Polycrystalline Al2O3 Research Articles

Influence of local structures on amorphous alumina exhibiting resistance random-access memory function

Amorphous alumina resistance random-access memory is a promising candidate as a next-generation nonvolatile memory. It is intriguing that the nonvolatile memory function emerges in only amorphous samples, unlike crystalline samples. We studied local structures of amorphous alumina samples and Al2O3 polycrystalline using atomic pair distribution function measurements. We derived the Al–Al, O–O, and Al–O atomic distances for each sample. By comparing them, we revealed that the subtle difference in the local structure significantly influences the performance of a nonvolatile memory function.

In situ studies on temperature‐dependent deformation mechanisms of Al2O3 prepared by spark plasma sintering

AbstractAlumina (α‐Al2O3) is one of the most versatile engineering ceramics, and its mechanical properties have been extensively studied. However, the micromechanical properties of Al2O3 with a fine microstructure are less well understood. Here, we present one of the first investigations that probe the micromechanical properties of fine‐grained polycrystalline Al2O3 fabricated via spark plasma sintering, employing in situ microcompression tests inside a scanning electron microscope. This study explores the influence of temperature variations on the deformation mechanisms, particularly the involvement of microcracks and dislocation activities throughout the deformation process. As temperature rises, substantial deformability occurs in the inherently brittle Al2O3 at intermediate temperature, where the improved plastic deformability mainly arose from prominent dislocation activities accompanied by grain boundary sliding. This study sheds light on understanding the relationship between defect evolution and mechanical behavior in Al2O3 with fine grain sizes.

Neutron irradiation impact on structural and electrical properties of polycrystalline Al2O3

High energy neutron irradiations impact on structural and electrical properties of alumina are studied with particular emphasis on real time in-situ radiation induced conductivity measurement in low flux region. Polycrystalline Al2O3 samples are subjected to high energy neutrons produced from D-T neutron generator and Am–Be neutron source. 14 MeV neutrons from D-T generator are chosen to study the role of fast neutron irradiation in the structural modification of samples. Real time in-situ electrical measurement is performed to investigate the change in insulation resistance of Al2O3 due to radiation induced conductivity at low flux regime. During neutron irradiation, a significant transient decrease in insulation resistance is observed which recovers relative higher value just after neutron exposure is switched off. XRD results of 14 MeV neutron irradiated samples suggest annealing effect. Impact of relatively low energy neutrons on the structural properties is also studied using Am–Be neutrons. In this case, clustering is observed on the sample surface after prolonged neutron exposure. The structural characterizations of pristine and irradiated Al2O3 samples are performed using XRD, SEM, and EDX. The results from these characterizations are analysed and interpreted in the manuscript.

Sintering, microstructure, and mechanical properties of ZrO2-doped Al2O3

ABSTRACT In the present work, the effect of Zr-doping on the sintering, microstructure development, and mechanical properties of polycrystalline Al2O3 was studied. Dopant concentrations of 830 and 2070ppm cationic ratio of Zr in Al2O3 corresponding to 2000 and 5000 wt. ppm of ZrO2 in Al2O3, respectively, were used. The sintering schedule of undoped as well as Zr-doped Al2O3 samples was optimised following a series of precoarsening experiments. The Zr-doped Al2O3 ceramics exhibited controlled grain size and improved density. Upon increasing the Zr-dopant concentration from 830 cat. ppm to 2070 cat. ppm. Zr-doped Al2O3 ceramics having a refined microstructure with a homogeneous distribution of controllably grown ZrO2 in Al2O3 matrix was obtained. Microhardness of the Zr-doped samples showed negligible dependence with Zr-doping, whereas mechanical strength was found to improve with it. The improvement in strength was attributed to the combined effect of improved sinter density, microstructural refinement, and grain-boundary strengthening.

Ultra-high quality factor of transparent Al2O3 ceramics fabricated by vacuum sintering and post hot isostatic pressing

Transparent polycrystalline Al2O3 (PCA) ceramics were prepared by vacuum sintering followed by hot isostatic pressing (HIP) using MgO as a sintering aid. The prepared sample with thickness of 0.8 mm exhibits a transmittance of 50% (λ = 640 nm) and 78% (λ = 4.5 μm) in the entire visible band and infrared band, respectively. Effects of additive content, sintering temperature as well as HIP method on the microstructure of Al2O3 ceramics were systematically investigated. The experimental results show that the effect of HIP on the transmittance of PCA in the visible light region is particularly obvious, which can increase the transmittance by almost two times. In particular, the quality factor of pure Al2O3 ceramics prepared by the same process is as high as 1,120,970 GHz (at 13.6 GHz) owing to extremely high density and reduction of the grain boundary, which render Al2O3 ceramics potential candidates for fabrication of multilayer electronic devices.

Enhanced Mechanical Properties of Al2O3 Nanoceramics via Low Temperature Spark Plasma Sintering of Amorphous Powders.

In this work, Al2O3 nanoceramics were prepared by spark plasma sintering of amorphous powders and polycrystalline powders with similar particle sizes. Effective comparisons of sintering processes and ultimate products depending on starting powder conditions were explored. To ensure near-full density higher than 98% of the Al2O3 nanoceramics, the threshold temperature in SPS is 1450 °C for polycrystalline Al2O3 powders and 1300 °C for amorphous powders. The low SPS temperature for amorphous powders is attributed to the metastable state with high free energy of amorphous powders. The Al2O3 nanoceramics prepared by amorphous powders display a mean grain size of 170 nm, and superior mechanical properties, including high bending strength of 870 MPa, Vickers hardness of 20.5 GPa and fracture toughness of 4.3 MPa∙m1/2. Furthermore, the Al2O3 nanoceramics prepared by amorphous powders showed a larger dynamic strength and dynamic strain. The toughening mechanism with predominant transgranular fracture is explained based on the separation of quasi-boundaries.

Scalable Al2O3–TiO2 Conductive Oxide Interfaces as Defect Reservoirs for Resistive Switching Devices

AbstractResistive switching devices herald a transformative technology for memory and computation, offering considerable advantages in performance and energy efficiency. Here, a simple and scalable material system of conductive oxide interfaces is employed, and their unique properties are leveraged for a new type of resistive switching device. An Al2O3–TiO2‐based valence‐change resistive switching device, where the conductive oxide interface serves both as the bottom electrode and as a reservoir of defects for switching, is demonstrated. The amorphous–polycrystalline Al2O3–TiO2 conductive interface is obtained following the technological path of simplifying the fabrication of the 2D electron gases (2DEGs), making them scalable for practical mass integration. Physical analysis of the device chemistry and microstructure with comprehensive electrical analysis of its switching behavior and performance is combined. The origin of the resistive switching is pinpointed to the conductive oxide interface, which serves both as the bottom electrode and as a reservoir of oxygen vacancies. The latter plays a key role in valence‐change resistive switching devices. The new device, based on scalable and complementary metal–oxide–semiconductor (CMOS)‐technology‐compatible fabrication processes, opens new design spaces toward increased tunability and simplification of the device selection challenge.

Fabrication of highly transparent thulium-doped Al2O3 nanocrystalline ceramics with broadband emission at 1.8 μm

Rare earth (RE) doped polycrystalline Al2O3 ceramics have attracted increasing research interest in the past decade for lasing and lighting applications because of their outstanding thermal conductivity and fracture strength enabling power scaling. However, the optically anisotropic nature of Al2O3 causes birefringent scattering in polycrystals, limiting the in-line transparency across visible and NIR wavelength range. To improve the transparency of Al2O3 ceramics, great control over powder processing and densification techniques are required to optimize its nanostructure. In this work, we present the first report of thulium incorporated nanocrystalline Al2O3 ceramic. The Tm:Al2O3 ceramic presented has the highest transparency among RE:Al2O3 reported to date, with a total loss coefficient of 0.6 cm−1 at the major emission wavelength 1.78 μm.

Nanostructured Manganite Films Grown by Pulsed Injection MOCVD: Tuning Low- and High-Field Magnetoresistive Properties for Sensors Applications.

The results of colossal magnetoresistance (CMR) properties of La0.83Sr0.17Mn1.21O3 (LSMO) films grown by pulsed injection MOCVD technique onto various substrates are presented. The films with thicknesses of 360 nm and 60 nm grown on AT-cut single crystal quartz, polycrystalline Al2O3, and amorphous Si/SiO2 substrates were nanostructured with column-shaped crystallites spread perpendicular to the film plane. It was found that morphology, microstructure, and magnetoresistive properties of the films strongly depend on the substrate used. The low-field MR at low temperatures (25 K) showed twice higher values (−31% at 0.7 T) for LSMO/quartz in comparison to films grown on the other substrates (−15%). This value is high in comparison to results published in literature for manganite films prepared without additional insulating oxides. The high-field MR measured up to 20 T at 80 K was also the highest for LSMO/quartz films (−56%) and demonstrated the highest sensitivity S = 0.28 V/T at B = 0.25 T (voltage supply 2.5 V), which is promising for magnetic sensor applications. It was demonstrated that Mn excess Mn/(La + Sr) = 1.21 increases the metal-insulator transition temperature of the films up to 285 K, allowing the increase in the operation temperature of magnetic sensors up to 363 K. These results allow us to fabricate CMR sensors with predetermined parameters in a wide range of magnetic fields and temperatures.

Formation of Ti2AlC-based thin films via magnetron sputtering and annealing under vacuum

High-purity Ti, Al and C targets were used to fabricate Ti2AlC-based thin films on industrial polycrystalline Al2O3 substrates via magnetron sputtering under argon, followed by vacuum annealing. The phase compositions of the Ti2AlC-based thin films were analysed, and scanning electron microscopy was performed to investigate their structures. Grain size and morphology depended on the deposition pattern. A material composed of ∼97.1% Ti2AlC and ∼2.9% TiC was obtained by sputtering Al for thirty minutes, then sputtering four alternating layers of Ti and C for ten minutes each. The thin film was subsequently annealed under vacuum at 900°C for 60 minutes. The samples contained lamellar hexagonal plate-shaped grains structures. The plate-shaped grains ranged from 200 nm to 400 nm in length depending on the sputtering mode. The long sides of the lamellar grains were predominantly oriented perpendicular to the alumina substrates. Regardless of the sputtering conditions, the coherent scattering region of Ti2AlC was ∼64 nm in size after annealing at 900°C. Changes in the size of the coherent scattering region of the TiC phase were non-linear and ranged from 9 to 60 nm.

Ab-initio study of paramagnetic defects in Mn and Cr doped transparent polycrystalline Al2O3 ceramics

Birefringence is a major source of difficulty in sintering of transparent polycrystalline alumina ceramics, especially as the grain size exceeds a few hundred nanometers, which ultimately leads to complete opacity, mainly due to scattering of light. Recent studies have made it clear that by application of a strong magnetic field, alumina grains can be aligned along a particular crystallographic orientation, which minimizes scattering due to birefringence, and enhances transparency. Defects that cause spin delocalization are known to induce a paramagnetic behavior in alumina ceramics. Therefore, such defects have become a focal point of research for magnetic field assisted sintering of transparent polycrystalline alumina, in order to reduce the necessary magnetic field strength during production process. In light of recent studies on paramagnetic potentials of transition metal doped alumina, we have applied Spin Polarized Density Functional Theory (SP-DFT) calculations on manganese and chromium doped and co-doped alumina to calculate the magnetic moments, density of states and defect formation energies, which should be expected from this system of dopants, along with their interactions with oxygen vacancies. The results clearly indicate that formation of a point defect comprised of chromium and manganese positioned substitutionally at adjacent aluminum sites, in vicinity of an oxygen vacancy can induce a magnetic moment equivalent to 5 Bohr magnetons (μβ), outperforming previously reported defects. Based on this study we find it likely that chromium and manganese co-doping in alumina can further reduce the required magnetic field strength for production of transparent polycrystalline alumina.

Effect of thermal annealing on the dielectric, passivation and pH detection properties of aluminium oxide thin films deposited by plasma-enhanced atomic layer deposition

The dielectric properties of aluminium oxide (Al2O3) thin films obtained by plasma-enhanced atomic layer deposition (PEALD) from Al(CH3)3/O2 precursors were investigated while focusing on the influences of thermal annealing under a dioxygen (O2) ambient. PEALD-Al2O3-based metal-insulator-silicon structures/capacitors and pH-sensitive chemical field effect transistors were fabricated to deal respectively with microelectronic applications and measurement in liquid phase. Antagonist results were thus evidenced. On the one hand, dealing with high-k gate materials, optimized dielectric properties, i.e. low fixed charge density (4 × 1012 cm−2), high dielectric constant (εr = 10.2), Fowler-Nordheim conduction and high breakdown electric fields (Ebd = 8.75 MV/cm) were obtained for polycrystalline PEALD Al2O3 films annealed at high temperature (T > 750 °C). On the other hand, pH detection properties, i.e. quasi-Nernstian sensitivity (59 mV/pH), low drift (< 1 mV/day) and long lifetime (> 180 days), were optimized for unannealed amorphous PEALD Al2O3 films. These phenomena were associated germination/crystallization phenomena in the deposited amorphous alumina structure as well as to related charge trapping and leakage currents in water-based solutions related to the final Al2O3 polycrystalline structure.

Precise tuning the CoMoOx/Al2O3 and CoMoSx/Al2O3 interfacial structures for efficient hydrodesulfurization of dibenzothiophene

In this work, we studied how the oxidic CoMoOx/Al2O3 and sulphurated CoMoSx/Al2O3 interfacial structures determine catalytic efficiency in selective hydrodesulfurization (HDS) of dibenzothiophene (DBT). To achieve this goal, we first synthesized the γ-Al2O3 materials of unique morphologies through the ionic liquid-assisted hydrothermal process under different conditions. The resulting γ-Al2O3 hexagonal plates (HP) and stacked hexagonal plates (SHP) are single crystallites with the dominant (110) and (111)/(100) facet structures, respectively; whereas the Al2O3 rods show polycrystalline feature. Through impregnation of Mo/Co salts of different loadings, the various CoMoOx precursors were generated on the different Al2O3 facets owing to the distinct interaction: mostly the two-dimensional (2D) poly-/mono-meric MoOx species together with the three-dimensional (3D) MoO3 on the Al2O3 (110) facet vs. the quasi 2D polymeric MoOx species on the Al2O3 polycrystalline facet. Such structural difference further affected the sulfidation of the MoCoOx as well as the quantity of active MoCoSx entities on different γ-Al2O3 supports: 10Mo3CoSx/Al2O3-HP had the highest level of MoCoOx sulfidation thus a much higher fraction of MoS2 plus Co9S8/CoMoS species whereas less quantity of partially sulphurated CoMoOxS2/CoOx together with SOx2-species. All the distinct interfacial domains of the sulphurated catalysts reasonably account for significantly different efficiency in HDS of DBT. Surface MoS2/CoMoS/Co9S8 species of the highest content over the Al2O3 (110) facet correspond to the highest DBT areal conversion rate and TOF value, owing to the enhanced MoS2-Co9S8 and MoS2-CoMoS conjunction and synergism.

Influences of TiO2 or Y2O3 doping on the homogeneity of polycrystalline Al2O3 produced by pulsed electric current sintering

ABSTRACT Influences of TiO2 or Y2O3 dopants on pressure-assisted sintering of polycrystalline Al2O3 were investigated to control the microstructure of sintered Al2O3 as well as to improve the homogeneity of transparent Al2O3 samples. The transparent polycrystalline Al2O3 samples with and without the dopants were fabricated by two-step pulsed electric current sintering (TS-PECS). By the grain boundary segregation by the dopants, both TiO2 and Y2O3 depressed the densification and grain growth processes during the sintering of polycrystalline Al2O3. The principle originalities of this work include the relationship between the homogeneities of microstructure and of optical transmittance of large-size polycrystalline Al2O3 and the consequent doping strategy for controlling of homogeneity of transparent polycrystalline Al2O3 prepared by two-step PECS. Doping of Y2O3 dramatically reduced the grain size of polycrystalline Al2O3 and improved its optical transparency. The homogeneity of the microstructure and the optical transparency of the large-sized polycrystalline Al2O3 with 0.1 mol% Y2O3 was improved. The addition of TiO2 also slightly retarded densification and reduced the grain size of the sintered Al2O3 samples. However, TiO2 dopant is not good for applications of transparent Al2O3 because it caused a dark color in transparent polycrystalline Al2O3 samples by uncertain reasons.

Formation of Hierarchical NiO Coatings on the Surface of Al2O3 Substrates under Hydrothermal Conditions

A hierarchical nickel oxide film was grown on the surface of a polycrystalline Al2O3 substrate by hydrothermal synthesis. The microstructure of the obtained coating was studied by scanning electron microscopy and atomic force microscopy, which showed that this coating comprises porous nanosheets (about 7 nm thick) arranged at various angles to each other and to the surface of the substrate. The lateral sizes of these nanosheets were in the range 3–5 μm. The local electrophysical characteristics of the obtained oxide coating were investigated by scanning capacitance microscopy and Kelvin probe force microscopy; using the obtained results, the maps of the distributions of the surface potential and capacitance contrast over the surface of the NiO film were constructed, and the work function of the surface of this film was calculated.

Features of Hydrothermal Growth of Hierarchical Co3O4 Coatings on Al2O3 Substrates

The formation of a hierarchical cobalt(II, III) oxide coating on the surface of a polycrystalline Al2O3 substrate under hydrothermal conditions was studied. Features of the microstructure of the obtained coatings were investigated by scanning electron microscopy; these coatings were shown to consist of Co3O4 nanosheets arranged perpendicular to the surface of the substrate with a blossoming flower-like structure. The production of the coating of the desired composition without crystalline impurities was confirmed by IR spectroscopy, X-ray powder diffraction analysis, and energy dispersive spectroscopy elemental analysis. The roughness of the formed Co3O4 film was estimated by atomic force microscopy. The local electrophysical characteristics of the produced oxide coating (work function of film surface, capacitance, and surface distribution maps of surface potential and capacitance contrast) were studied by scanning capacitance microscopy and Kelvin probe force microscopy. A local analysis of the current–voltage characteristics of the studied samples was made.

Thickness- and substrate-dependent magnetotransport properties of lanthanum–strontium manganite films with overstoichiometric manganese content

Electrical and magnetoresistive properties of the La0.6Sr0.2Mn1.2O3±δ (LSMO) films deposited on SrTiO3 single-crystalline and Al2O3 polycrystalline substrates by magnetron sputtering have been studied. The aim of the study is to understand the effect of substrate type on the thickness-dependent evolution of magnetotransport properties of the films of lanthanum–strontium manganites with overstoichiometric manganese content. It is shown that the decrease in the LSMO film thickness below 1000 nm causes the reduction of Curie temperature (TC) and magnetoresistance value at 300 K concomitant with the increase in electrical resistivity. At the same time, the dependence of electrical and magnetoresistance parameters on the substrate structure and type becomes stronger. It is found that TC of LSMOSTO films exceeds and resistivity is smaller than the respective parameters of LSMOAO films, and the difference grows with the decrease in the film thickness. All the results show that the behavior of the doped manganite films with overstoichiometric manganese content differs from that characteristic of stoichiometric in manganese films. These observations can be attributed to the effect of manganese excess on the film growth mechanism, making it different from the manganite films of other compositions.

High quality sapphire crystal by advanced chemical transport process

Single crystal sapphire was synthesized by chemical transport of Al-O generated by the reaction of polycrystalline Al2O3 ceramic and carbon. Using C-axis oriented polycrystalline Al2O3 ceramics as a seed crystal in the deposition temperature range, a C-axis sapphire crystal (Φ5xL35 mm) was grown at a temperature range of 700–1000 °C, and the growth rate in the C-axis direction was about 3.5 mm/h. The transmittance in the visible to infrared region of the synthesized sapphire is a theoretical value (transmission loss is lower than 0.1 %/cm), and the absorption edge was less than 200 nm (the band gap is 6.2 eV), which is shorter than the absorption edge (240 nm) of the commercially available single crystal (band gap 5.2 eV) synthesized by the Czochralski method. The dislocation density in this material was extremely low, and it was confirmed by lattice image observation that it was a high-quality single crystal with very few defects.

Noble Metal/Mn3O4 Nanocomposite Systems:from Design to Applications As Ultra-Sensitive Gas Sensors for Nitrogen Mustard Simulant

The efficient detection of chemical warfare agents (CWAs) is of paramount importance in the development of reliable sensing devices for safety applications. In particular, the increased threats of chemical attacks by terrorist organizations have stimulated a significant interest in the early detection of CWAs. Due to the high toxicity and necessity of special infrastructures for storage/manipulation of CWAs, the activities aimed at their detection are carried out on mimicking compounds compatible with lab test security level. In this context, the recognition of di(propylene glycol) monomethyl ether (DPGME), a simulant of the vesicant nitrogen mustard, is of key importance. Among the various devices adopted for CWA sensing, chemoresistive gas sensors based on nanostructured metal oxides offer various advantages, encompassing low cost, limited power consumption, and good stability/sensitivity. To date, DPGME detection has been performed only in a few cases by the use of SnO2-based systems, and the development of highly efficient sensors for its selective monitoring is of primary importance for next-generation devices.Among metal oxides, p-type Mn3O4, a mixed-valence and environmentally friendly multifunctional system, is an attractive candidate for CWAs detection thanks to the tunable Mn redox chemistry and the inherent catalytic properties. On the other hand, gas sensors based on pure p-type materials like Mn3O4 typically feature responses lower than n-type ones, as well as modest sensitivity/selectivity. To circumvent these drawbacks, a viable alternative is the functionalization with noble metals particles, like Ag and Au, that can act as catalytic promoters of the involved chemical reactions at the nanoscale and/or favor the formation of metal/oxide Schottky junctions, improving charge carrier separation.Based on preliminary results on the development of pure Mn3O4 nanomaterials as gas sensors [1,2], we have reported for the first time on the gas-sensing performances of nanocomposite systems based on Mn3O4-M (M = Ag, Au) in DPGME detection [3]. The target materials were obtained by the initial chemical vapor deposition (CVD) of Mn3O4 on polycrystalline Al2O3 substrates, followed by silver or gold radio frequency (RF) sputtering, with the aim of obtaining a high dispersion of metal nanoparticles.The results yielded by a multi-technique characterization highlighted the formation of phase-pure Mn3O4 nanostructures eventually functionalized with Ag or Au and an intimate contact between metal oxide matrix and nanoparticles. These features enabled us to enhance the system sensitivity in the detection not only of standard volatile organic compounds (acetone and ethanol), but also of DPGME, down to the ppb scale. Moreover, the possibility of discriminating between DPGME, on one side, and acetonitrile (CH3CN) or dimethyl methyl phosphonate (DMMP), simulants of HCN (blood agent gas) and Sarin nerve gas, respectively, demonstrates the system selectivity toward the target warfare gas simulant. To shed light on the origin of enhanced selectivity due to functionalization, density functional theory (DFT) based calculations were performed. The results, providing a molecular insight into the interactions between the target systems and DPGME, enabled us to elucidate a dual-site activation mechanism based on the binding of DPGME to both manganese oxide and gold nanoparticles, accounting for the higher functional performances registered for Au-Mn3O4 sensors.As a whole, this study represents an important step forward in the fabrication of highly efficient sensors based on dual metal oxide-metal NP composites. The obtained results pave the way to mastering next-generation sensing devices for real world applications, which will be pursued by additional investigation of the system long-term stability and humidity influence on the resulting functional performances.

Manganese Oxide Nanoarchitectures as Chemoresistive Gas Sensors to Monitor Fruit Ripening.

The efficient detection of low-concentration ethylene is a challenging issue of key importance for food quality control end-uses. Herein, we report on the fabrication of MnO₂-based nanoarchitectures by a two-step plasma-assisted process, consisting in the initial chemical vapor deposition of MnO₂ (host) on polycrystalline Al₂O₃ substrates and the subsequent functionalization with Ag and Au-based nanoparticles (guest) by sputtering processes. The resulting composites, characterized by a high Ag/Au dispersion and an effective host-guest contact, were tested for the first time as chemoresistive gas sensors for ethylene recognition at low temperatures. The high sensitivity and promising responses, enhanced by metal particle introduction, candidate the target systems as attractive platforms for the eventual monitoring of vegetables/fruits ripening and ageing.