Pulse Plasma Sintering of NiAl-Al2O3 Composite Powder Produced by Mechanical Alloying with Contribution of Nanometric Al2O3 Powder

NiAl-Al₂O₃ composites, fabricated from the prepared composite powders by mechanical alloying and then consolidated by pulse plasma sintering, were presented. The use of nanometric alumina powder for reinforcement of a synthetized intermetallic matrix was the innovative concept of this work. Moreover, this is the first reported attempt to use the Pulse Plasma Sintering (PPS) method to consolidate composite powder with the contribution of nanometric alumina powder. The composite powders consisting of the intermetallic phase NiAl and Al₂O₃ were prepared by mechanical alloying from powder mixtures containing Ni-50at.%Al with the contribution of 10 wt.% or 20 wt.% nanometric aluminum oxide. A nanocrystalline NiAl matrix was formed, with uniformly distributed Al₂O₃ inclusions as reinforcement. The PPS method successfully consolidated NiAl-Al₂O₃ composite powders with limited grain growth in the NiAl matrix. The appropriate sintering temperature for composite powder was selected based on analysis of the grain growth and hardness of Al₂O₃ subjected to PPS consolidation at various temperatures. As a result of these tests, sintering of the NiAl-Al₂O₃ powders was carried out at temperatures of 1200 °C, 1300 °C, and 1400 °C. The microstructure and properties of the initial powders, composite powders, and consolidated bulk composite materials were characterized by SEM, EDS, XRD, density, and hardness measurements. The hardness of the ultrafine-grained NiAl-Al₂O₃ composites obtained via PPS depends on the Al₂O₃ content in the composite, as well as the sintering temperature applied. The highest values of the hardness of the composites were obtained after sintering at the lowest temperature (1200 °C), reaching 7.2 ± 0.29 GPa and 8.4 ± 0.07 GPa for 10 wt.% Al₂O₃ and 20 wt.% Al₂O₃, respectively, and exceeding the hardness values reported in the literature. From a technological point of view, the possibility to use sintering temperatures as low as 1200 °C is crucial for the production of fully dense, ultrafine-grained composites with high hardness.     

Al2O3/ZrO2 Materials as an Environmentally Friendly Solution for Linear Infrastructure Applications

The present work deals with the evaluation of the effect of ZrO₂ on the structure and selected properties of shapes obtained using the centrifugal slip casting method. The samples were made of alumina and zirconia. The applied technology made it possible to produce tubes with a high density reaching 99–100% after sintering. Very good bonding was obtained at the Al₂O₃/ZrO₂ interphase boundaries with no discernible delamination or cracks, which was confirmed by STEM observations. In the case of Al₂O₃/ZrO₂ composites containing 5 vol.% and 10 vol.% ZrO₂, the presence of equiaxial ZrO₂ grains with an average size of 0.25 µm was observed, which are distributed along the grain boundaries of Al₂O₃. At the same time, the composites exhibited a very high hardness of 22–23 GPa. Moreover, the environmental influences accompanying the sintering process were quantified. The impacts were determined using the life cycle analysis method, in the phase related to the extraction and processing of raw materials and the process of producing Al₂O₃/ZrO₂ composites. The results obtained show that the production of 1 kg of sintered composite results in greenhouse gas emissions of 2.24–2.9 kg CO₂ eq. which is comparable to the amount of emissions accompanying the production of 1 kg of Polyvinyl Chloride (PVC), Polypropylene (PP), or hot-rolled steel products.     

Effect of Al2TiO5 Content and Sintering Temperature on the Microstructure and Residual Stress of Al2O3–Al2TiO5 Ceramic Composites

A series of Al₂O₃–Al₂TiO₅ ceramic composites with different Al₂TiO₅ contents (10 and 40 vol.%) fabricated at different sintering temperatures (1450 and 1550 °C) was studied in the present work. The microstructure, crystallite structure, and through-thickness residual stress of these composites were investigated by scanning electron microscopy, X-ray diffraction, time-of-flight neutron diffraction, and Rietveld analysis. Lattice parameter variations and individual peak shifts were analyzed to calculate the mean phase stresses in the Al₂O₃ matrix and Al₂TiO₅ particulates as well as the peak-specific residual stresses for different hkl reflections of each phase. The results showed that the microstructure of the composites was affected by the Al₂TiO₅ content and sintering temperature. Moreover, as the Al₂TiO₅ grain size increased, microcracking occurred, resulting in decreased flexure strength. The sintering temperatures at 1450 and 1550 °C ensured the complete formation of Al₂TiO₅ during the reaction sintering and the subsequent cooling of Al₂O₃–Al₂TiO₅ composites. Some decomposition of AT occurred at the sintering temperature of 1550 °C. The mean phase residual stresses in Al₂TiO₅ particulates are tensile, and those in the Al₂O₃ matrix are compressive, with virtually flat through-thickness residual stress profiles in bulk samples. Owing to the thermal expansion anisotropy in the individual phase, the sign and magnitude of peak-specific residual stress values highly depend on individual hkl reflection. Both mean phase and peak-specific residual stresses were found to be dependent on the Al₂TiO₅ content and sintering temperature of Al₂O₃–Al₂TiO₅ composites, since the different developed microstructures can produce stress-relief microcracks. The present work is beneficial for developing Al₂O₃–Al₂TiO₅ composites with controlled microstructure and residual stress, which are crucial for achieving the desired thermal and mechanical properties.     

Elaboration of Alumina-Zirconia Composites: Role of the Zirconia Content on the Microstructure and Mechanical Properties

Alumina-zirconia (AZ) composites are attractive structural materials, which combine the high hardness and Young’s modulus of the alumina matrix with additional toughening effects, due to the zirconia dispersion. In this study, AZ composites containing different amounts of zirconia (in the range 5–20 vol %) were prepared by a wet chemical method, consisting on the surface coating of alumina powders by mixing them with zirconium salt aqueous solutions. After spray-drying, powders were calcined at 600 °C for 1 h. Green bodies were then prepared by two methods: uniaxial pressing of spray-dried granules and slip casting of slurries, obtained by re-dispersing the spray dried granulates. After pressureless sintering at 1500 °C for 1 h, the slip cast samples gave rise to fully dense materials, characterized by a quite homogeneous distribution of ZrO₂ grains in the alumina matrix. The microstructure, phase composition, tetragonal to monoclinic transformation behavior and mechanical properties were investigated and are here discussed as a function of the ZrO₂ content. The material containing 10 vol % ZrO₂ presented a relevant hardness and exhibited the maximum value of K_I0, mainly imputable to the t → m transformation at the crack tip.     

Al2O3/ZrO2/Y3Al5O12 Composites: A High-Temperature Mechanical Characterization

An Al₂O₃/5 vol%·ZrO₂/5 vol%·Y₃Al₅O₁₂ (YAG) tri-phase composite was manufactured by surface modification of an alumina powder with inorganic precursors of the second phases. The bulk materials were produced by die-pressing and pressureless sintering at 1500 °C, obtaining fully dense, homogenous samples, with ultra-fine ZrO₂ and YAG grains dispersed in a sub-micronic alumina matrix. The high temperature mechanical properties were investigated by four-point bending tests up to 1500 °C, and the grain size stability was assessed by observing the microstructural evolution of the samples heat treated up to 1700 °C. Dynamic indentation measures were performed on as-sintered and heat-treated Al₂O₃/ZrO₂/YAG samples in order to evaluate the micro-hardness and elastic modulus as a function of re-heating temperature. The high temperature bending tests highlighted a transition from brittle to plastic behavior comprised between 1350 and 1400 °C and a considerable flexural strength reduction at temperatures higher than 1400 °C; moreover, the microstructural investigations carried out on the re-heated samples showed a very limited grain growth up to 1650 °C.     

Manufacturing of Al2O3/Ni/Ti Composites Enhanced by Intermetallic Phases

In this study, ceramic–metal composites in the Al₂O₃/Ti/Ni system were fabricated using the slip casting method. Two series of composites with 15 vol.% metal content and different solid phase contents were obtained and examined. A proper fabrication process allows obtaining composites enhanced by intermetallic phases. The microstructure of the base powders, slurries, and sintered composites was analyzed by scanning electron microscope. Analysis of the sedimentation tendency of slurries was carried out. The phase composition of the sintered samples was examined by X-ray diffraction analysis. A monotonic compression test was used to investigate the mechanical properties of the composites. A fractography investigation was also carried out. The research conducted revealed that the slip casting method allows the obtaining of composites enhanced by intermetallic phases (TiNi, Ni₃Ti). The results show the correlation between solid-phase content, microstructure, and mechanical properties of the composites.     

Study on Manufacturing via Slip Casting and Properties of Alumina-Titanium Composite Enhanced by Thialite Phase

This paper aims to study the Al₂O₃/Ti ceramic-metal composite obtained by the slip casting method. Samples containing 50% volume of the solid phase, including 10% volume of the metallic phase, were investigated. The rheological properties were analyzed. Thermogravimetric analysis was performed. The properties of the obtained composite determined the phase composition using and SEM/EDS microstructural analysis and the XRD method. The size of the titanium particles equals 20.6 ± 10.1 mm, which corresponds to 27.5% of the initial size and indicates significant fragmentation of the titanium powder during the manufacturing of the composite. The relative density of the fabricated composites was equal to 99%. The slip casting method allows to obtain the proposed composite additionally enhanced by the presence of TiO₂ and Al₂TiO₅ (thialite). Research results revealed a non-Newtonian character of the composite suspension flow with clear thinning under the influence of increasing shear forces. The obtained composites are characterized by the lack of visible defects (cracks, microcracks and delamination) on the surface.     

Phase Formation,Mechanical Strength,and Bioactive Properties of Lithium Disilicate Glass–Ceramics with Different Al2O3 Contents

Owing to its excellent mechanical properties and aesthetic tooth-like appearance, lithium disilicate glass–ceramic is more attractive as a crown for dental restorations. In this study, lithium disilicate glass–ceramics were prepared from SiO₂–Li₂O–K₂O–P₂O₅–CeO₂ glass systems with various Al₂O₃ contents. The mixed glass was then heat-treated at 600 °C and 800 °C for 2 h to form glass–ceramic samples. Phase formation, microstructure, mechanical properties and bioactivity were investigated. The phase formation analysis confirmed the presence of Li₂Si₂O₅ in all the samples. The glass–ceramic sample with an Al₂O₃ content of 1 wt% showed rod-like Li₂Si₂O₅ crystals that could contribute to the delay in crack propagation and demonstrated the highest mechanical properties. Surface treatment with hydrofluoric acid followed by a silane-coupling agent provided the highest micro-shear bond strength for all ceramic conditions, with no significant difference between ceramic samples. The biocompatibility tests of the material showed that Al₂O₃-added lithium disilicate glass–ceramic sample was bioactive, thus activating protein production and stimulating the alkaline phosphatase (ALP) activity of osteoblast-like cells.     

Heat Treatment-Induced Microstructure and Property Evolution of Mg/Al Intermetallic Compound Coatings Prepared by Al Electrodeposition on Mg Alloy from Molten Salt Electrolytes

A method of forming an Mg/Al intermetallic compound coating enriched with Mg₁₇Al₁₂ and Mg₂Al₃ was developed by heat treatment of electrodeposition Al coatings on Mg alloy at 350 °C. The composition of the Mg/Al intermetallic compounds could be tuned by changing the thickness of the Zn immersion layer. The morphology and composition of the Mg/Al intermetallic compound coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscattered diffraction (EBSD). Nanomechanical properties were investigated via nano-hardness (nHV) and the elastic modulus (EIT), and the corrosion behavior was studied through hydrogen evolution and potentiodynamic (PD) polarization. The compact and uniform Al coating was electrodeposited on the Zn-immersed AZ91D substrate. After heat treatment, Mg₂Al₃ and Mg₁₇Al₁₂ phases formed, and as the thickness of the Zn layer increased from 0.2 to 1.8 μm, the ratio of Mg₂Al₃ and Mg₁₇Al₁₂ varied from 1:1 to 4:1. The nano-hardness increased to 2.4 ± 0.5 GPa and further improved to 3.5 ± 0.1 GPa. The Mg/Al intermetallic compound coating exhibited excellent corrosion resistance and had a prominent effect on the protection of the Mg alloy matrix. The control over the ratio of intermetallic compounds by varying the thickness of the Zn immersion layer can be an effective approach to achieve the optimal comprehensive performance. As the Zn immersion time was 4 min, the obtained intermetallic compounds had relatively excellent comprehensive properties.     

Preparation and Characterization of Mg–Al–B Alloy (Mg0.5Al0.5B2) Via High-Temperature Sintering

Boron and its alloys have long been explored as potential fuel and increasingly replace pure aluminum powder in high-energy formulations. The ignition and burning properties of boron can be improved by making boron alloys. In this study, an Mg–Al–B alloy was synthesized from magnesium, aluminum and boron powders in a 1:1:4 molar ratio by preheating to 600 °C for 30 min, followed by high-temperature sintering in a tube furnace. The effects of sintering temperature (700–1000 °C) and holding time (0.5–10 h) on the phase composition of mixed powders were studied. After the samples were cooled to room temperature, they were ground into powder. The phase composition, micromorphology and the bonding forms of elements of the synthesized samples were studied using XRD, SEM and XPS. The results show that each element exists in the form of simple substance in the alloy. The influence of the sintering temperature on the synthesis reaction of Mg_0.5Al_0.5B₂ is very important, but holding time has little effect on it. With the increase of sintering temperature, the content of the Mg_0.5Al_0.5B₂ phase gradually increases, and the phase content of residual metal gradually decreases. The phase and morphology analyses show that the optimum sintering temperature is 1000 °C with a minimum holding time of 0.5 h. It is expected to be used in gunpowder, propellant, explosives and pyrotechnics with improved characteristics.     

Peculiarities of the Phase Formation during Electroconsolidation of Al2O3–SiO2–ZrO2 Powders Mixtures

This paper is devoted to the sintering process of Al₂O₃–SiO₂–ZrO₂ ceramics. The studied method was electroconsolidation with directly applied electric current. This method provides substantial improvements to the mechanical properties of the sintered samples compared to the traditional sintering in the air. The research covered elemental and phase analysis of the samples, which revealed phase transition of high-alumina solid solutions into mullite and corundum. Zirconia was represented mainly by tetragonal phase, but monoclinic phase was present, too. Electroconsolidation enabled samples to reach a density of 3.0 g/cm³ at 1300 °C, while the sample prepared by traditional sintering method obtained it only at 1700 °C. For the composite Al₂O₃—20 wt.% SiO₂—10 wt.% ZrO₂ fabricated by electroconsolidation, it was demonstrated that fracture toughness was higher by 20–30%, and hardness was higher by 15–20% compared to that of samples sintered traditionally. Similarly, the samples fabricated by electroconsolidation exhibited elastic modulus E higher by 15–20%. The hypothesis was proposed that the difference in mechanical and physical properties could be attributed to the peculiarities of phase formation processes during electroconsolidation.     

Mechanical Properties of Titanium Nitride Nanocomposites Produced by Chemical Precursor Synthesis Followed by High-P,T Treatment

We investigated the high-P,T annealing and mechanical properties of nanocomposite materials with a highly nitrided bulk composition close to Ti₃N₄. Amorphous solids were precipitated from solution by ammonolysis of metal dialkylamide precursors followed by heating at 400–700 °C in flowing NH₃ to produce reddish-brown amorphous/nanocrystalline materials. The precursors were then densified at 2 GPa and 200–700 °C to form monolithic ceramics. There was no evidence for N₂ loss during the high-P,T treatment. Micro- and nanoindentation experiments indicate hardness values between 4–20 GPa for loads ranging between 0.005–3 N. Young''s modulus values were measured to lie in the range 200–650 GPa. Palmqvist cracks determined from microindentation experiments indicate fracture toughness values between 2–4 MPa·m^1/2 similar to Si₃N₄, SiC and Al₂O₃. Significant variations in the hardness may be associated with the distribution of amorphous/crystalline regions and the very fine grained nature (~3 nm grain sizes) of the crystalline component in these materials.     

Effects of Annealing and Thickness of Co60Fe20Yb20 Nanofilms on Their Structure,Magnetic Properties,Electrical Efficiency,and Nanomechanical Characteristics

X-ray diffraction (XRD) analysis showed that metal oxide peaks appear at 2θ = 47.7°, 54.5°, and 56.3°, corresponding to Yb₂O₃ (440), Co₂O₃ (422), and Co₂O₃ (511). It was found that oxide formation plays an important role in magnetic, electrical, and surface energy. For magnetic and electrical measurements, the highest alternating current magnetic susceptibility (χ_ac) and the lowest resistivity (×10⁻² Ω·cm) were 0.213 and 0.42, respectively, and at 50 nm, it annealed at 300 °C due to weak oxide formation. For mechanical measurement, the highest value of hardness was 15.93 GPa at 200 °C in a 50 nm thick film. When the thickness increased from 10 to 50 nm, the hardness and Young’s modulus of the Co₆₀Fe₂₀Yb₂₀ film also showed a saturation trend. After annealing at 300 °C, Co₆₀Fe₂₀Yb₂₀ films of 40 nm thickness showed the highest surface energy. Higher surface energy indicated stronger adhesion, allowing for the formation of multilayer thin films. The optimal condition was found to be 50 nm with annealing at 300 °C due to high χ_ac, strong adhesion, high nano-mechanical properties, and low resistivity.     

Al13Fe4-Al Composites with Nanocrystalline Matrix Manufactured by Hot-Pressing of Milled Powders

The paper describes composites with the matrix containing a nanocrystalline intermetallic Al₁₃Fe₄ phase and microcrystalline aluminium. Mechanically alloyed Al₈₀Fe₂₀ powder, containing a metastable nanocrystalline Al₅Fe₂ phase, was mixed with 20, 30, and 40 vol.% of Al powder and consolidated at 750 °C under the pressure of 7.7 GPa. During the consolidation, the metastable Al₅Fe₂ phase transformed into a nanocrystalline Al₁₃Fe₄ phase. In the bulk samples, Al₁₃Fe₄ areas were wrapped around by networking Al regions. The hardness of the Al₁₃Fe₄-Al composites was in the range of 4.52–5.50 GPa. The compressive strength of the Al₁₃Fe₄-30%Al and Al₁₃Fe₄-40%Al composites was 805 and 812 MPa, respectively, and it was considerably higher than that of the Al₁₃Fe₄-20%Al composite (538 MPa), which failed in the elastic region. The Al₁₃Fe₄-30%Al and Al₁₃Fe₄-40%Al composites, in contrast, showed some plasticity: namely, 1.5% and 9.1%, respectively. The density of the produced composites is in the range of 3.27–3.48 g/cm³ and decreases with the increase in the Al content.     

Role of Liquid-Phase Amount in Ceramization of Silicone Rubber Composites and Its Controlling

The reliable mechanical properties of ceramizable silicone rubber composites during pyrolysis are necessary for their application in the fire-resistant fields. The effects of liquid-phase amount on the mechanical properties of silicone rubber composites are investigated. The results show a positive correlation between the liquid-phase amount and the flexural strength of the residual products pyrolysis below 800 °C. The nano-γ-Al₂O₃ in the fillers reacts with liquid B₂O₃ to form aluminum borate above 800 °C, which consumes the liquid phase and strengthens the residual products to a certain extent. Increasing the B₂O₃ addition and introducing nano-γ-Al₂O₃ can control the liquid-phase amount in the range of 15% to 30%, which makes the composites have better residual strength and support performance. The residual strength of composites pyrolysis at 500 °C to 1000 °C is higher than 2.50 MPa, and the maximum is up to 18.7 MPa at 1000 °C.     

Microstructural Evolution,Tensile Failure,Fatigue Behavior and Wear Properties of Al2O3 Reinforced Al2014 Alloy T6 Heat Treated Metal Composites

The paper focused on an experimental study on the microstructural, mechanical, and wear characteristics of 15 wt.% alumina (Al₂O₃) particulates with an average particle size of 20 µm, reinforced in Al2014 alloy matrix composite as-cast and heat-treated samples. The metal matrix composite (MMC)samples were produced via a novel two-stage stir-casting technique. The fabricated composite samples were subjected to evaluate hardness, tensile strength, fatigue behavior and wear properties for both as cast and T6 heat-treated test samples. The Al2014 alloy and Al2014-15 wt.% Al₂O₃ MMCs were in solution for 1 h at a temperature of 525 °C, quenched instantly in cold water, and then artificially aged for 10 h at a temperature of 175 °C. SEM and X-ray diffraction analyses were used to investigate the microstructure and dispersion of the reinforced Al₂O₃ particles in the composite and the base alloy Al2014. The obtained results indicated that the hardness, tensile and fatigue strength and wear resistance increased when an amount of Al₂O₃ particles was added, compared to the as-cast Al2014 alloy and it was observed that after subjecting the same composite samples to heat treatment, there was further enhancement in the mechanical and wear properties in the Al2014 matrix alloy and Al2014-15 wt.% Al₂O₃ composite samples.     

Synthesis and Characterization of a Nearly Single Bulk Ti2AlN MAX Phase Obtained from Ti/AlN Powder Mixture through Spark Plasma Sintering

MAX phases are an advanced class of ceramics based on ternary carbides or nitrides that combine some of the ceramic and metallic properties, which make them potential candidate materials for many engineering applications under severe conditions. The present work reports the successful synthesis of nearly single bulk Ti₂AlN MAX phase (>98% purity) through solid-state reaction and from a Ti and AlN powder mixture in a molar ratio of 2:1 as starting materials. The mixture of Ti and AlN powders was subjected to reactive spark plasma sintering (SPS) under 30 MPa at 1200 °C and 1300 °C for 10 min in a vacuum atmosphere. It was found that the massive formation of Al₂O₃ particles at the grain boundaries during sintering inhibits the development of the Ti₂AlN MAX phase in the outer zone of the samples. The effect of sintering temperature on the microstructure and mechanical properties of the Ti₂AlN MAX phase was investigated and discussed.     

Characterization of Al2O3 Samples and NiAl–Al2O3 Composite Consolidated by Pulse Plasma Sintering

The paper describes an investigation of Al₂O₃ samples and NiAl–Al₂O₃ composites consolidated by pulse plasma sintering (PPS). In the experiment, several methods were used to determine the properties and microstructure of the raw Al₂O₃ powder, NiAl–Al₂O₃ powder after mechanical alloying, and samples obtained via the PPS. The microstructural investigation of the alumina and composite properties involves scanning electron microscopy (SEM) analysis and X-ray diffraction (XRD). The relative densities were investigated with helium pycnometer and Archimedes method measurements. Microhardness analysis with fracture toughness (K_IC) measures was applied to estimate the mechanical properties of the investigated materials. Using the PPS technique allows the production of bulk Al₂O₃ samples and intermetallic ceramic composites from the NiAl–Al₂O₃ system. To produce by PPS method the NiAl–Al₂O₃ bulk materials initially, the composite powder NiAl–Al₂O₃ was obtained by mechanical alloying. As initial powders, Ni, Al, and Al₂O₃ were used. After the PPS process, the final composite materials consist of two phases: Al₂O₃ located within the NiAl matrix. The intermetallic ceramic composites have relative densities: for composites with 10 wt.% Al₂O₃ 97.9% and samples containing 20 wt.% Al₂O₃ close to 100%. The hardness of both composites is equal to 5.8 GPa. Moreover, after PPS consolidation, NiAl–Al₂O₃ composites were characterized by high plasticity. The presented results are promising for the subsequent study of consolidation composite NiAl–Al₂O₃ powder with various initial contributions of ceramics (Al₂O₃) and a mixture of intermetallic–ceramic composite powders with the addition of ceramics to fabricate composites with complex microstructures and properties. In composites with complex microstructures that belong to the new class of composites, in particular, the synergistic effect of various mechanisms of improving the fracture toughness will be operated.     

Study on Electromagnetic Performance of La0.5Sr0.5CoO3/Al2O3 Ceramic with Metal Periodic Structure at X-Band

A radar absorbing material (RAM) is designed by combining the La_0.5Sr_0.5CoO₃/Al₂O₃ ceramic and the metal periodic structure. The phase constitution and the microscopic morphology of the La_0.5Sr_0.5CoO₃/Al₂O₃ ceramic are examined, respectively. The electrical properties and magnetic properties of the La_0.5Sr_0.5CoO₃/Al₂O₃ ceramic are also measured at the temperature range of 25~500 °C. Based on the experimental and simulation results, the changes in the reflection loss along with the structure parameters of RAM are analyzed at 500 °C. The analytical results show that the absorption property of the RAM increases with the increase in the temperature. When the thickness of the La_0.5Sr_0.5CoO₃/Al₂O₃ ceramic is 1.5 mm, a reflection loss <−10 dB can be obtained in the frequency range from approximately 8.2 to 16 GHz. More than 90% microwave energy can be consumed in the RAM, which may be applied in the high temperature environment.