Morphology-controlled synthesis and field-emission properties of patterned SnO2 nanostructures with different morphologies

Tin oxide nanostructured arrays with different morphologies were grown on stainless-steel mesh substrates by a simple thermal evaporation process. It was found that the SnO₂ nanostructures could be easily changed from nanobelts to nanocones, nanoneedles, micro-rods, ultra-long nanowires, and slim nanorods by controlling the parameters of growth temperature and N₂/O₂ flow. A model combining vapor-liquid-solid (VLS)-base growth and vapor-solid (VS)-tip growth was proposed to explain the growth of SnO₂ nanostructures with manifold morphologies. Field-emission (FE) studies revealed that the morphologies of these patterned SnO₂ nanostructures had considerable effects on the FE properties. Among these nanostructures, ultra-long nanowire arrays had the lowest turn-on field (~0.47 V/um) and the highest field enhancement factor (~8848). More importantly, the ultra-long nanowire emitters showed excellent FE stability with fluctuations within 2.7%. The enhanced FE properties may be attributed to synergic effects arising from the aligned structures of the ultra-long nanowire emitters, their smaller areal density and their highest aspect ratio (~12000).     

Sn-doped In2O3 nanowires: enhancement of electrical field emission by a selective area growth

Selective area growth of single crystalline Sn-doped In₂O₃ (ITO) nanowires synthesized via vapor–liquid–solid (VLS) method at 600°C was applied to improve the field emission behavior owing to the reduction of screen effect. The enhanced field emission performance reveals the reduction of turn-on fields from 9.3 to 6.6 V μm⁻¹ with increase of field enhancement factors (β) from 1,621 to 1,857 after the selective area growth at 3 h. Moreover, we find that the screen effect also highly depends on the length of nanowires on the field emission performance. Consequently, the turn-on fields increase from 6.6 to 13.6 V μm⁻¹ with decreasing β values from 1,857 to 699 after the 10-h growth. The detailed screen effect in terms of electrical potential and NW density are investigated in details. The findings provide an effective way of improving the field emission properties for nanodevice application.     

Self-catalytic crystal growth,formation mechanism,and optical properties of indium tin oxide nanostructures

In-Sn-O nanostructures with rectangular cross-sectional rod-like, sword-like, and bowling pin-like morphologies were successfully synthesized through self-catalytic growth. Mixed metallic In and Sn powders were used as source materials, and no catalyst layer was pre-coated on the substrates. The distance between the substrate and the source materials affected the size of the Sn-rich alloy particles during crystal growth in a quartz tube. This caused In-Sn-O nanostructures with various morphologies to form. An X-ray photoelectron spectroscope and a transmittance electron microscope with an energy-dispersive X-ray spectrometer were used to investigate the elemental binding states and compositions of the as-synthesized nanostructures. The Sn doping and oxygen vacancies in the In₂O₃ crystals corresponded to the blue-green and yellow-orange emission bands of the nanostructures, respectively.     

Morphological evolution,growth mechanism,and magneto-transport properties of silver telluride one-dimensional nanostructures

Single crystalline one-dimensional (1D) nanostructures of silver telluride (Ag₂Te) with well-controlled shapes and sizes were synthesized via the hydrothermal reduction of sodium tellurite (Na₂TeO₃) in a mixed solution. The morphological evolution of various 1D nanostructures was mainly determined by properly controlling the nucleation and growth process of Ag₂Te in different reaction times. Based on the transmission electron microscopy and scanning electron microscopy studies, the formation mechanism for these 1D nanostructures was rationally interpreted. In addition, the current–voltage (I-V) characteristics as a function of magnetic field of the highly single crystal Ag₂Te nanowires were systematically measured. From the investigation of I-V characteristics, we have observed a rapid change of the current in low magnetic field, which can be used as the magnetic field sensor. The magneto-resistance behavior of the Ag₂Te nanowires with monoclinic structure was also investigated. Comparing to the bulk and thin film materials, we found that there is generally a larger change in R (T) as the sample size is reduced, which indicates that the size of the sample has a certain impact on magneto-transport properties. Simultaneously, some possible reasons resulting in the observed large positive magneto-resistance behavior are discussed.     

Structural,electrical and optical properties of stoichiometric In2Te3 thin films

In₂Te₃ thin films were grown by thermal evaporation technique. The annealing of films played a major role to obtain stoichiometry, regardless of substrate temperature. Annealing at 300 ⁰C resulted in well oriented, mono-phased and nearly stoichiometric In₂Te₃ thin films. The variation in grain size of In₂Te₃ films associated with the substrate temperatures provides a significant control over the resistivity of the films, and the resistivity decreased with an increase in the grain size. The activation energy and optical band gap of stoichiometric In₂Te₃ films were found to be 0.01±0.005 eV and 0.99±0.02 eV, respectively. The absorption co-efficient of these films was found to be of the order of 10⁵ cm⁻¹.     

Highly stable field emission properties from well-crystalline 6-Fold symmetrical hierarchical ZnO nanostructures

Herein, we report the growth, characterization and field emission application of well-crystalline 6-fold symmetrical hierarchical ZnO nanostructures grown on silicon substrate by thermal evaporation process. The detailed morphological characterizations revealed that the prepared material possess six-fold structures in which ZnO nanoneedles are symmetrically grown on each facets of core hexagonal ZnO nanorods in such a manner that they made beautiful 6-fold symmetrical hierarchical structure. The detailed structural studies confirmed that the grown hierarchical structures possess well-crystallinity with wurtzite hexagonal phase. The room-temperature photoluminescence (PL) spectrum exhibited a strong UV emission confirming good optical properties. The Raman-scattering revealed the wurtzite hexagonal phase for as-grown hierarchical structures. The field emission properties of the 6-fold symmetrical hierarchical ZnO nanostructures were tested and a turn-on voltage equal to 2.8 kV, corresponds to emission current of 65 nA, was observed. A threshold voltage of 4.6 kV with a maximum emission current of about 9.36 µA was also recorded. A high emission current stability profile over a period of ~7000 s was noted for the fabricated FE device.     

Time dependent growth of ZnO nanoflowers with enhanced field emission properties

Herein, we report the facile growth of ZnO nanoflowers composed of nanorods on silicon substrate by non-catalytic thermal evaporation process. The grown nanoflowers were examined in terms of their morphological, structural, optical and field emission properties. The detailed characterizations revealed that the nanoflowers are grown in high density, possessing well-crystallinity and exhibiting wurtzite hexagonal phase. The Raman-scattering spectrum shows a sharp optical-phonon E₂ mode at 437 cm⁻¹ which confirmed the wurtzite hexagonal phase for the grown nanoflowers. The room-temperature PL spectrum depict a strong ultraviolet emission at 381 nm, revealed good optical properties for the ZnO nanoflowers. The field emission studies revealed that a turn-on field for the ZnO nanoflowers based field emission device was 4.3 V/μm and the emission current density reached to 0.075 mA/cm² at an applied electric field of 7.2 V/μm and exhibit no saturation. The field enhancement factor ‘β’ for the fabricated device was estimated from the F-N plot and found to be ~2.75×10³. Finally, systematic time-dependent experiments were performed to determine the growth process for the formation of ZnO nanoflowers composed of nanorods.     

Enhanced Field Emission from Argon Plasma-Treated Ultra-sharp α-Fe2O3 Nanoflakes

Hematite nanoflakes have been synthesized by a simple heat oxide method and further treated by Argon plasmas. The effects of Argon plasma on the morphology and crystal structures of nanoflakes were investigated. Significant enhancement of field-induced electron emission from the plasma-treated nanoflakes was observed. The transmission electron microscopy investigation shows that the plasma treatment effectively removes amorphous coating and creates plenty of sub-tips at the surface of the nanoflakes, which are believed to contribute the enhancement of emission. This work suggests that plasma treatment technique could be a direct means to improve field-emission properties of nanostructures.     

Microstructure and mechanical properties of CaAl12O19 reinforced Al2O3-Cr2O3 composites

Al₂O₃-Cr₂O₃ refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. However, Al₂O₃-Cr₂O₃ refractories have poor mechanical properties and sintering properties. In order to improve the mechanical properties of Al₂O₃-Cr₂O₃ materials, the CaAl₁₂O₁₉ reinforced Al₂O₃-Cr₂O₃ composites were prepared by pressureless sintering process, and the influences of CaO content on the sintering properties, mechanical properties, and microstructure evolution of the composites were studied. The results show that a small amount of CaO can significantly improve the compactness of the composites, which is mainly due to the formed sheet-like CA6 fill the gap between the solid solutions, and reduces the porosity of the composites. In addition, the sheet-like CA6 makes the connection between solid solutions closer, and the intergranular fracture gradually transforms into a mixed mode of intergranular and transgranular fracture. The best mechanical propertie is observed at S4 with the CaO content of 2 wt.%. Compared with sample S0 without CaO, the hardness, compressive strength and flexural strength of the S4 were increased by 35.19 %, 49.69 %, and 68.34 %, respectively. The addition of excessive CaO will deteriorate the mechanical properties of the composites, because the formation of a large number of layered CA6 increases the porosity of the composites. Furthermore, a small amount of CaO addition can significantly improve the thermal shock resistance of the composites. After 10 and 20 thermal shock cycles, the strength loss rates of S4 are only 5.83 % and 8.74 %, respectively.     

Hydrothermal synthesis of 3D TiO2 nanostructures using nitric acid: Characterization and evolution mechanism

Various morphologies of TiO₂ nanostructures were synthesized by HNO₃ assisted hydrothermal treatment with respect to the acid molarity (1 M, 3 M, and 8 M), temperature (110, 140, and 180 °C), and time (1, 3, and 6 h). An additional sample was synthesized inside the protonated titanate nanoribbon coated vessel with the acid molarity of 8M at 140 °C for 3 h. The crystal structure and morphology of the nanostructures synthesized were investigated using X-Ray diffractometer, scanning electron microscope, and transmission electron microscope. The results revealed that lower acid concentrations, longer synthesis durations and higher temperatures favored anatase phase formation. Meanwhile, a phase pure 3D lotus structure rutile TiO₂ could be obtained by hydrothermal synthesis at 8M HNO₃ concentration at 140 °C for 3 h using protonated H-titanate nanoribbons. A probable mechanism for the evolution of 3D rutile lotus structure was highlighted.     

Design of a narrow-band blue-emitting Rb2Ba3(P2O7)2:Eu2+ phosphor with low thermal quenching for plant growth LEDs

Phosphor-converted light-emitting diodes (pc-LEDs) are commonly used to regulate the light environment to control the growth rates and improve the production efficiency of plant. Among them, the exploration of blue-emitting phosphors with high efficiency, low thermal quenching and excellent spectrum resemblance matching with the plant response spectrum is still challenging. Herein, a narrow-band blue-emitting Rb₂Ba₃(P₂O₇)₂:Eu²⁺ phosphor with high color purity of 93.4% has been developed. Under 345 nm excitation, it exhibits a blue emission band centered at 413 nm with a full width at half-maximum (FWHM) of 36 nm, and the emission spectrum of Rb₂Ba₃(P₂O₇)₂:0.060Eu²⁺ sample shows 85.7% spectrum resemblance with the absorption spectrum of chlorophyll-a in the blue region from 400 to 500 nm. In addition, the temperature-dependent emission spectra demonstrate that the Rb₂Ba₃(P₂O₇)₂:0.060Eu²⁺ phosphor has good thermal stability and small chromaticity shift, with the emission intensity dropping to 72.5% at 423 K of the initial intensity at 298 K and a chromaticity shift of 38 × 10^-3 at 498 K. All results suggest that the blue-emitting Rb₂Ba₃(P₂O₇)₂:Eu²⁺ phosphor has potential application in plant growth LEDs.     

Comparisons on properties and growth mechanisms of carbon nanotubes fabricated by high-pressure and low-pressure plasma-enhanced chemical vapor deposition

Effects of plasma pressure and the presence of nitrogen on growth of carbon nanotubes (CNTs) and their properties were studied by using microwave plasma chemical vapor deposition (MPCVD) (pressure=600–3300 Pa) and electron cyclotron resonance chemical vapor deposition (ECR-CVD) (pressure=0.3–0.6 Pa) systems. CH₄/H₂ and CH₄/N₂ were used as source gases, and Co as the catalyst. The structures and properties of CNTs were characterized by using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Raman spectra, and field emission I–V measurements. The results show that CNTs made by higher plasma pressure system have a higher growth rate (typically 1–3 μm/min), smaller tube diameter, better field emission properties, and better tube quality. The growth rate is related to the availability of carbon source. The morphology change from spaghetti-like to well-aligned CNTs is discussed in terms of directed ions. The change in field emission properties is reasoned in terms of geometric enhancement factor and screening effect for different tube morphologies. The presence of nitrogen plasma can have the following effects: increasing tube diameter, increasing straightness of CNTs, forming of bamboo-like CNTs, deterioration of field emission properties, and shifting of Raman peak toward lower-frequency side (or increasing residual tensile stress).     

MgCO3·3H2O晶须的制备及其生长机制的研究

以MgCl₂·6H₂O和NH₄HCO₃为原料,CH₃COONa·3H₂O为形貌控制剂,采用沉淀结晶法制备MgCO₃·3H₂O晶须,考察了不同添加量的CH₃COONa·3H₂O对晶须结晶过程和形貌的影响,并研究了晶须在该体系中的生长机制。结果表明：当体系中加入质量分数为0.23%的CH₃COONa·3H₂O时可以成功制备长径比约为30的棒状MgCO₃·3H₂O晶须,CH₃COONa·3H₂O的存在促进了MgCO₃·3H₂O晶须的形成。该体系中晶须的生长过程：首先形成无定形的4MgCO₃·Mg（OH）₂·4H₂O,之后无定形4MgCO₃·Mg（OH）₂·4H₂O逐渐转变为MgCO₃·3H₂O并生长成较大长径比的棒状MgCO₃·3H₂O晶须。这是因为CH₃COONa·3H₂O电离产生的Na ⁺选择性吸附在MgCO₃·3H₂O晶体轴向的（101）晶面,抑制了该晶面生长,而径向晶面生长速率未受到影响,从而促使无定形MgCO₃·3H₂O生长成棒状晶须。     

A study of phase evolution and crystal growth for nano-sized monoclinic Y4Al2O9 as a novel thermal barrier coatings

Nano-sized monoclinic Y₄Al₂O₉ was produced by sol-gel process as a novel potential candidate material for thermal barrier coatings. The thermal behavior, structural evolution of the products and the morphological characteristics of the compacted bodies were investigated by Thermogravimetric analysis and differential scanning calorimeter (TG-DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and Field emission scanning electron microscopy (FESEM). Qualitative analyses indicate that monoclinic Y₄Al₂O₉ was formed at about 1000 °C, and exhibited good phase stability throughout the annealing temperature ranging from 1000 °C to 1400 °C. The thermophysical properties of Y₄Al₂O₉ ceramics were also evaluated compared with 8YSZ and La₂Zr₂O₇. The determined activation energy of crystal growth is about 72.71 ± 0.31 kJ mol⁻¹. Meanwhile, Y₄Al₂O₉ represents low thermal conductivity (1.71 W m⁻¹ K⁻¹), moderate thermal expansion coefficient (8.73 × 10⁻⁶ K⁻¹), and high sintering-resistance ability. Such results reveal that nano-sized Y₄Al₂O₉ is favorable for the application of TBCs.     

Thermo-physical and mechanical properties of Yb2O3 and Sc2O3 co-doped Gd2Zr2O7 ceramics

Ceramic materials for the thermal barrier coating (TBC) application of Gd₂Zr₂O₇ (GZO), (Gd_0.94Yb_0.06)₂Zr₂O₇ (GYb_0.06Z), (Gd_0.925Sc_0.075)₂Zr₂O₇ (GSc_0.075Z), (Gd_0.865Sc_0.075Yb_0.06)₂Zr₂O₇ (GSc_0.075Yb_0.06Z), and (Gd_0.8Sc_0.1Yb_0.1)₂Zr₂O₇ (GSc_0.1Yb_0.1Z) were successfully synthesized by chemical co-precipitation. The effects of the doping of Sc₂O₃ and Yb₂O₃ on the phases, thermo-physical and mechanical properties of the ceramics were investigated. The results show that both Yb₂O₃ and Sc₂O₃ doping promoted the phase transition of GZO from pyrochlore to fluorite. All the Sc₂O₃-doped samples exhibited enhanced fracture toughness, as compared to the undoped sample. Furthermore, the GSc_0.075Yb_0.06Z sample revealed a thermal conductivity of ~0.8 W/mK at 1200 °C, which was nearly 30% lower than that of the undoped sample. The associated mechanisms related to the effects of the doping on the thermophysical and mechanical properties are discussed.     

Thermoelectricity and antivibration properties of screen-printed nanodoped In1.35ZnO2.11/In2O3 thin-film thermocouples on alumina substrates

Owing to the low flow field disturbances and fast response, thin-film thermocouples (TFTCs) are used to measure the service temperature of aero-engines. Indium tin oxide (ITO) and In₂O₃ are widely used in high–temperature measurements. However, ITO undergoes phase transition and consequent thermoelectric failure at above 1300 °C. In this study, In_1.35ZnO_2.11/In₂O₃ TFTCs were prepared on alumina substrates via screen printing method through introduction of ZnO nanopowder followed by annealing treatment. Results show that prepared TFTCs exhibited good thermoelectric properties at 1500 °C. The morphology, structure, and electrical coefficients of TFTCs were investigated. The average Seebeck coefficient was 39.8 μV/°C at 1500 °C with a drift rate (DT) of 0.84 °C/h, which was significantly improved with respect to that of ITO/In₂O₃, corresponding to 44.5 μV/°C at 1270 °C with a DT of 5.44 °C/h and failed at higher temperature. The result of lumped capacity method test show that the response time was 4.8 ms at 100 °C. Preliminary engine gas temperature measurements with a heat load of 1000 °C at 1 Mach show that these TFTCs are promising candidates for engineering applications. Finally, the structural reliability under high-magnitude vibration and impact tests (10–2000 Hz/20 g and 100 g/11 ms) was also investigated. As a result of the excellent bonding strengths of 47.36 and 59.83 N between the film and the substrate for both In_1.35ZnO_2.11 and In₂O₃, respectively, destructive cracking and peeling of the film were not observed, and no change in the Seebeck coefficient of the sample occurred after impact and vibration tests. These results provide an important basis for the potential application of In_1.35ZnO_2.11/In₂O₃ TFTCs in aero-engine high-temperature measurements of flow channel components.     

Improvement on thermophysical and mechanincal properties of LaMgAl11O19 magnetoplumbite by Nd2O3 and Sc2O3 co-doping

LaMgAl₁₁O₁₉-type magnetoplumbite holds great promise to be used above 1300 °C as thermal barrier coatings (TBCs), but its practical application has been restricted because of inferior thermophysical properties. Herein, we focus on optimizing the thermophysical properties of LaMgAl₁₁O₁₉ by simultaneously substituting La³⁺ and Al³⁺ ions with Nd³⁺ and Sc³⁺ ions, respectively. Results show that the effects of co-substitution on reducing thermal conductivity are pronounced. The thermal conductivities of La_1-xNd_xMgAl_11-xSc_xO₁₉ (x = 0, 0.1, 0.2, 0.3) ceramics decrease progressively with dopant concentration and a lowest thermal conductivity of 2.04 W/(m·K) is achieved with x = 0.3 at 1000 °C, which is a value superior to pure LMA and even lower than YSZ. The mechanisms behind the lowered thermal conductivity are investigated. Increase of the thermal expansion coefficient is also realized (8.53 × 10⁻⁶ K⁻¹ for pure LMA, 9.07 × 10⁻⁶ K⁻¹ for x = 0.3, 1300 °C). Most importantly, Nd³⁺ and Sc³⁺ combination doping indeed facilitates mechanical properties of La_1-xNd_xMgAl_11-xSc_xO₁₉ solid solutions as well. It should be noted that Sc³⁺ doping at Al³⁺ site plays more effective role in improving thermal properties than Nd³⁺ does at La³⁺ site. This work provides a path to simultaneously integrate low thermal conductivity, good phase stability, moderate thermal expansion behavior and excellent mechanical properties on LMA for the next generation TBCs.     

Field emission properties and stability of thermally treated photosensitive carbon nanotube paste with different inorganic binders

To investigate emission properties depending on the inorganic binders, two types of photosensitive CNT paste were formulated using glass frit and spin on glass (SOG) as an inorganic binder. For uniform thickness and patterning of CNT paste, backside exposure and development process were carried out after screen printing and drying process. CNT paste films with different inorganic binders were fired at various conditions and their emission properties were evaluated. We also investigated emission stabilities of printed CNT emitter after the electrical aging treatment.     

Field emission property of arrayed nanocrystalline diamond

Arrays of nanocrystalline diamond (NCD) stripes were fabricated by plasma etching of a NCD film. Electron field emission (EFE) of NCD arrays with 100-μm-wide stripes separated by different spacings was analyzed. The NCD arrays had higher EFE efficacy than the non-patterned blanket NCD film. The turn-on electric field (E_on) decreased from 5.4 V/μm^-1 for the blanket NCD film to 4.2, 4.4 and 4.7 V/μm^− 1 for the NCD arrays with 100, 500 and 1000 μm of spacing, respectively. Both the effective emitting area and the field enhancement factor for the NCD emitters were increased by patterning. The enhanced EFE from arrayed NCD stripes was possibly attributed to the edge effect and reduction of electrostatic screening.     

Thermal and mechanical properties of Ta2O5 doped La2Ce2O7 thermal barrier coatings prepared by atmospheric plasma spraying

La₂Ce₂O₇ (LC) is a new promising thermal barrier coating (TBC) material for high-temperature applications. However, the sudden decrease of thermal expansion coefficient (TEC) at ∼623 K limits its application. In this study, the plasma-sprayed La₂Ce_1.7Ta_0.3O_7.15 (LCT) coating was developed by partial substitution of Ce⁴⁺ in LC with Ta⁵⁺. LCT coating shows lower thermal conductivity between 298 K and 1273 K (0.54–0.71 W/(m·K)) than LC coating (0.65–0.85 W/(m·K)) and the traditional yttria partially stabilized zirconia (YSZ) coating (1.53–1.72 W/(m·K)). It also exhibits excellent thermal stability at least up to 1573 K for 1000 h. What is more, the sudden TEC drop is suppressed owing to the reduced oxygen vacancy concentration governed by Ta⁵⁺-substitution content. As a result, LCT TBC shows an improved thermal cycling lifetime in an air furnace as compared to LC TBC.