Single Crystalline Magnesium Oxide Research Articles

An LC Wireless Passive Pressure Sensor Based on Single-Crystal MgO MEMS Processing Technique for High Temperature Applications.

An LC wireless passive pressure sensor based on a single-crystalline magnesium oxide (MgO) MEMS processing technique is proposed and experimentally demonstrated for applications in environmental conditions of 900 °C. Compared to other high-temperature resistant materials, MgO was selected as the sensor substrate material for the first time in the field of wireless passive sensing because of its ultra-high melting point (2800 °C) and excellent mechanical properties at elevated temperatures. The sensor mainly consists of inductance coils and an embedded sealed cavity. The cavity length decreases with the applied pressure, leading to a monotonic variation in the resonant frequency of the sensor, which can be retrieved wirelessly via a readout antenna. The capacitor cavity was fabricated using a MgO MEMS technique. This MEMS processing technique, including the wet chemical etching and direct bonding process, can improve the operating temperature of the sensor. The experimental results indicate that the proposed sensor can stably operate at an ambient environment of 22–900 °C and 0–700 kPa, and the pressure sensitivity of this sensor at room temperature is 14.52 kHz/kPa. In addition, the sensor with a simple fabrication process shows high potential for practical engineering applications in harsh environments.

MgO Single Crystals MEMS-Based Fiber-Optic Fabry–Perot Pressure Sensor for Harsh Monitoring

A single crystalline magnesium oxide (MgO) MEMS-based fiber-optic Fabry-Perot (FP) pressure sensor was designed, fabricated and tested for 800 °C high-temperature environmental applications. In this article, MgO was selected as the sensitive materials of the optical sensor owing to its high melting point, and excellent mechanical and optical properties at elevated temperatures. The sensor head was constructed by combining wet chemical etching with direct bonding. This MEMS process technique realized all-MgO sensor head structure, which is beneficial to reduce the temperature gradient within the sensor structure, and avoid sensor failures. In addition, a non-scanning cross-correlated interrogation method is used to realize the demodulation of FP pressure cavity length having multiple reflectors. The experimental tests demonstrate that the proposed sensor can stably operate at an ambient environment of 22-800 °C and 0-0.6 MPa with a pressure sensitivity of 5.21 μm/MPa (room temperature), a repeatability error of 1.67% and a zero drift of 0.00353 μm/°C. In addition, the high-temperature testing proves that the adhesive-free well-sealed pressure cavity is conducive to improve the operating temperature and the performance of the sensor. To our best knowledge, this is the first research to realize fabrication of high-temperature pressure sensor by developing MgO micromachining technology. Our work is of fundamental importance in realization of pressure detecting in harsh environments.

Hydrophilic Direct Bonding of MgO/MgO for High-Temperature MEMS Devices

Single-crystalline magnesium oxide (MgO) is an attractive material of substrates for high-temperature devices and high-temperature superconducting films. However, it is difficult to achieve the direct bonding of MgO/MgO because of its high brittleness and hardness, as well as the weak bonding force between the crystal faces. In this paper, we presented a hydrophilic direct bonding method of MgO by using two-step surface activation and a high-temperature annealing process. The bonding strengths under different annealing temperatures, pressures and times were measured. A high bonding strength (~7 MPa) and a fine bonding interface without any microcracks and voids were obtained after annealing at 1200 °C, 140 min and 4 MPa. The bonding interface was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The bonding mechanism of MgO/MgO was also clearly clarified. For the demo application, a MgO sealed cavity was formed by using the direct bonding method which is commonly used in the microelectromechanical systems (MEMS) devices for harsh environment applications.

Surface characterization of patterning on MgO single crystals using wet chemical etching process to advance MEMS devices

Single-crystalline magnesium oxide (MgO) is a material with outstanding high-temperature resistance and huge potential for use in high-temperature devices, light emitting devices and optical display fields. The investigation of fabricating a microstructure on the MgO substrate using wet etching process is conducted. The temperature and concentration dependence of the etching rate on the materials, and the surface roughness of the microstructure, are explored and analyzed. A microcavity with good profile and low roughness, 80.7 µm in depth and 4 mm in diameter, has been generated on a MgO substrate with a 50% H3PO4 etchant solution at ~100 °C. Optical microscopy, atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscope and x-ray diffraction analysis are employed to demonstrate the successful application of wet etching for improving the etching rate and surface morphology without the deterioration of the surface roughness. Our work is of fundamental importance in the fabrication of MgO-based devices (such as pressure sensors, vibration sensors and photonic resonators) and the improvement of growth condition of oxide films on MgO substrates.

Growth of vertically aligned single-walled carbon nanotubes with metallic chirality through faceted FePt-Au catalysts

Direct synthesis of vertically aligned metallic single-walled carbon nanotubes (m-SWCNT forests) is a difficult challenge. We have successfully synthesized m-SWCNT forests using faceted iron platinum-gold catalysts epitaxially grown on a single crystalline magnesium oxide substrate. The metallic content of the forests estimated by Raman spectroscopy reaches 90%. From the standpoint of growth rate of the forests, the growth mechanism is probably based on the catalyst of solid state. It is suggested that preferential growth of m-SWCNTs is achieved when both factors are satisfied, namely, {111} dominant octahedral facet and ideal size (fine particles) of FePt particles.

Plasmonic arrays of titanium nitride nanoparticles fabricated from epitaxial thin films.

We have fabricated two-dimensional periodic arrays of titanium nitride (TiN) nanoparticles from epitaxial thin films. The thin films of TiN, deposited on sapphire and single crystalline magnesium oxide substrates by a pulsed laser deposition, are metallic and show reasonably small optical loss in the visible and near infrared regions. The thin films prepared were structured to the arrays of nanoparticles with the pitch of 400 nm by the combination of nanoimprint lithography and reactive ion etching. Optical transmission indicates that the arrays support the collective plasmonic modes, where the localized surface plasmon polaritons in TiN nanoparticles are radiatively coupled through diffraction. Numerical simulation visualizes the intense fields accumulated both in the nanoparticles and in between the particles, confirming that the collective mode originates from the simultaneous excitation of localized surface plasmon polaritons and diffraction. This study experimentally verified that the processing of TiN thin films with the nanoimprint lithography and reactive ion etching is a powerful and versatile way of preparing plasmonic nanostructures.

Synthesis of vertically aligned single-walled carbon nanotubes with metallic chirality through facet control of catalysts

For nanotube synthesis, iron platinum (FePt) alloy particles have been prepared on a single crystalline magnesium oxide (MgO) substrate by alternate sputter deposition of FePt and MgO. Partially facetted {111}-nano particles of FePt have been epitaxially formed on the substrate and periodically exposed on the surface. The particles of FePt were half-buried between deposited MgO showed superior thermal stability and microparticulations were also achieved by optimization of film layer thickness. By using the substrates for growth of carbon nanotubes, vertically aligned single-walled carbon nanotubes (forest) have been successfully grown on the substrate containing the faceted FePt nanoparticles. Raman spectra of the forest have revealed prominent features of metallic nanotubes in the radial breathing-mode region.

Preparation of photoluminescent single crystalline MgO nanobelts by DC arc plasma jet CVD

Magnesium oxide (MgO) nanostructures were synthesized by DC arc plasma jet chemical vapor deposition, which possesses the advantages of being simple, economical, fast, effective and environmentally benign. The formation of “tadpole”-, dendrite-, belt- and rod-like MgO nanostructures was confirmed by scanning electron microscopy and high-resolution transmission electron microscopy. Powder X-ray diffraction analysis revealed that the nanostructures consist of cubic phase MgO. Nanobelts that were 30–50nm wide with a width/thickness ratio of 1–2 were synthesized in just 5min. Most of the nanobelts were connected to others, and the connected nanobelts possessed a single-crystal structure. A formation mechanism for MgO nanostructures was proposed. Fourier transform infrared spectra indicated the adsorption of water and CO2 on the MgO surface. The nanobelts exhibited relatively strong blue-green luminescence.

Magnetic anisotropy and field switching in cobalt ferrite thin films deposited by pulsed laser ablation

We report the observation of contrasting magnetic behavior in cobalt ferrite (CFO) thin films deposited on single crystalline magnesium oxide (MgO) and strontium titanate (STO). Epitaxial films on MgO (100) with a lattice mismatch of 0.35% showed out-of-plane anisotropy whereas the films on STO (100) with a lattice mismatch of 7.4% displayed in-plane anisotropy. Stress anisotropy calculated from angle-dependent x-ray diffraction analysis confirmed that the change in anisotropy originates from the lattice mismatch. An additional low-field switching characteristic is observed in the M-H loops of the CFO films, which became prominent with lowering temperature as also evidenced from the rf transverse susceptibility measurements. The obtained results revealed that the low field switching is associated with the film-substrate interface.

Improved epitaxy of barium titanate by molecular beam epitaxy through a single crystalline magnesium oxide template for integration on hexagonal silicon carbide

Crystalline MgO(111) has the potential to be an effective template for the heteroepitaxial integration of BTO(111) and other functional oxides on 6H-SiC(0001). Deposition of MgO on 6H-SiC(0001) at 140°C resulted in a twinned structure with only (111) orientation. By heating the MgO(111) after deposition to 650°C at a background pressure of 1.0×10−9Torr or depositing the MgO at 650°C, the twinned structure can be minimized, resulting in a reflection high energy electron diffraction (RHEED) pattern characteristic of random epitaxial islands. The use of a 2nm MgO(111) template layer proved optimal for deposition of crystalline BTO(111) by molecular beam epitaxy on 6H-SiC(0001). The BTO was found to be twinned with a 60° in-plane rotation. The grain formation of the BTO resulted in a transmission dominated RHEED pattern. Deposition of BTO at a higher substrate temperature resulted in large grain formation, 50nm in size, but an increased surface roughness of 1.4±0.1nm over a 1μm2 area. In order to integrate BTO in a multiferroic device with multilayers of BTO(111) and hexagonal ferrites, it will be necessary to optimize the processing conditions to establish a smoother BTO surface for the subsequent heteroepitaxy of the magnetic film.

Selective area synthesis of magnesium oxide nanowires

Single crystalline magnesium oxide (MgO) nanowires exhibiting a square cross section have been grown on (001)-oriented MgO and Si substrates using the vapor-liquid-solid growth mechanism. While the nanowires grow vertically aligned on MgO, they display random orientations on the silicon substrate. For growth on MgO substrates, the selective placement and density of the nanowires can be controlled by using electron beam lithography for prepatterning the gold catalyst layer. The nanowire samples have been characterized using field-emission scanning electron microscopy and transmission electron microscopy. The described process for selective placement of the nanowires is attractive for their use as templates for coaxial coatings and also for their manipulation for potential device fabrication.

Low temperature growth of crystalline magnesium oxide on hexagonal silicon carbide (0001) by molecular beam epitaxy

Magnesium oxide (111) was grown epitaxially on hexagonal silicon carbide (6H-SiC) (0001) substrates at low temperatures by molecular beam epitaxy and a remote oxygen plasma source. The films were characterized by reflection high-energy electron diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy. Crystal structure, morphology, and growth rate of the magnesium oxide (MgO) films were found to be dependent on the magnesium flux, indicating a magnesium adsorption controlled growth mechanism. The single crystalline MgO thin films had an epitaxial relationship where MgO (111)‖6H-SiC (0001) and were stable in both air and 10−9Torr up to 1023K.

Observation of two- and three-dimensional magnesium oxide nanostructures formed by thermal treatment of magnesium diboride powder

A simple thermal treatment has been used to produce a variety of elongated single crystalline magnesium oxide nanostructures. Incipient evaporation of a magnesium diboride starting powder combined with a mixed oxygen/argon process gas provided the constituents for nanostructure growth. Electron diffraction and energy dispersive x-ray spectroscopy confirm the single crystalline nature and chemical composition of the objects, and the atomic surface plane is observed to be of the (001) type. Transmission electron microscopy and scanning electron microscopy reveal several MgO morphologies including linear nanobelts and two- and three-dimensional entities that contain approximately ninety degree angles. Atomic steps are observed on the inner edges of these angles, but are absent from the outer rims. The experimental conditions used to form these nanoscale materials and possible applications are described.

Time-resolved high-resolution electron microscopy of structural stability in mgo clusters

Structure and stability of atomic clusters have been studied by time-resolved high-resolution electron microscopy (TRHREM). Typical examples are observations of structural fluctuation in gold (Au) clusters supported on silicon oxide films, graphtized carbon films and magnesium oxide (MgO) films. All the observations have been performed on the clusters consisted of single metal element. Structural stability of ceramics clusters, such as metal-oxide, metal-nitride and metal-carbide clusters, has not been observed by TRHREM although the clusters show anomalous structural and functional properties concerning to solid state physics and materials science.In the present study, the behavior of ceramic, magnesium oxide (MgO) clusters is for the first time observed by TRHREM at 1/60 s time resolution and at atomic resolution down to 0.2 nm.MgO and gold were subsequently deposited on sodium chloride (001) substrates. The specimens, single crystalline MgO films on which Au particles were dispersed were separated in distilled water and observed by using a 200-kV high-resolution electron microscope (JEOL, JEM2010) equipped with a high sensitive TV camera and a video tape recorder system.

FTIR studies of the adsorption of CO on supported metallic clusters [formula omitted

Recently, we have evidenced a strong size effect in the adsorption energy of CO on small palladium clusters supported on single crystalline magnesium oxide surfaces. One possible interpretation is the presence of several types of adsorption sites on the metallic clusters. In order to investigate more precisely the adsorption of CO, a new experiment has been developed. The main technique is the fast Fourier transform infrared spectroscopy (FTIR) used to measure the stretching vibration frequencies of the adsorbed CO molecules. New results demonstrating the availability of FTIR to determine adsorption sites of CO on Pd MgO(100) model catalysts are presented. Actually, two distinct absorption bands are observed, tentatively assigned one to the facets of the cluster and the other to the edges.

Nanometre scale electron beam processing andin situatomic observation of vacuum-deposited MgO films in high-resolution transmission electron microscopy

Abstract Nanometre scale processing was performed on vacuum-deposited single crystalline magnesium oxide films by using a nanometre-sized electron beam in highresolution transmission electron microscopy (HRTEM). Two kinds of processing, drilling and edge-flattening, were demonstrated. The resolution of the present processing was demonstrated by the minimum size of the holes being about 4 nm and the minimum distance between holes being 14nm. The shape modulation during the processing was observed for the first time in situ dynamically at atomic resolution. The dynamic observations in HRTEM showed that the processing is caused by desorption of constituent atoms from the solid state surface of the magnesium oxide films. It was found that the size of the processing area could be controlled at the atomic scale by adjusting the irradiation time during the in situ observation of the processing.

Preparation and characterization of composite films of magnesium oxide embedding nanometer-sized metal crystallites

Composite films of magnesium oxide and metals (Au, Pd, Pb, Fe, Cr and Ti) are prepared by a simultaneous deposition technique. The mean thicknesses of magnesium oxide and metal are 20 to 25 nm and 1 to 3 nm, respectively. All metals except titanium grow epitaxially as small crystallites of a few nm in size inside single crystalline magnesium oxide films, the thickness of metal crystallites being estimated to be a few atomic layers. For iron-magnesium oxide composite films, the transformation from α-iron to γ-iron is observed by the heat treatment at 500°C for two hours. For titanium-magnesium oxide composite films, titanium grows as crystallites less than 1 nm in size with the structure of Ti 1− x O.

Nanometer-area diffraction of small iron crystallites in single crystalline Fe-MgO composite films

Nanometer-sized iron(Fe) crystallites can be prepared in a single crystalline magnesium oxide(MgO) film by a simultaneous vacuum deposition of Fe and MgO. The crystallites are grown epitaxially and almost coherently in the film, the orientation being (001) [110]Fe//(001)[100]MgOand (011) [100]Fe//(001) [100]MgO. A heat treatment of the as-grown composite films at 500-1000°C brings about a phase-transformation from α -iron(b.c.c.) to γ -iron(f.c.c.). In the present study, the phase-transformation and the structure of the γ-iron crystallites are studied by nanometer-area electron diffraction(nanodiffraction) in TEM and STEM as well as high-resolution electron microscopy.The specimens were single crystalline Fe-MgO composite films prepared on a NaCl (001 ) surface by co-evaporation of Fe and MgO. The films were separated from the substrate in water and mounted on a perforated carbon film. Nanodiffraction in TEM was performed in a 200 kV transmission electron microscope(JEM- 2000FX)2 and that in STEM3was carried out in a 100 kV scanning transmission electron microscope (VG-HB5) equipped with a specimen-preparation chamber.