Growth of AlN Crystals on SiC Substrates by Thermal Nitridation of Al2O3

The growth of AlN crystals on c‐plane 6H–SiC substrates by thermal nitridation of Al₂O₃ pellets in the presence of graphite and ZrO₂ was demonstrated. Addition of graphite and ZrO₂ effectively accelerated the evaporation of Al₂O₃, yielding c‐axis oriented AlN films on SiC substrates. The SiC substrate was severely deteriorated at 2173 K, which produced a porous interface between the AlN film and substrate, resulting in low‐quality AlN crystals. The deterioration of SiC was successfully suppressed by introducing a pre‐deposited homo‐buffer layer, allowing two‐dimensional‐like growth of AlN. The buffer layer promoted the formation of a high‐quality AlN film. At 2173 K, the full‐width at half maximum of the X‐ray rocking curves of the (0002) and (10–10) planes of the AlN film was 360 and 425 arcsec, respectively.     

Superheating nature coupled with Mg-doping effect of high thermal stable NdBa2Cu3O7-δ film

NdBa₂Cu₃O_7-δ thin film deposited on a MgO substrate has been verified to remain solid above its peritectic melting temperature (T_p). Such a superheating nature is attributed to its low energy surface and epitaxial interface with substrate. Here, combining superheating nature with doping effect, we report a novel structure, Mg-doped NdBa₂Cu₃O_7-δ film with YBa₂Cu₃O_7-δ buffer layer for the first time. Remarkably, this film presents a higher thermal stability level than heretofore possible when acting as a seed for preparing SmBa₂Cu₃O_7-δ bulks by melt growth, enduring a temperature of 1128°C, 43 K above its T_p for 1 hour. The utilization of such a high T_max in melt growth is beneficial to the fabrication of large-sized and high-performance bulk in terms of effectively broadening the growth window and suppressing the heterogeneous nucleation. More importantly, some high thermal stability required technological applications, such as batch growth and failed bulk recycling, are likely to be realized by this novel Mg-doped NdBa₂Cu₃O_7-δ film seed. Finally, we show how the observed metastable phase is linked to the distinctive film architecture.     

Chemical compatibility of rare earth apatite with yttria-stabilized zirconia

The chemical compatibility of a series of rare earth apatite (RE-apatite), with Y₂O₃-stabilized ZrO₂ (YSZ) has been investigated. Three types of RE-apatite powders with different ionic radius (RE = Gd, Nd and La) were prepared, and bulks prepared from the powder mixtures of RE-apatite and YSZ were heat-treated at 1300 °C up to 100 h in this study. It was found that Gd-apatite reacted with YSZ and formed a reaction layer (Gd₂Si₂O₇) at the Gd-apatite/YSZ interface. Meanwhile, the intense Gd³⁺ diffusion resulted in the formation of Gd solid solutions in YSZ and much YSZ phase transformation. In contrary, as for Nd- or La-apatite/YSZ composite, which has larger ionic radius, no reaction product was observed at interface and there was less RE diffusion into YSZ as well as YSZ transformation. These results clearly indicated that large ionic radius of RE³⁺ could enhance the chemical compatibility of RE-apatite with YSZ.     

Effects of Bismuth Oxide Buffer Layer on BiFeO3 Thin Film

Bismuth ferrite (BiFeO₃) thin films with Bi₂O₃ buffer layers were prepared on Si/SiO₂/TiO₂/Pt substrates by sol–gel‐derived spin‐coating method. The structural and electrical properties of BiFeO₃ was effectively improved by adding a Bi₂O₃ buffer layers either at Pt/BiFeO₃ interface or on BiFeO₃ surface, also strongly depending on the positions and the annealing conditions of buffer layers. A 500°C‐annealed Bi₂O₃ buffer layer could act as a Bi source for compensating Bi volatilization and a diffusion barrier for species from BiFeO₃. A near stoichiometric BiFeO₃ with less defects and substrate contamination was obtained by employing a 500°C‐annealed Bi₂O₃ buffer layer in between Pt substrate and BiFeO₃. The structure change in BiFeO₃ led by such a buffer layer should result from the interfacial constraint between buffer layer and BiFeO₃. Furthermore, this crystalline BiFeO₃ specimen exhibited a highly (100)‐textured, where this preferred orientation was attributed to the accumulation of Bi at Pt/BFO interface. Therefore, the Pt/500°C‐annealed Bi₂O₃/BiFeO₃/Pt thin film exhibited the good ferroelectric and magnetic properties. As compared to the usual method for controlling BiFeO₃ composition by adding excess Bi, this study indicates the more advantages using a Bi₂O₃ buffer layer.     

Role of Alumina Buffer Layer on the Dielectric and Piezoelectric Properties of PZT System Thick Films

Piezoelectric properties of screen‐printed thick films, 0.01Pb(Mg_1/2W_1/2)O₃–0.41Pb(Ni_1/3Nb_2/3)O₃–0.35PbTiO₃–0.23PbZrO₃ + 0.1 wt% Y₂O₃ + 1.5 wt% ZnO (PMW–PNN–PT–PZ+YZ) on alumina (Al₂O₃) buffer layers deposited on Si substrates, were studied. To improve piezoelectric properties of and integrate the PMW–PNN–PT–PZ+YZ thick films, the Al₂O₃ buffer layers on silicon (Si) substrates were used. The Al₂O₃ buffer layer on the Si substrate suppressed the pyrochlore phases of the piezoelectric thick films and prevented interdiffusion of Si and Pb. The PMW–PNN–PT–PZ+YZ thick films with 900 nm thick Al₂O₃ buffer layer showed piezoelectric properties such as P_r = 32 μC/cm², E_c = 25 kV/cm, and d₃₃ = 32 pC/N. These significant piezoelectric properties of our screen‐printed PMW–PNN–PT–PZ+YZ thick films by the Al₂O₃ buffer layers can be applied to functional thick film in many micro‐electromechanical system (MEMS) applications such as micro actuators and sensors.     

A promising molten silicate resistant material: Rare-earth oxy-apatite RE9.33(SiO4)6O2 (RE = Gd,Nd or La)

In this work we reported a new class of rare earth oxy-apatite, RE_9.33(SiO₄)₆O₂, with superior molten silicate resistant capability which shows promising application for thermal barrier coatings (TBCs). Three RE elements with different ion radius, i.e., Gd, Nd and La, were selected to prepare RE_9.33(SiO₄)₆O₂ bulk using co-precipitation and pressless-sintering. After 8-h CMAS attack at 1300 °C, the specimens exhibited a dense, continuous reprecipitated layer at RE-apatite/CMAS interface, which is predominantly controlled by a dissolution-reprecipitation mechanism distinguishing from the sacrificial material which is usually with reaction layer being formed. With an increase of ion radius, apatite is easier to crystallize in the melt.     

Corrosion products evolution and hot corrosion mechanisms of REPO4 (RE= Gd,Nd, La) in the presence of V2O5 + Na2SO4 molten salt

REPO₄ (RE = Gd, Nd, La) ceramics with a monazite structure were fabricated by a chemical co-precipitation and calcination method. Hot corrosion tests were carried out in V₂O₅+Na₂SO₄ molten salt at 800 °C, 900 °C and 1000 °C for 2 h and 10 h. The temperature and heat duration had little effect on the type of corrosion products in this study. However, GdPO₄ and REPO₄ (RE = Nd, La) revealed different hot corrosion behavior. Exposed to the molten salt, GdVO₄ and Gd₄(P₂O₇)₃ formed as the corrosion products for the GdPO₄ case, while an RE(P,V)O₄ (RE = Nd, La) solid solution was generated for NdPO₄ and LaPO₄ cases. The formation of the solid solution had less damage to the original microstructure, which benefited the hot corrosion resistance of the ceramics. From the crystallographic characteristics of rare earth phosphates/vanadates and a thermodynamics perspective, the hot corrosion mechanisms of REPO₄ (RE = Gd, Nd, La) are discussed.     

Reactivity between rare‐earth oxides based thermal barrier coatings and a silicate melt

The reactivity between rare‐earth (RE‐) oxide stabilized ZrO₂ or HfO₂ thermal barrier coatings (TBCs) and a calcium‐magnesium‐aluminum‐silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top‐coating system. Thermodynamic calculations of the phase assemblage of the (1?x) ZrO₂‐xY₂O₃ coating materials and CMAS melt are compared with the experimental findings. CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total RE‐oxide content higher than 16 mol% in the reaction zone for the ZrO₂ system. In general, samples with nominal compositions ZrO₂‐9Dy₂O₃, HfO₂‐7Dy₂O₃, ZrO₂‐8Y₂O₃, HfO₂‐6Er₂O₃, ZrO₂‐9.5Y₂O₃‐2.25Gd₂O₃‐2.25Yb₂O₃, and ZrO₂‐30Y₂O₃ exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions.     

Al2O3 films with Ni-Based buffer layer prepared by plasma-ion assisted deposition on Cu substrate

In this study, Al₂O₃ films with an Ni-based buffer layer were prepared on a Cu substrate by plasma-ion assisted deposition (PIAD). The main purpose of this study is to develop a novel electrical insulating film to be used at high temperature. X-ray diffraction (XRD) spectra show the Al₂O₃ films prepared by this method are amorphous. The results of atomic force microscopy (AFM), scanning electron microscopy (SEM), and auger electron spectroscopy (AES) analyses reveal that the Al₂O₃ films perfectly adhere to the substrate through the buffer layer, no visible defects were observed, and no impurity from Ni or Cu was detected. The diffusion of Cu into the Al₂O₃ film at high temperature is suppressed. Al₂O₃ films with an Ni-based buffer layer exhibit excellent resistivity (>10¹⁰Ω·cm) even after experiencing a high temperature environment as high as 600°C 10 times.     

Ferroelectric Phase Transition,Interfaces Quality,and Stress Evolution of TiOx/BaxSr1−xTiO3 Structures

Ba(Sr,Ti)O₃ material presents a remarkable property that lies in the possibility to change the permittivity by applying a dc electric field, i.e., BST is a tunable material. That makes BST a very interesting material for the development of reconfigurable devices in microelectronics. In this study, we focus our work on Ba(Sr,Ti)O₃ with Ba/Sr = 30/70, the films are deposited by radio‐frequency magnetron sputtering on Al₂O₃ (0001). A buffer layer of TiO_x is used to control the film orientation. The influence of this buffer layer on the dielectric properties, the interfaces quality with respect to the film thickness, and the temperature is analyzed. An increase of 30% of the relative permittivity was measured and a tunability of 50% was attained at 300 KV/cm. The dielectric measurements on BST/TiO_x as a function of the temperature show a shift of the Curie temperature (T_c = ?40°C) in comparison to BST without TiO_x layer (T_c = ?80°C). We demonstrate that the Curie temperature does not correspond to the maximum permittivity. The important stress measured on the films (930 MPa) could explain this behavior.     

High‐energy storage density and excellent temperature stability in antiferroelectric/ferroelectric bilayer thin films

The antiferroelectric/ferroelectric (PbZrO₃/PbZr_0.52Ti_0.48O₃) bilayer thin films were fabricated on a Pt(111)/Ti/SiO₂/Si substrate using sol‐gel method. PbZr_0.52Ti_0.48O₃ layer acts as a buffered layer and template for the crystallization of PbZrO₃ layer. The PbZrO₃ layer with improved quality can share the external voltage due to its smaller dielectric constant and thinner thickness, resulting in the enhancements of electric field strength and energy storage density for the PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer thin film. The greatly improved electric breakdown strength value of 2615 kV/cm has been obtained, which is more than twice the value of individual PbZr_0.52Ti_0.48O₃ film. The enhanced energy storage density of 28.2 J/cm³ at 2410 kV/cm has been achieved in PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer film at 20°C, which is higher than that of individual PbZr_0.52Ti_0.48O₃ film (15.6 J/cm³). Meanwhile, the energy storage density and efficiency of PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer film increase slightly with the increasing temperature from 20°C to 120°C. Our results indicate that the design of antiferroelectric/ferroelectric bilayer films may be an effective way for developing high power energy storage density capacitors with high‐temperature stability.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.     

Superconducting properties of binary rare-earth HoREBCO thin films prepared by pulsed layer deposition

The superconducting properties of REBa₂Cu₃O_y (REBCO or RE123, RE = rare earth elements) coated conductors could essentially depend on the rare earth elements, which could be the premise of all potential superconducting applications. In this paper, Ho_0.75RE_0.25Ba₂Cu₃O_7-σ (HoREBCO, RE = Dy, Er, Yb) high temperature superconducting films were fabricated successfully by a reel-to-reel pulsed laser deposition (PLD) system for the first time. By replacing Ho elements with heavy rare earth elements in a certain mole ratio, the critical currents of the HoREBCO superconducting films were improved significantly compared with pure HoBCO films. The lattice distortions are considered as the determinative factors of the changes. The replacement of Ho atoms by Dy, Er or Yb also influenced the structure and the surface morphology of the superconducting films. The decrease of the lattice constant in HoREBCO superconducting film and the variation of the interatomic force play the decisive role in the improvement of the superconducting property.     

Role of polymer chain end groups in plasma modification for surface metallization of polymeric materials

How to improve adhesion between poly(oxybenzoate‐co‐oxynaphthoate) (Vecstar OC and FA films) and copper metal by Ar, O₂, N₂ and NH₃ plasma modification was investigated. The mechanism of adhesion improvement is discussed from the viewpoint of chemical and physical interactions at the interface between the Vecstar film and copper metal layer. The adhesion between Vecstar OC film and copper metal was improved by chemical rather than physical interactions. Polymer chain end groups that occur at Vecstar OC film surfaces contribute effectively to adhesion. This improvement in adhesion is due to interactions between copper metal and O?C groups formed by plasma modification. Aggregation of the O?C groups to the copper metal/Vecstar OC film interface is a key factor for good adhesion. From this aspect, heat treatment of plasma‐modified Vecstar OC films on glass plates is effective in the aggregation, and the peel strength for the copper metal/Vecstar OC film system reached 1.21 N (5 mm)^?1. Copyright © 2009 Society of Chemical Industry     

Pellet‐buffered film seed to grow single grain bulk YBCO

Epitaxial thin film seeds, exhibit a higher peritectic temperature than the bulk, have been recognized as a possible way to obtain large‐sized single‐grain REBCO bulks. Recently, use of a pellet as a buffer layer inserted between the film‐seed and the main pellet has been reported. This approach can prevent Mg diffusion from the MgO substrate to the film seed, and enable the thin film to tolerate a higher maximum temperature in the TSMG (Top Seed Melt Growth) method. However, we found that the liquid phase decomposed from YBa₂Cu₃O_7?x(YBCO) contacted the MgO substrate, due to surface tension, and caused undesirable grain nucleation. To resolve this problem, this study used a mini‐pellet, smaller than the seed, as a buffer layer. This new technique can avoid the liquid phase contacting the MgO substrate, thereby allowing the YBCO single‐grain bulk to be grown more easily. Moreover, the thin film seed can be re‐used by removing the buffer layer after the crystal growing process.     

Microstructure and ferroelectric properties of Ta-doped Bi3.25La0.75Ti3O12/ZnO thin film capacitors

Ta-doped Bi_3.25La_0.75Ti₃O₁₂(BLTT)/ZnO films were fabricated on Pt(111)/Ti/SiO₂/Si substrates by a magnetron sputtering method. Firstly, ZnO crystal thin films were grown on the substrates by a reactive sputtering method. Then, BLTT thin films were deposited on the ZnO layers at room temperature and post-annealed at 600 °C. The micromorphology, ferroelectric and dielectric properties of BLTT/ZnO films were analyzed. The XRD analysis shows that ZnO buffer layer significantly reduces the crystallization temperature of BLTT thin film. The TEM results show that lamellar BLTT grains are grown on ZnO layer at a certain angle with few elements diffusion at the interface of ZnO phase and Bi₄Ti₃O₁₂ phase. The ferroelectric properties indicate that BLTT/ZnO films exhibit different remanent polarization and coercive fields under electric field with different directions. The novel mechanism of tailoring ferroelectric properties may open new possibilities for designing special ferroelectric devices.     

High‐performance varistors prepared by hot‐dipping tin oxide thin films in Sb2O3 powder: Influence of temperature

A series of SnO_x–Sb₂O₃ thin film varistors were fabricated through hot‐dipping tin oxide films deposited by radio‐frequency magnetron sputtering in Sb₂O₃ powder at varied temperatures in air. With the increase in hot‐dipping temperature (HDT) from 200°C to 600°C, the nonlinear coefficient (α) of the samples increased first and then decreased, reaching the maximum at 500°C, which was mainly determined by the completeness of high‐resistant Sb₂O₃ layer at tin oxide grain boundary and the chemical composition of tin oxide films. Correspondingly, the leakage current (I_L) decreased first and increased later. The breakdown electric field (E_100 mA) decreased constantly with increasing HDT. The SnO_x–Sb₂O₃ film varistors prepared at 500°C exhibited the optimum nonlinear properties with the maximum α of 10.88, the minimum I_L of 36.3 mA/cm², and an E_100mA of 0.0188 V/nm. The obtained nanoscaled film varistors would be promising in electrical/electronic devices working in low voltage.     

Crystal Structure and Microwave Dielectric Properties of LiRE9(SiO4)6O2 Ceramics (RE = La,Pr, Nd,Sm, Eu,Gd, and Er)

The crystal structure and microwave dielectric properties of apatite‐type LiRE₉(SiO₄)₆O₂ ceramics (RE = La, Pr, Nd, Sm, Eu, Gd, and Er) have been investigated. The densification of lithium apatites has been greatly improved with the addition of 1 wt% LiF. Selected area electron diffraction and X‐ray diffraction (XRD) Rietveld analysis confirm that these compounds belong to the P6₃/m (No. 176) space group with hexagonal crystal symmetry. The porosity‐corrected relative permittivity was found to decrease with decreasing ionic polarizability of RE³⁺ ions. Relationships between the structural parameters and microwave dielectric properties have been examined. The observed variation in the quality factor of LiRE₉(SiO₄)₆O₂ + 1 wt% LiF ceramics (RE = La, Pr, and Nd) was correlated with average cation covalency (%). The temperature coefficient of resonant frequency was found to depend on the bond valence sum of cations. LiEr₉(SiO₄)₆O₂ + 1 wt% LiF ceramics showed good microwave dielectric properties with ε_r = 12.8, Q_u × f = 13000 GHz and τ_f = +17 ppm/°C. All the compositions showed low coefficient of thermal expansion with thermal conductivity in the range 1.3–2.8 W (m K)^?1.     

Atomic layer deposition of Al2O3 thin films on diamond

Atomic layer deposition (ALD) of aluminum oxide thin films on diamond was demonstrated for the first time, and the film properties as a gate insulator for diamond field effect transistor (FET) were examined. The interface between the aluminum oxide and the diamond was abrupt, and the ratio of aluminum to oxygen in the film was confirmed to be stoichiometric by Rutherford back scattering. Even a bumpy surface of polycrystalline diamond film was conformally covered by the Al₂O₃ films. To evaluate the feasibility of the film for FET gate insulator, the electrical characteristics of the Al₂O₃ films deposited by ALD on diamond were measured using metal–insulator–semiconductor structure. It was found that the Al₂O₃ films deposited by ALD were better than those deposited by conventional methods, which indicates that the ALD-Al₂O₃ films are feasible for gate insulators of diamond FETs.     

Role of Interface(s) for the Growth of Ultra‐Thin Amorphous Oxides on Al–Si Alloys: A Thermodynamic Analysis

This study presents a thermodynamic analysis to predict the type of initial, amorphous oxide overgrowth (i.e., am‐Al₂O₃ or am‐SiO₂) on bare Al–Si alloy substrates. This analysis have taken into account the energies associated with both its interfaces (interface between the Al–Si alloy substrate and the thin oxide film and interface between the thin oxide film and vacuum) along with the bulk Gibbs free energy of oxide formation. This developed analysis is then applied for various parameters, such as, Si alloying element content at the substrate/oxide interface, the growth temperature, the oxide film thickness (up to 1 nm), and various low‐index crystallographic surfaces of the substrate. It is found that am‐SiO₂ overgrowth is thermodynamically preferred for a combination of lower oxide film thickness, lower growth temperature, and lower Si alloying content at the alloy/oxide interface. This is because of the overcompensation of the lower energies of both the interfaces over the bulk Gibbs free energy. Furthermore, it is found that for all cases, am‐Al₂O₃ forms a more stable interface with Al–Si alloy than am‐SiO_2.