Small Zirconia Research Articles

Surfactant-stabilized copper paticles for low-temperature sintering: Paste preparation using a milling with small zirconia beads: Effect of pre-treatment with the disperse medium

Copper paste is considered as a promising candidate for printed electronics in replacement for silver paste. This is owing to copper which has anti-electromigration property, lower cost, and similar conductivity and compared with silver. We synthesize a copper nanoparticle (NP) paste that can be sintered at low temperature for high conductivity. The copper NP paste composes of 50 wt% copper NPs and dipropylene glycol (DPG) as the disperse medium. The effect of DPG coating and various conditions of milling with small beads on improving the dispersity of copper NPs has been investigated. The optimum conditions for milling are at 1000 and 2000 rpm for 30 min. This results in a volume resistivity of 6.62 × 10-6 Ω·cm after sintering the copper NP paste at 200 °C.

Contamination Study of Zirconia on the Densification Process and Properties of Transparent MgAl₂O₄ Ceramics.

In this paper, transparent magnesium aluminate (MgAl2O4) spinel ceramics are fabricated through pressureless sintering combined with hot isostatic pressing (HIPing). To investigate the effect of zirconia on sintering behavior, microstructure, and optical properties of transparent spinel ceramics, different contents of zirconia were added. The results show that zirconia can promote the densification process by the formation of anion vacancies. The resulting zirconia exists as tetragonal phase along the grain boundaries. The average grain size of resulting ceramics depends on the content of zirconia and HIPing condition. Small zirconia content did not deteriorate the optical properties of samples. A 5-mm-thick sample with 0.05 wt% ZrO2 pre-sintered at 1500 °C followed by HIPing treatment at 1550 °C achieved a high in-line transmittance of 75.5% at 400 nm and 85% at 1100 nm.

The role of oxide location in HMF etherification with ethanol over sulfated ZrO2 supported on SBA-15

The etherification of 5-hydroxymethyl-2-furfural (HMF) over ZrO2 and sulfated ZrO2-SBA-15 was chosen as a case study to analyze (i) the quantitative relationship between the concentration of Lewis and Brønsted acid sites and the catalytic behavior in the above reaction, which is also of industrial relevance for the production of biodiesel additives, and (ii) how the location of zirconia nanoparticles inside or outside the mesoporous channels of SBA-15 could significantly influence the specific reactivity in this reaction, both before and after sulfation. Depending on the loading of zirconia (about 10 or 35wt%), the characterization data by different techniques (TEM, XRD, BET, Dr-UV–vis, and XPS) agree in indicating that zirconia is located predominantly outside the mesoporous channels as small zirconia nanoparticles for the lower loading, and predominantly inside the mesoporous channels for the higher loading. The concentration of medium–strong Lewis and Brønsted acid sites were determined by pyridine chemisorption monitored by IR spectroscopy. While the concentration of Brønsted acid sites (formed after sulfation) is linearly dependent on the amount of zirconia in SBA-15, a marked deviation is observed for Lewis acid sites. The same conclusion was derived from analysis of the dependence of the catalytic activity in Lewis- or Brønsted-acid-site-promoted reactions. The analysis of these results indicated that the characteristics of the zirconia nanoparticles deposited outside or inside the mesoporous silica channels differ in terms of acid features and in turn of catalytic reactivity.

Microstructure and mechanical properties of zirconia-toughened lithium disilicate glass–ceramic composites

The lithium disilicate glass–ceramics composites reinforced and toughened by tetragonal zirconia (3Y-TZP) were prepared by hot-pressing at 800 °C with varying zirconia content from 0 to 30 wt.%. In the case of the composites of small zirconia content (below 10 wt.%), zirconia acted as nucleation agent primarily, and the microstructure was refined continuously. The morphology of Li2Si2O5 crystals transformed from rod-shaped to spherical structure, and the mechanical properties decreased inevitably. For the composites of large zirconia content (from 15 wt.% to 30 wt.%), however, zirconia restrained the phase separation of glass. The morphology of Li2Si2O5 crystals transformed to rod-shaped structure again. The mechanical properties of the composite at zirconia content of 15 wt.% increased up to 340 MPa and 3.5 MPa m1/2 which were much higher than those of zirconia-free glass–ceramics. The improved properties were attributed mainly to compressive stress reinforcement, phase transformation and bridging toughening mechanisms.

중공형 미세구를 이용한 마이크로셀룰라 지르코니아의 가공 특성 고찰

In this study, a novel-processing route for producing microcellular zirconia ceramics has been developed. The proposed strategy for making the microcellular zirconia ceramics involves hollow microsphere as a pore former which has extremely low density of 0.025 g/㎤. Effects of hollow microsphere content and sintering temperature on microstructure, porosity, pore distribution, and compressive strength were investigated in the processing of microcellular zirconia ceramics. By controlling the content of hollow microsphere, it was possible to make the porous zirconia ceramics with porosities ranging from 45% to 75%. Typical compressive strength value of microcellular zirconia ceramics with ~65% porosity was over 50㎫. By adjusting the mixing ratio of large and small zirconia powders, it was possible to control the pore structure from close to open pores.

Development of mullite/zirconia composites from a mixture of aluminum dross and zircon

In order to obtain mullite/zirconia composites, mixtures of aluminum dross and zircon were sintered. Aluminum dross was collected and purified by a milling, sieving and washing process. Stoichiometric mixtures of aluminum dross and zircon were sintered at several temperatures (1400, 1450 and 1500 °C) for several periods of time (2, 4 and 6 h). After the purifying treatment the dross contained mainly Al 2O 3, AlN, MgAl 2O 4, SiO 2 and metallic Al. A homogeneous mullite matrix with small zirconia particles was obtained by sintering the aluminum dross–zircon samples at 1500 °C for 6 h.

Grain Growth Behavior of Al2O3-ZrO2 with small zirconia content

Grain Growth Behavior of Al2O3-ZrO2 with small zirconia content

Microstructure after superplastic creep of alumina–zirconia composites prepared by powder alcoxide mixtures

The microstructural evolution during superplastic creep of alumina–zirconia composites prepared from alumina powder–zirconium alcoxide mixtures was studied, in order to determine the changes in the grain size and shapes. Zirconia reinforced alumina composites with up to 20 wt.% ZrO 2 content were deformed in compression creep tests at constant load in air at 1350 °C. The microstructure was studied by means of SEM. The distribution of both phases, grain sizes and shapes were determined on sintered and tested materials. Grain size distributions are narrow, with typical zirconia particle sizes several times finer than alumina ones, resulting in intergranular small zirconia grains occupying most alumina triple points. Every composition was deformed in stationary creep to 100% true strain. Superplastic strains were achieved by the coupled dynamic movement of grains during grain boundary sliding and simultaneous accommodation at triple points by zirconia particles. The shape and size changes in alumina and zirconia grains revealed that transport of matter took place also by diffusion at the grain boundaries. Grain boundary diffusion at alumina–zirconia interphases resulted in significant grain shapes in the zirconia particles, with zirconia grains exhibiting protruding parts along the alumina grain boundaries aligned perpendicularly with the compression axis.

HVOF Thermal Spray Deposited Y<SUB>2</SUB>O<SUB>3</SUB>-Stabilized ZrO<SUB>2</SUB> Coatings for Thermal Barrier Applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (P o) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures T p that are high enough for partial melting of small (∼10 µm) zirconia particles, T p=(1.10−0.95)T m. Further analysis shows that for larger particles (38 µm), adherent coatings are produced when the particle temperature, T p=0.59−0.60 T m, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.

Electrochemical characterisation of 3Y-TZP–Fe 2O 3 composites

The influence of the addition of ferric oxide to 3Y-TZP on the conductivity and microstructure of sintered Y-stabilised tetragonal zirconia ceramics (3Y-TZP) was investigated. A comparison was made between two different dense 3Y-TZP–α-Fe 2O 3 composites. Compacts were made by pressureless sintering at 1150 °C or by sinterforging at 1000 °C and 100 MPa. The sinterforging process resulted in smaller zirconia and hematite grains and a higher monoclinic zirconia content as compared to the compact that was sintered pressureless. The high monoclinic content led to loss of ionic conductivity. The addition of ferric oxide caused electronic conductivity. The sinterforging resulted in a high concentration of metastable defects in the zirconia–hematite composite, leading to a relatively high electronic conductivity. Heating above 380 °C caused irreversible loss of these defects and a large decrease in electronic conductivity.

Size Effects on the Stabilization of Ultrafine Zirconia Nanoparticles

The spontaneous oxidation of crystallized ZrAu at room temperature leads to the formation of ultrafine monoclinic zirconia (MZ) particles (about 3.6 nm in diameter). The presence of tetragonal phase (TZ), generally observed in very small zirconia particles, was investigated by X-ray diffraction and Raman experiments but was not observed. Thermodynamical calculations considering the finite size effects in zirconia were introduced to explain the formation of pure MZ considering experimental and theoretical surface energies. The results of these calculations associated with the near equilibrium conditions used for synthesis demonstrate that the monoclinic structure is the stable crystallographic structure of zirconia nanocrystals at room temperature.

Tetragonality of Nanosized 3Y‐TZP Powders

The tetragonality of small zirconia particles has been established by accurately measuring the tetragonality ratio (c/a) of decomposed zirconyl hydroxide, which was then calcined over a range of temperatures to produce a range of average‐sized zirconia crystallite particles. The zirconia initially crystallized as a finely divided cubic phase, which, as the calcination temperature increased, gradually developed an increase in c/a.At temperatures >800°C, the c/avalue reached a critical value; thereafter, the tetragonal symmetry was stabilized.

Interaction of Sulfate Groups with the Surface of Zirconia: An HRTEM Characterization Study

High-resolution transmission electron microscopy (HRTEM) has been used to characterize the morphology and the surface structure at an atomic level of sulfated zirconia and sulfate-free zirconia. Our study shows that HRTEM can be used to directly observe sulfate layers adsorbed on the surface of zirconia crystallites. The results indicate that the presence of sulfate groups not only stabilizes the tetragonal zirconia phase, but also induces the formation of well-faceted small zirconia crystallites. In particular, it is observed that the presence of sulfate groups induces the preferential formation of relatively long-flat (110) plane of tetragonal zirconia. HRTEM images of this plane revealed the presence of an adsorbed sulfate layer. It is proposed that the geometry of the (110) plane is such that it can accommodate sulfate groups in a two- or threefold coordination. Although, zirconia crystallites with relatively long-flat {110} planes containing adsorbed sulfate groups were clearly predominant in the sulfated zirconia catalyst; its role in the formation of the highly acidic sites is not clear. Our study also reveals the presence of few zirconia crystallites containing rough surfaces, crystallographically speaking high-Miller-index surfaces, which if they were to contain sulfate groups, they could be the locus of the highly acidic sites. Thus, it is suggested that caution must be taken when performing spectroscopic studies using techniques such as IR, NMR, XPS, and Raman to distinguish between spectator sulfate groups, which could be a majority if they were the ones observed on the (110) plane, and those participating in the formation of the highly acidic site, which could be associated to the few high-Miller-index-containing zirconia crystallites.

Surface atomie resolution microscopy of sulfated-zirconia and Fe/Mn promoted sulfated zirconia

ABSTRACTBy using high-resolution transmission electron microscopy (HRTEM), it was possible to elucidate edge-on views of the surface atomic structure of sulfated zirconia and Fe/Mn promoted sulfated-zirconia catalysts. The results indicate that the presence of sulfate groups and Fe/Mn species not only stabilizes the tetragonal zirconia phase, but also induces the formation of well-faceted small zirconia crystallites. The shape of the sulfated zirconia crystallites exhibits two types of surfaces: long smooth and relatively few short rough planes, with the long smooth (110) plane being clearly predominant. HRTEM images of this plane revealed the presence of an adsorbed sulfate layer. On the other hand, the presence of Fe and Mn species were apparent on the (001) and (110) surfaces of zirconia particles.

Products of reactions of Mo and Zr compounds in molten nitrate

Reactions of Mo and Zr compounds in molten nitrate media have been studied. It has been shown that an interaction between ZrIV and MoVI species occurs in nitrate melts, leading to the formation of high-surface-area mixed-oxide solids containing dispersed molybdate species grafted onto small zirconia particles. The physico-chemical properties of the products were studied vs. reaction temperature, nature of the alkali-metal cation and Mo reactant. The reaction temperature was of primary importance in determining the composition and structure of the product. At 30 °C, amorphous zirconia with randomly distributed monomolybdate species was formed, whereas at 500 °C crystalline ZrO2 superficially modified by polymolybdate was obtained. The alkali-metal cation influenced the amount of Mo in the solid and its dispersion.

A zirconia near-blackbody radiation source, 2500 K in air

A near-blackbody radiation source was designed for operating in an oxidizing atmosphere up to 2500 K. As a small zirconia furnace, the radiation source has a blackbody cavity, provided by a lateral hole formed on an yttria-stabilized zirconia tube. A good transfer radiation source should have an emissivity independent of the wavelength. This condition depends on the emissivity of the material and on the thermal characteristics of the source. The emissivity of the cavity has been calculated for different experimental conditions. The influence of temperature gradients in the cavity has been outlined. The aperture of the source, which is given by the sizes of the holes in the radiation shields, should not be too large. to avoid a large temperature gradient. even though some compensation occurs. For special applications. the aperture can be as large as 60°. The stability of the source has been studied.

Preparation of Na3Zr2Si2PO12–sodium aluminosilicate composite and its application as a solid-state electrochemical CO2gas sensor

A composite of sodium aluminosilicate and Na3Zr2Si2PO12 has been examined as a dense solid-state sodium-ion conductor. For the composite sintered at 1050 °C, the electrical properties are mainly based on those of Na3Zr2Si2PO12. When the composite was sintered at a higher temperature, the activation energy of the resistance increased toca. 55 kJ mol–1 and the densification was well progressed with the formation of small zirconia particles. A good junction between the composite and Y-stabilized zirconia was achieved when the sintering was carried out at1175 °C. The formation of zirconia particles in the composite layer resulted in an enhancement of the mechanical strength at the junction. By using the ceramics, a solid-state electrochemical CO2 sensor with an Na2CO3 layer was fabricated. Whilst the sensitivity of this sensor to CO2 was slightly influenced by the coexistence of the water vapour in the test gas, a good sensing characteristic based on two-electron electrochemical reaction was confirmed.

Effects of Ball Size and Particle Size on the Rate of Wet Ball-Milling

A lead-based relaxor material for multilayer ceramic capacitors was ground by ball-milling with partially stabilized zirconia balls of 1, 3 and 15mm in diameter and the effects of ball size and particle size of powder on milling rate were studied. Particle size distribution and specific surface area of powders were examined as a function of grinding time. Small zirconia balls in ball milling were effective to grind the powder to submicronmeter. In order to explain these experimental results on the milling rate, an equation for expressing the milling rate was proposed. The new equation is characterized by two parameters, ball size and particle size of the powder. The calculated and observed milling rates agreed well, and the effects of ball size and particle size on the milling rate were quantitatively confirmed It was found by estimating the proposed equation that milling rate of a powder had a maximum value at a ball size. Tha ball size which showed the maximum milling rate increased as particle size and mechanical strength of the powder increased.

Sintering of Submicron Pb(Zn, Nb, Fe, W)O<sub>3</sub> Powders and Dielectric Properties of the Ceramics Fabricated Therefrom

Lead-based relaxor material, Pb(Zn, Nb, Fe, W)O3 for multilayer ceramic capacitors was ground by ball-milling with partially stabilized zirconia balls of 1, 3 and 15mm in diameter in order to examine the effects of particle size on the sintering and dielectric properties. Particle size distribution, contamination from the balls and the residual strain of the powder were examined. The use of small zirconia balls in ball-milling was effective to grind the powder to submicronmeter. Average particle sizes of powders ball-milled for 128 hours using 1, 3, 15mmφ balls were 0.24, 0.35 and 0.68μm, respectively. The structural distortion of the ball-milled powders increased with decreasing particle size in contrast with little change in the lattice strain. The contamination from zirconia balls in ball-milling was examined with fluorescent X-ray analysis, and it was made clear that the ball-milled powders were free from the contamination. The average grain size of sintered ceramics from the fine powder with average particle size of 0.24μm was rather fine than that of sintered ceramics at the same temperature from 0.68μm powder. The densification of the fine powder was enhanced at lower firing temperature, and the dense dielectric ceramics were obtained at the firing temperature of 750°C which was lower by 100°C compared with the optimum firing temperature for the powders with average particle size of about 1μm. Relative permittivity of the fine grained sample fabricated from 0.24μm powder at 750°C was above 104 at room temperature and less DC bias field and less temperature dependences compared to that of coarse grained sample fabricated from the usual powder.

Sintering of Submicron Powders Prepared by Ball Milling

The calcined powders of PLZT 2/50/50 and low temperature sinterable dielectric ceramics were ball milled for a maximum of 240 h using small zirconia balls of 1 mm and 3 mm in diameter. The powders were ground down to 0.2 µm in average particle size. The sintering temperature could be lowered 100∼200°C by making the particle size of powders very fine. Furthermore, the combination of the fine powder and the liquidus sintering aid was able to make the sintering temperature low additionally. Dielectric and piezoelectric properties of the ceramics fabricated at relatively low temperature from the fine powders are compared with those of the ceramics fabricated from the usual powders.