Dye Using Nanocomposites of Copper Sulfide Doped with Fe/Cd/Zr as Nanophotocatalyst

Objectives: To characterize and to know the nature of the synthesized products and identify the photocatalytic response of the hybrid nanocomposites was investigated by the decolourization of (MB) dye under natural Sunlight radiation. Methods/statistical analysis: We report fabrication of mesoporous nanocomposites (Fe–CuS, Cd–CuS, Zr–CuS) with uniform nanostructures via a hydrothermal method employing copper nitrate trihydrate (Cu (NO3).3H2O), Thiourea (Tu,Sc(NH2)2, Iron trinitrate (Fe(No3)3), (Cd(No3)3) and (Zr(No3)3) as starting materials with cationic surfactant cetyltrimethylammonium bromide (CTAB) as stabilizer/ size controller and ethylene glycol as solvent at 130˚C temperature. The products were characterized by X-ray diffraction (XRD), Transmission Electron Microscopy TEM, and UV-Vis spectrum (UV) analysis. Findings: X-ray diffraction (XRD) spectra confirmed the Fe doped copper sulfide nanocomposites are crystalline in nature with the hexagonal CuS phase. XRD pattern showed Fe doped spherical structure of CuS with a grain size of 21 nm. An intense surface Plasmon resonance between 250–300 nm in the UV–Vis spectrum clearly reveals the formation of Fe doped copper sulfide nanocomposites. The XRD pattern of Cd incorporated CuS shows crystallite nature and crystallinity increases with the addition of cadmium on CuS. The observed results show that Zr is homogeneously dispersed on CuS and well separated from one another. The XRD results confirm the presence of well-dispersed cubic ZrS2 on the covellite CuS surface. Photo reactivity of copper sulfide nanoparticles as undoped and doped with ferrous, cadmium and Zirconium were studied in the decolorization of methylene blue under natural sunlight. The concentration of dopants and concentration of dye are variables that influence the photoreactivity of nanocatalysts. Application/improvements: The prepared nanoparticles CuS, Fe–CuS, Cd–CuS and Zr–CuS show the most degradation efficiency. The optimum dosage of the photocatalyst was 120 mg/L. In the optimized conditions, the degradation efficiency was 86–95% for methylene blue in irradiation time of 150 min.Keywords: Nanophotocatalyst, Copper Sulfide, Methylene Blue, Photodegradation.

Study and Prepared of CuS Thin Films by Ultrasonic Nebulizer Method

Thin Film of copper sulfide with different thickness (100, 200 and 300) nm have been prepared on pre-heated glass substrates up to (330oC) by Ultrasonic Nebulizer Deposition (UND). The effect of thickness on the structural, optical, and electrical properties of films has been investigated. The results of the XRD show that the film which deposited with thickness CuS phase with (103) orientation. Atomic force measurement showed the grain size increase with thickness in the range of (76.91-101.32 nm). The optical properties of the films have been studied over a wavelength (370-1100)nm. The calculated optical energy band gap values were between 2.55 and 2.62eV, depending on the film thickness and in which phase crystallized. The effect of thickness on the electric properties the films have a positive Hall coefficient (p-type), the hall effect increases as thickness increase this due to inversely depend RH on carrier concentration.

One-pot Preparation of Cu2(OH)3NO3 Nanosheets and Cu(OH)2 Nanowires

Introduction: By using Cu(NO3)2 as precursor and polyvinylpyrrolidone (PVP) as surfactant, nanosheets of Cu2(OH)3NO3, nanowires of Cu(OH)2 or the mixture of the two were prepared under different molar ratios of OH− to Cu2+. Materials and Methods: The crystal structures and morphologies of the products were characterized by X-Ray Diffraction (XRD) and Transmission Electron Microscope (TEM). Results: When the molar ratio of OH− to Cu2+ in reaction solution is lower than 1.28, pure Cu2(OH)3NO3 nanosheets were obtained. The thickness of one piece of nanosheet is about 167 nm. The Cu2(OH)3NO3 nanosheets consists of two types of crystal structures, monoclinic phase and orthorhombic phase. With increase of the molar ratio of OH− to Cu2+, the monoclinic phase of Cu2(OH)3NO3 was transferred to the orthorhombic phase of Cu2(OH)3NO3. When the molar ratio of OH− to Cu2+ is within 1.28-2.24, the product is the mixture of Cu2(OH)3NO3 nanosheets and Cu(OH)2 nanowires. And when this molar ratio is higher than 2.24, only Cu(OH)2 nanowires were produced. The lengths and the diameters of the Cu(OH)2 nanowires are in the region of 50-250 nm and 10 nm, respectively. Conclusion: The reason of the Cu2(OH)3NO3 nanosheets changing into the Cu(OH)2 nanowires is that the OH− anions replace the NO3 − anions in the layered Cu2(OH)3NO3 nanosheets, which causes the rupture of hydrogen bonds connecting the adjacent layers. The Cu(OH)2 nanowires were not stable and found to become spindled CuO nanosheets in air at room temperature.

Development of high-coercivity state in high-energy and high-temperature Sm-Co-Fe-Cu-Zr magnets upon step cooling

The work compares the peculiarities of the high-coercivity state formation in the Sm-Co-Fe-Cu-Zr high-temperature and high-energy permanent magnets (HTPM and HEPM) in the course of the heat treatment with the stepwise decreasing temperature from 830 to 400 °C. Two types of magnets with varying Fe concentration, i.e., Sm(Co0.88-xFexCu0.09Zr0.03)7 with x = 0–0.12 (the HTPMs) and Sm(Co0.91-xFexCu0.06 Zr0.03)7.5 with x = 0.24–0.33 (the HEPMs) were studied at different temperatures of heat treatment for phase formation by x-ray diffraction followed by magnetic property measurements. Microstructure characterization was performed using transmission electron microscopy, whereas the three-dimensional elemental distribution at near-atomic scale was obtained using atom probe tomography. In HEPMs, the main increase in coercivity and relaxation of stresses accompanied by intensive enrichment of the 1:5 phase in Cu are observed at high temperatures (Т ≈ 700 °C). In HTPMs, the coercivity monotonously increases in the entire temperature range of the slow cooling from 700 to 400 °C at a rate of 0.5 °C/s. At the temperature close to the Curie temperature (∼550 °C) of the Sm(Co,Cu)5-type phase, the anomaly of the coercivity increment has been observed. The interphase stresses grow and the elemental redistribution appears to be accelerated simultaneously. The non-uniform Cu distribution in the 1:5 phase can be described by the formation of Cu-rich interlayers at the interface of the Sm(Co,Cu)5 and Sm2(Co,Fe)17-type phases.

Enhancing the Efficiency of Dye Sensitized Solar Cells by Using Anatase and Brookite Mixed Phases of TiO2

The phases of TiO2 have an important role in the efficiency of dye sensitized solar cells (DSSCs). In this research, the effects of anatase and mixed phases (brookite and anatase) of TiO2 on the efficiency of DSSCs have been reported. 2% Cu doped TiO2 (Cu:TiO2) thin films are prepared by sol–gel dip coating technique. These films are annealed at the temperatures of "350 °C, 450 °C, 550 °C and 650 °C." XRD showed that at 550 °C and 650 °C, films have mixed phases of TiO2. Also, it showed that grain size is increased by increasing temperature. No extra phases of Cu like CuO etc. were observed in XRD graphs, which confirmed that Cu is completely dissolved into TiO2. UV-Vis results showed that all the films have more than 80% transmittance in the visible region. The optical band gap energy (Eg) of films at "350 °C, 450 °C, 550 °C and 650 °C" is 3.96, 3.81, 3.75 and 3.64 eV, respectively. The electrical resistivity of these thin films by four point probe technique is calculated as 121.8 × 102 Ω-m, 95.69 × 102 Ω-m, 86.65 × 102 Ω-m and 76.3 × 102Ω-m respectively. Solar simulator results showed that the current density (Jsc) of mixed phase is higher than single phase, causing higher efficiency. The maximum Jsc 5.12 mAcm–2 is achieved at 650 °C (mixed phase), resulting in higher efficiency of 2.35% which is more than 100 times the efficiency of anatase phase which has 1.18%. This work will provide a way to enhance the efficiency of DSSCs by using mixed phases of TiO2. In future, if 1-D structure of these mixed phases is used in DSSCs then efficiency of DSSCs will improve definitely (or positively).

Towards phase pure CZTS thin films by SILAR method with augmented Zn adsorption for photovoltaic applications

Cu2ZnSnS4 (CZTS) thin films were deposited from a single cationic bath by Successive Ionic Layer Adsorption and Reaction (SILAR) method. Regular SILAR route for CZTS had the drawback of preferential adsorption of copper and tin cations in comparison with zinc. This resulted in Cu3SnS4 (CTS) and Cu2S phases rather than phase pure CZTS films. A modified SILAR route, with a separate bath for Zn2+ ions, circumvented the difficulty and hence led to phase pure CZTS thin films. UV–visible absorption spectra of the CZTS thin films showed absorption coefficients of ~ 104 cm−1 and a band gap of 1.5 eV. Combined van der Pauw and Hall measurements of CZTS thin films showed a resistivity of approximately 1.51 × 102 Ωcm, carrier density of ~ 1.28 × 1017 cm−3, and mobility ~ 0.32 cm2 V−1s−1. A completely solution processed P–N junction was fabricated and characterized by forming glass/FTO/TiO2/CdS/CZTS multilayer.

Cu and Zn isotope fractionation during extreme chemical weathering

Copper and Zn are trace metal micronutrients whose stable isotope systematics are receiving increasing attention as possible paleoenvironmental tracers. However, to realise this potential, their behaviour during chemical weathering must be better constrained. We present coupled Cu and Zn isotope data for a well-characterised Indian laterite weathering profile, which includes a full suite of samples from unaltered greywacke bedrock to indurated lateritic duricrust. This sample set provides an exceptional opportunity to interrogate Cu and Zn isotope compositions during an extreme example of chemical weathering. Despite their occurrence in different host phases within the parent greywacke, Cu and Zn isotopes behave coherently during weathering. We observe preferential loss of heavy isotopes at increasing degrees of alteration, with 0.6‰ total variability in δ66Zn and 0.9‰ in δ65Cu. Given the absence of evidence for CuS or ZnS phases in the parent lithology, we attribute the liberation of heavy isotopes to organic complexation in the aqueous phase and/or incorporation of light isotopes in secondary aluminous Fe-oxides. Strong enrichment of both metals is also associated with a peak in Mn at a previously identified paleo-water table horizon. This dataset confirms that weathering under oxygenated conditions releases isotopically heavy Cu, regardless of the host phase. Meanwhile, Zn isotopes are only fractionated to any significant extent at the most extreme degrees of chemical weathering reached during lateritization. We conclude that the isotopic composition of the weathering-derived input of Zn to rivers should be largely insensitive to climate change on geological timescales.

High temperature–high pressure phase transformation of Cu

DFT calculations (typically done at 0 K), Helmholtz free energy and Gibbs free energy calculations (for high temperature) within the quasi-harmonic approximation have been done in this manuscript to probe a metastable phase of Cu observed at high temperature and high pressure. The high electronic thermal conductivity observed for this new metastable phase of Cu may be beneficial for high temperature engineering applications in electronic and spacecraft device designs.

Underpotential deposition of SnBi thin films for sodium ion batteries: The effect of deposition potential and Sn concentration

Bimetallic SnBi film was deposited on a Cu foil substrate via the electrochemical atomic layer deposition (E-ALD) technique. The deposition attainment of Sn and Bi were investigated using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The deposition potential of Bi was varied in the underpotential deposition (UPD) region and the concentration of Sn was varied in the SnBi bimetallic material. The materials were characterised using field emission scanning electron microscopy coupled with energy dispersive spectroscopy (FE-SEM/EDS) for morphology and elemental distribution, focused ion beam scanning electron microscopy (FIBSEM) for thickness, X-ray diffraction (XRD) for crystallinity and inductively coupled plasma mass spectroscopy (ICP-MS) for composition measurements. Bi deposited at different UPD regions was structurally different. The deposits were crystalline SnBi materials containing Sn, Bi and other phases of Cu and Sn. Bi was concentrated on the surface, while Sn was distributed evenly across the film. The SnBi electrodes were tested as anode materials in Na-ion batteries using galvanostatic cycling (GC), CV and electrochemical impedance spectroscopy (EIS). Initial discharge capacities of 1900 mAh g−1 for SnBi (1:1) and 341 mAh g−1 for SnBi (3:1) electrodes at 38.5 mA g−1 were obtained, while the electrodes suffered capacity loss after 10 cycles.

МИНЕРАЛОГО-ГЕОХИМИЧЕСКИЕ ОСОБЕННОСТИ ИСТОЧНИКА ЯМКУН (ЗАБАЙКАЛЬСКИЙ КРАЙ)

Актуальность. Источник Ямкун известен как один из немногих радиоактивных источников на территории Забайкальского края. Его воды ранее использовались для лечения болезни Кашина–Бека (уровской болезни), в настоящее время на его базе функционирует краевой центр медицинской реабилитации для лечения заболеваний костно-мышечной, нервной систем, в том числе ДЦП, а также заболеваний кожи. Цель: выявление минералого-геохимических особенностей компонентов источника Ямкун. Объекты: воды озера, ручья, травертины, железистые осадки в русле ручья, солевые отложения на трубе каптажного колодца, растительность (лишайники, мхи, хвоя сосны, листья тополя). Методы: масс-спектрометрия с индуктивно связанной плазмой, инструментальный нейтронно-активационный анализ, рентгеновская дифрактометрия, сканирующая электронная микроскопия. Результаты. Воды источника Ямкун являются гидрокарбонатными кальциево-магниевыми, радиоактивными радоновой природы Они существенно обогащены Mn, Fe, Be, As, Zr, Cs, Hg, Tl, Li, Co, Ba относительно воды оз. Байкал. Обогащение воды редкими щелочными и щелочноземельными элементами (Li, Be, Cs), а также Hg, Tl, As и Zr позволяет предположить, что их источником являются редкометалльные гранитоиды. При разгрузке вод наблюдается современное минералообразование в виде «ямкунской пены» (арагонит) на поверхности водных ванн, трубах каптажного колодца (несквегонит) и ручье (железистые осадки). Железистые осадки и травертины представлены в основном магнезиальным кальцитом. При этом в железистых осадках выявлены в больших количествах соединения Fe и Mn, вероятно гидроксиды. В гидроксидах Mn встречаются также соединения Ba, K, Cl, в гидроксидах Fe – As, P, Si, Al. Из редких микровключений в травертинах отмечены интерметаллические соединения Cu и Sn, La-Ce карбонат, а также сульфид As (леллингит?). Установлена унаследованность состава современных железистых осадков и травертинов. Образующиеся травертины обогащены теми же элементами, что и вода в источнике.

Effects of iron content on tribological properties of Cu-Fe-based friction material

Purpose The purpose of this study is to investigate the effect of iron content on the friction and wear performances of Cu–Fe-based friction materials under dry sliding friction and wear test condition. Design/methodology/approach Cu–Fe-based friction materials with different iron content were prepared by powder metallurgy route. The tribological properties of Cu–Fe-based friction materials against GCr15 steel balls were studied at different applied loads and sliding speeds. Meanwhile, microstructure and phases of Cu–Fe-based friction materials were investigated. Findings Cu–Fe-based friction materials with different iron content are suitable for specific applied load and sliding speed, respectively. Low iron content Cu–Fe-based friction material is suitable for a high load 60 N and low sliding speed 70 mm/min and high iron content Cu–Fe-based friction material will be more suitable for a high load 60 N and high sliding speed 150 mm/min. The abrasive wear is the main wear mechanism for two kinds of Cu–Fe-based friction materials. Originality/value The friction and wear properties of Cu–Fe-based friction materials with different iron content were determined at different applied loads and sliding speeds, providing a direction and theoretical basis for the future development of Cu–Fe-based friction materials.