Thermal cycle behavior of plasma sprayed La2O3, Y2O3 stabilized ZrO2 coatings

The effects of La₂O₃ addition on thermal conductivity, phase stability and thermal cycle life of Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La₂O₃ addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.     

Crystallisation of oxygen-stabilised amorphous phase in a Zr50Cu50 alloy

The crystallisation of the oxygen-stabilised amorphous phase in a Zr₅₀Cu₅₀ alloy has been investigated by means of neutron diffraction and electron microscopy. The crystallisation microstructure consists of ZrO₂, Zr₂Cu and Zr₇Cu₁₀. A two-stage crystallisation mechanism is suggested: (i) primary crystallisation of Zr₂Cu and (ii) formation of nanocrystals ZrO₂ and Zr₇Cu₁₀. In (i), it is proposed, Zr₂Cu crystallises from the oxygen-stabilised amorphous phase, leaving an oxygen- and copper-enriched matrix ; Zr₂Cu rapidly grows and eventually attains a grain size of about 100 nm. In (ii), it is suggested, the residual amorphous matrix crystallises into nanocrystals ZrO₂ and Zr₇Cu₁₀ due to the sluggish growth of ZrO₂ and to the already formed ZrO₂ which acts as a growth barrier to Zr₇Cu₁₀. In this case there is no particular orientation relationship between Zr₂Cu and Zr₇Cu₁₀.     

Ni-CeO2复合镀层厚度对δ-Ni2Al3-CeO2/Ni-CeO2涂层体系扩散行为的影响

在Ni基体电沉积不同厚度的Ni-CeO₂复合镀层并对其进行620 °C低温渗铝,制备了剩余复合镀层厚度分别约为10和45 μm厚的δ-Ni₂Al₃-CeO₂/Ni-CeO₂涂层体系。1000 °C真空退火5 h后,以上两种涂层体系均可在铝化物/镀层界面形成CeO₂富集层。但剩余复合镀层厚度为10 μm的涂层体系由于氧化物富集程度不足,涂层退化程度较剩余复合镀层厚度为45 μm的涂层体系严重。可见,在镀层氧化物颗粒含量一定的情况下,渗铝后剩余镀层厚度是影响扩散障有效性的重要因素。     

Thermoelectric properties of Fe0.98Co0.02Si2 with ZrO2 and rare-earth oxide dispersion by mechanical alloying

The n-type Co-doped β-FeSi₂ (Fe_0.98Co_0.02Si₂) with dispersion of several oxides, such as ZrO₂ or several rare-earth oxides (Y₂O₃, Nd₂O₃, Sm₂O₃ and Gd₂O₃), was synthesized by mechanical alloying and subsequent hot pressing. The effects of these oxide dispersions on the thermoelectric properties of Fe_0.98Co_0.02Si₂ were investigated. ZrO₂ was decomposed in the β phase, and the ZrSi and -FeSi phases, which are metallic phases, were formed in the samples with ZrO₂ addition. The Seebeck coefficient and the electrical resistivity were significantly decreased with increasing amount of ZrO₂, indicating that a part of the Zr atoms was substituted for Fe atoms in the β phase. In the case of the samples with rare-earth oxide addition, a decomposition of a large amount of these added oxides did not occur. However, the rare-earth oxide addition caused a slight increase in the amount of the phase. The Seebeck coefficient was significantly enhanced by the rare-earth oxide addition especially in the low temperature range. These facts indicated that a small amount of rare-earth oxides was decomposed in the β phase, and rare-earth elements were substituted for Fe atoms as a p-type dopant, resulting in the decrease in the carrier concentration. The rare-earth oxide addition was also effective in reducing the thermal conductivity.     

Oxygen release behaviour of Ce(1−x)ZrxO2 powders and appearance of Ce(8−4y)Zr4yO(14−δ) solid solution in the ZrO2–CeO2–CeO1.5 system

To clarify the existence of metastable phases in the ZrO₂–CeO₂–CeO_1.5 system, evolved-oxygen gas analyses, (EGA), by heating a single phase of t′ and t″ (Ce_(1−x)Zr_xO₂) with various compositions, x, in a reducing gas and successive oxidation were carried out repeatedly. The oxygen release behaviour of the t′ and t″ phases was very complicated. The single κ phases, (Ce_(1−x)Zr_xO₂) with the composition, x=0.5 and 0.6, which were obtained by oxidizing the resulting pyrochlore as a precursor in O₂ gas at 873 K, exhibited a sharp oxygen release at the lowest temperature; the composition range of κ phase may be x=0.450.65. A new tetragonal phase t*, (Ce_(1−x)Zr_xO₂), which was attained by cyclic redox process together with annealing in O₂ gas at 1323 or 1423 K, exhibited a sharp oxygen release at the highest temperature; the composition range of t* phase may be as wide as x=0.200.65. A metastable solid solution expressed by a chemical formula of Ce_(8−4y)Zr_4yO_(14−δ) (y=01) possessing a CaF₂-related structure appeared on deoxidation of the t* phase. A ternary phase diagram containing the t* and Ce_(8−4y)Zr_4yO_(14−δ) solid solution was proposed.     

pH值对CeO2/SiO2纳米复合磨粒分散性的影响

以胶体SiO₂溶液作硅源,采用均相沉淀工艺制备壳-核结构完整的CeO₂/SiO₂纳米复合磨粒。采用X射线衍射仪和透射电子显微镜对复合磨粒样品进行物相组成分析和微观形貌观察;通过粒径分布、Zeta电位分析,研究水相分散系中pH值对CeO₂/SiO₂复合磨粒分散性的影响。结果表明:所制备的CeO₂/SiO₂复合磨粒为壳-核包覆结构完整的纳米微球,粒径约110 nm,内核为无定形SiO₂,壳层为立方萤石型CeO₂;CeO₂/SiO₂复合磨粒的等电位点pH值约为5,其值由SiO₂等电位点向CeO₂等电位点明显偏移。CeO₂/SiO₂复合磨粒在酸性水相介质中分散性差,容易出现严重的团聚现象;而在碱性环境下,CeO₂/SiO₂复合磨粒分散性良好。      

含ZrO2类Ds夹杂物的来源及控制分析

对某厂产品中发现的含ZrO2类Ds夹杂物Ds类夹杂物来源进行了系统性的分析和研究,研究结果表明含ZrO2类Ds夹杂物产生的直接原因在于含Zr浸入式水口发生脱碳,脱碳通道内CaO-Al2O3-SiO2(-MgO)相中的SiO2将被钢液中的Al还原,最终成为CaO-Al2O3(-MgO)相,ZrO2仅起到了类似示踪剂的作用。     

Achieving tensile superplasticity in 8 mol% Y2O3 cubic stabilized ZrO2 through the addition of intergranular silica

The addition of 5 wt.% SiO₂, a viscous second phase, to 8 mol% Y₂O₃ cubic stabilized ZrO₂ (8Y-CSZ) made superplastic 8Y-CSZ. This material had a fine grain size of 0.4 μm and exhibited deformations in tension as large as 520% at 1430 °C with a strain rate of 1.0 × 10⁻⁴ s⁻¹.     

Phase diagram of the CuGaSe2–SiSe2 and CuInSe2–SiSe2 systems

The phase diagrams of the CuGaSe₂–SiSe₂ and CuInSe₂–SiSe₂ systems were constructed using the results of differential thermal and X-ray phase analysis. Both systems are of the eutectic type with the eutectic point coordinates 75 mol% SiSe₂, 1042 K (CuGaSe₂–SiSe₂); 67 mol% SiSe₂, 1083 K (CuInSe₂–SiSe₂). Solid solutions based on CuGaSe₂ and CuInSe₂ were discovered in these systems; their extent at 670 K being 24 and 25 mol% SiSe₂, respectively. The crystal structure of the limiting compositions of these solid solutions was refined.     

Microstructure and composition analysis in plasma sprayed coatings of Al2O3/ZrSiO4 mixtures

Plasma spraying of Al₂O₃/ZrSiO₄ was performed using spray dried and plasma spheroidised powder feedstock. The mixtures were sprayed using different spray stand-off distances and plasma power levels. X-Ray diffraction (XRD) was used to characterise the phase composition and scanning electron microscopy (SEM) examined the morphology of the sprayed surface and polished cross-sections. The results showed that the plasma spray process parameters played an important role in the final outcome of microstructures of the coatings. The coatings produced with spheroidised powders displayed a much denser structure than those produced with the spray-dried powders. The phase composition analysis showed the presence of amorphous phases in addition to crystalline alumina, zircon and tetragonal (t) zirconia (ZrO₂). Transmission electron microscopy (TEM) showed that amorphous phases and t-ZrO₂ crystals with particle size 100–200 nm could coexist within a single splat due to the relatively low local cooling rate.     

Double SHS of ZrB2 powder

Production of ZrB₂ powder through self-propagating high-temperature synthesis (SHS) from ZrO₂, Mg and H₃BO₃ mixture often leads to incomplete conversion. A new technique, called DSHS (double SHS) has been developed, wherein the reaction product of the first SHS is mixed with calculated amounts of Mg and H₃BO₃ powder and subjected to a second SHS. The ZrB₂ powder produced by DSHS technique yields increased conversion. The NaCl is used as a diluent during SHS to control the particle size of the product. The average particle size of SHS ZrB₂ powder found to be 75–125 nm in range, which decrease to 25–40 nm after DSHS.     

Dielectric relaxation and ionic conductivity studies of [N(CH3)4]2Cu0.5Zn0.5Cl4

WO₃-doped zinc titanate ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. The effects of WO₃ additions on the phase structure and phase transitions of zinc titanate ceramics were investigated by high-temperature X-ray diffractometry (HTXRD) and transmission electron microscopy (TEM). The results showed that the major phase of zinc titanate ceramics transformed from zinc orthotitanate phase to zinc metatitanate phase with the amounts of WO₃ additions increasing. Small WO₃ (<1.00 mass%) addition accelerated the transition of ZnTiO₃ to Zn₂TiO₄, while excessive WO₃ addition restrained the transition. HTXRD showed that WO₃ enhanced the stability of Zn₂Ti₃O₈ and weakened the stability of ZnTiO₃. A precipitate within the Zn₂TiO₄ matrix was observed. Viewed along the orientation of Zn₂TiO₄, the precipitate was found to have a rectangular shape and to be nanometer level in size; its composition was concluded to be Zn₂Ti₃O₈. The dielectric properties of WO₃-doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed maximum values when the amount of WO₃ was 0.50 mass%.     

Stress analysis of high temperature ZrO2 insulation coatings on Ag using finite element method

The aim of this study is to investigate residual stresses occurred during cooling procedure of ZrO₂ insulation coating on Ag substrate for magnet technologies. ZrO₂ coatings were produced on Ag tape substrate by using a reel-to-reel sol–gel technique. SEM inspection showed that ZrO₂ coatings had mosaic structures. ANSYS finite element software was used to calculate the temperature and stress distribution of the ZrO₂/Ag structure. The effect of coating thickness on residual stresses was also examined. The results obtained showed that thermal stresses in ZrO₂ coating and Ag substrate were considerably affected by the cooling time and coating thickness. It is concluded the thermal stresses increase with increase of film thickness.     

Size and pressure effects on solid transition temperatures of ZrO2

Transition temperatures between tetragonal and monoclinic polymorphs of ZrO₂ nanoparticles, thin films and nanostructured materials are calculated by considering energetic contributions of surface (interface) energy and surface stress on total Gibbs free energy. The transition temperatures drop as the size of the nanocrystals decreases, which is consistent with available results.     

Formation of zirconium diboride (ZrB2) by room temperature mechanochemical reaction between ZrO2, B2O3 and Mg

A mixture of magnesium, boric oxide and zirconium dioxide were mechanically milled under argon for up to 15 h in a laboratory scale ball mill. X-ray diffraction showed that there was an increasing conversion of ZrO₂ to ZrB₂ with milling time with >98% reaction after 15 h. Differential thermal analysis revealed there were multiple, overlapping reactions all of which seemed to be formation of ZrB₂. The energy evolved decreased with milling time and the sample after 15 h milling showed no thermal reaction. After milling, separation of the ZrB₂ from the coproduct MgO was easily achieved by a mild acid leaching leaving essentially pure ZrB₂ with a crystallite size of 75 nm.     

ZrO2–SiO2 gradient multilayer oxidation protective coating for SiC coated carbon/carbon composites

In order to eliminate the mismatch of thermal expansion coefficient between the ZrO₂ outer layer and the internal bonding SiC layer, ZrO₂–SiO₂ composition-gradient transition layers were prepared by a sol–gel technique using tetraethoxysilane (TEOS) and zirconyl chloride as source materials. Energy dispersive spectroscopy (EDS) analysis displays that the gradient composition ZrO₂–SiO₂ outer coating could be obtained by immersing the SiC precoated carbon/carbon (C/C) composites into the gradient composition zirconia-silica sols (ZS sol) in turn. Oxidation test shows that, after 10 h oxidation in air at 1773 K, the weight loss of the gradient ZrO₂–SiO₂ coating coated SiC-C/C is only 1.97%.     

Influence of Al2O3 or ZrO2 particulate addition on the microstructure aspects of AlNi and AlSi alloys

Different metal/ceramic composites (Al7Si0.3Mg, Al3Ni, Al6Ni, Al9Ni) reinforced with Al₂O₃ or ZrO₂ were prepared by vortex method. Metallographic investigations reveal that in all the composites -Al did not nucleate on the reinforcement particulates. The particulates were generally observed to be located in the last freezing regions regardless of matrix alloy, particulate type or size. The reason for that was the mismatch in the thermal diffusivity between the ceramic particulates and matrix alloys. SEM micrographs show that the presence of the particulates in the AlSi alloy tends to modify the silicon eutectic. In contrast, the addition of the particulates into AlNi alloys did not result in a significant modification of the NiAl₃ phase, but it displaced the eutectic point to lower Ni content.     

Role of strontium addition on the phase transition of lanthanum copper oxide from K2NiF4 to perovskite structure

Without Sr addition, the sintered La₂O₃ and CuO powder mixture in a mole ratio of 1:2 formed K₂NiF₄-structured La₂CuO₄ with excess CuO. When 15% of strontium was added, La₂CuO₄ transformed into the single perovskite La_1−xSr_xCuO_2.5−δ phase with orthorhombic structure. As the strontium addition increased to 20%, the perovskite lattice changed from orthorhombic to tetragonal. These phase transitions may be attributed to the enhanced oxidation of the divalent cupper ions (Cu²⁺) to trivalent ones (Cu³⁺) by the strontium addition. Based on the electroneutrality in an ABO₃ perovskite lattice, a divalent cation is unstable in the B-site cation sub-lattice when the A-site is occupied by a trivalent cation such as La³⁺. As strontium was added into the A-site cation sub-lattice, the oxidation of Cu²⁺ ion into trivalent Cu³⁺ ion was enhanced. The increase of Cu³⁺ concentration strengthened the electrostatic bonding (ESB) of copper ions with their neighboring anions. Consequently, the symmetrical tetragonal Sr-doped lanthanum copper oxide was obtained.     

Effects of plasma-sprayed NiCrAl/ZrO2 intermediate on the combination ability of coatings

A NiCrAl/ZrO₂ composite coating was deposited on the surface of metal carrier FeCrAl alloy by a plasma-spray technique. After static-state oxidation at 800°C, the transitions in structure and composition of the coating was analyzed by XRD, SEM and EDX. The results showed that the surface phases of the as-sprayed coating were mainly composed of Ni and ZrO₂. When the oxidation time was extended from 8 to 50 h, NiO crystallites were formed and these grew coarse on the coating surface, and alloy elements were diffused between the NiCrAl/ZrO₂ coating and the FeCrAl substrate. With the pretreatment, an intermediate coating was prepared with a coarse and porous structure, high cohesive strength and high heat resistance. These developed properties could provide high geometric surface area for a catalytic γ-Al₂O₃ washcoat, and enhance the adhesive strength between ceramic washcoat and metal substrate so as to extend the lifetime of the washcoat.