Sintering Cycle Research Articles

Microstructure-property relations in composite yttria-substituted zirconia solid electrolytes

A study of composite 8 mol% yttria stabilized zirconia (8YSZ) and 3 mol% yttria tetragonal zirconia polycrystal (3YTZP) solid electrolytes sintered under isothermal and two-step sintering cycles is reported. The nominal phase composition is retained for composites with up to 25 wt.% 3YTZP. These composites show a combination of beneficial effects with respect to pure 8YSZ, including slight improvement in sinterability, gains in bulk and grain boundary conductivity and also enhanced fracture toughness. Impedance spectroscopy revealed an enhancement of the specific grain boundary conductivity for samples with finer grain sizes, attained by increasing the fraction of 3YTZP or by hindering grain growth under two-step sintering cycles. This effect is rationalized in terms of a decrease of the grain boundary space-charge potential. The conductivity gains decrease with increasing temperature, but even at 700 °C the total ionic conductivity of ceramics with 25 wt.% 3YTZP is still higher than that of pure 8YSZ, whereas at 900 °C there is a performance loss of less than 10%. The improved mechanical and electrical performance in the intermediate temperature range represents an important advantage of the heterostructured electrolytes for low/intermediate temperature SOFC operation.

In-Situ X-Ray Diffraction Observations of Low-Temperature Ag-Nanoink Sintering and High-Temperature Eutectic Reaction with Copper

Nanoinks, which contain nanometer-sized metallic particles suspended in an organic dispersant fluid, are finding numerous microelectronic applications. One characteristic of nanoinks is that they sinter at much lower temperatures than bulk metals due to their high surface area to volume ratio and small radius of curvature, which reduces their melting points significantly below their bulk values. The unusually low sintering temperatures have unique potential for materials joining, since their melting points increase dramatically afterward. In this article, the sintering kinetics of Ag nanoink is studied using in-situ synchrotron methods to determine diffraction peak characteristics during the sintering cycle, and to subsequently calculate particle size and growth during sintering. Ag nanoink is further explored as a eutectic bonding medium by tracking phase transformations between sintered Ag nanoink and a Cu substrate to high temperatures, where melting occurs at the Ag-Cu eutectic, demonstrating nanoinks as a viable eutectic bonding medium.

Dielectric and viscoelastic properties of KNbO3 doped BaTiO3

Fine grain (1–2 μm) 2%, 3% KNbO3-BaTiO3 ceramics have been synthesized via the solid state reaction method. Fine and coarse grain ceramics sintered at different temperatures exhibit peaks in dielectric constant and internal friction in the vicinity of transition temperatures. Doping lowered the Curie point and raised the temperatures for the structural transformations between orthorhombic and rhombohedral symmetry and broadened the response near the transformations. Dielectric and viscoelastic responses sharpened with increasing sintering cycles. This effect is attributed to a reduced core-shell effect. Doped ceramic exhibited a relaxation peak due to oxygen vacancy with a similar activation energy and relaxation time as pure material. The steep increase in dielectric constant and internal friction in the ferroelectric phase and the reduced dielectric and mechanical anomaly in the vicinity of the transformations in the fine grain ceramics are attributed to the internal stress built up at the grain boundaries. Other possible mechanisms involved such as oxygen vacancy pinning effect, constrained negative stiffness 90° domain bands and the liquid phase effect have also been discussed.

Consolidation of (Ti,Mo)(C,N)–Ni cermets by glass encapsulated hot isostatic pressing

Microstructural evolution of (Ti,Mo)(C,N)–Ni cermets consolidated by hot isostatic pressing (HIP) has been analysed. HIP processing allows full densification of these materials at lower temperatures than those normally employed in vacuum sintering cycles (VS). Solution and reprecipitation phenomena are limited and a nanometric fraction of (Ti,Mo)(C,N) grains is retained in the microstructure leading to a significant increase in hardness with respect to vacuum sintered materials (from 11 to 14 GPa). HIP-ed cermets show more tendency to uncombined carbon precipitation than those obtained by VS. In studied systems, carbon precipitation can be related either to an excess of carbon in the initial mixture of powders or to destabilization of carbonitride phase during HIP. Control of the C/N ratio has been carried out by the adequate selection of powder mixtures and the design of the thermal treatments. It has been proved that free carbon in these cermets can be avoided including presintering cycles under hydrogen before encapsulation.

Micro‐Raman spectroscopy of indentation induced phase transformation in nanozirconia ceramics

Micro‐Raman spectroscopy has been employed as an effective technique to determine the phase transformations in nanostructured yttria stabilised zirconia (YSZ) ceramics with different yttria contents. Samples have been prepared with varying mean grain sizes by a slip casting route followed by a microwave assisted two‐step sintering cycle starting with aqueous nanozirconia suspensions. Indents were generated using a Vickers pyramidal indenter at different loads, and the resulting phase transformations were mapped using micro‐Raman spectroscopy. The results were compared to those of a commercial submicrometre 3 mol.% YSZ. The amount of transformation was found to be much lower for nanozirconia compared to the submicrometre zirconia with similar yttria content.

Stainless Steels Sintered Form the Mixture of Prealloyed Stainless Steel and Alloying Element Powders

The aim of the presented paper is to describe the sintered duplex stainless steels manufactured in sinter-hardening process and their structural and mechanical properties. Duplex stainless steels were obtained through powder metallurgy starting from austenitic 316L or ferritic 410L prealloyed base powders by controlled addition of alloying elements powder. Prepared mixes were compacted at 700MPa and sintered in a vacuum furnace with argon backfilling at temperature of 1240°C for 1h. After sintering different cooling cycles were applied: rapid cooling (6°C/s) using nitrogen under pressure and slow cooling (0.1°C/s) with furnace in argon atmosphere. Produced sintered duplex stainless steels were studied by scanning and optical microscopy and EDS chemical analysis of microstructure components as well as X-ray analysis. Mechanical properties were studied through tensile and three-point bending tests and Charpy impact test. It was demonstrated that austenitic-ferritic microstructures with regular arrangement of both phases and absence of precipitates can be obtained with properly designed powder mix composition as well as sintering cycle with rapid cooling rate. Produced sintered duplex steels show good mechanical properties which depend on austenite/ferrite ratio in the microstructure and elements partitioning (Cr/Ni) between phases. The optimal mechanical properties were obtained for compositions based on ferritic 410L powder where the balanced distribution of α and γ is present and the tensile strength can reach value about 500MPa with 16% of elongation and impact energy about 120J. The precipitations of hard intermetallic σ-FeCr phase take place when sintering with slow cooling cycle what cause substantial decrease of plastic properties, including reduce of elongation to 7% and in particular decrease of impact energy to 68 J.

Powder Metallurgy Processing of Al/TiAl<sub>3</sub>+Al<sub>2</sub>O<sub>3</sub>

This paper aims to present a way of obtaining aluminum based composite material obtained by powder metallurgy technique. We start mixing and milling the powders, compressing and sintering the samples. We established a sintering cycle for this type of composite material and studied obtained samples by SEM microscopy. The reaction between the matrix and the precursor of the reinforcing materials was studied with the aid of HSC 4.0 program, available at Engineering and Management of Metallic Materials department.

In situ synthesis and densification of submicrometer-grained B 4C–TiB 2 composites by pulsed electric current sintering

B 4C composites with 15 and 30 vol% TiB 2 were pulsed electric current sintered from B 4C–TiO 2–carbon black mixtures in vacuum at 2000 °C. Full densification could be realised when applying an optimized loading cycle in which the maximum load is applied after completion of the B 4C–TiB 2 powder synthesis, allowing degassing of volatile species. The influence of the sintering temperature on the phase constitution and microstructure during synthesis and densification was assessed from interrupted sintering cycles. The in situ conversion of TiO 2 to TiB 2 was a complex process in which TiO 2 is initially converted to TiB 2 with B 2O 3 as intermediate product at 1400–1700 °C. At 1900–2000 °C, B 2O 3 reacted with C forming B 4C and CO. The B 4C and TiB 2 grain size in the fully densified 30 vol% TiB 2 composite was 0.97 and 0.63 μm, combining a Vickers hardness of 39.3 GPa, an excellent flexural strength of 865 MPa, and modest fracture toughness of 3.0 MPa m 1/2.

Colloidal Processing and Characterization of Aluminum-Doped Lanthanum Oxyapatite, La10AlSi5O26.5

Lanthanum apatite is one of the most promising materials as electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFC), those operating close to 700°C. The complexity of microstructures, composition, and geometries of SOFC devices, make it necessary to have a precise control of processing parameters to obtain the desired combination of properties. This work involves the preparation and optimization of La10AlSi5O26.5 materials by reaction sintering of the raw materials (La2O3, Al2O3, and SiO2) to be used as IT-SOFCs electrolytes. Homogeneous mixed suspensions of those ceramic oxides were prepared. A heterocoagulation process was followed for ensuring a better reactivity during sintering. All the parameters involved in the process, such as deflocculant content, particle size of the initial powders, sonication time of the mixed suspensions, compaction, and sintering cycle were optimized. The effect of all these parameters were studied and discussed. Finally, La10AlSi5O26.5 electrolytes with density values of ∼88% of the theoretical density and total conductivities of 1.7 × 10−2 S/cm at 700°C were obtained. This study opens a path for the preparation of concentrated suspensions for obtaining dense thin films.

Experimental features affecting the transparency of YAG ceramics

This article focuses on the importance of the ceramic production process for the final transparency and overall optical quality of materials to be used as laser hosts. YAG-based ceramics are prepared starting from commercial powders. The materials are prepared by reactive sintering in a clean atmosphere and under high vacuum. Nd or Yb are selected as active elements as the more appropriate for high energy and high peak power lasers. The powder type and treatment and the solvent removal technique are described in detail as well as the experimental conditions adopted during shaping. The influence of the pre-sintering and sintering cycles on the reaction among the involved oxides and on the microstructure after sintering is shown. The optical characterization is also reported.

Microstructure and mechanical properties of pulsed electric current sintered B4C–TiB2 composites

Monolithic B4C, TiB2 and B4C–TiB2 particulate composites were consolidated without sintering additives by means of pulsed electric current sintering in vacuum. Sintering studies on B4C–TiB2 composites were carried out to reveal the influence of the pressure loading cycle during pulsed electrical current sintering (PECS) on the removal of oxide impurities, i.e. boron oxide and titanium oxide, hereby influencing the densification behavior as well as microstructure evolvement. The critical temperature to evaporate the boron oxide impurities was determined to be 2000°C. Fully dense B4C–TiB2 composites were achieved by PECS for 4min at 2000°C when applying the maximum external pressure of 60MPa after volatilization of the oxide impurities, whereas a relative density of 95–97% was obtained when applying the external pressure below 2000°C. Microstructural analysis showed that B4C and TiB2 grain growth was substantially suppressed due to the pinning effect of the secondary phase and the rapid sintering cycle, resulting in micrometer sized and homogeneous microstructures. Excellent properties were obtained for the 60vol% TiB2 composite, combining a Vickers hardness of 29GPa, a fracture toughness of 4.5MPam1/2 and a flexural strength of 867MPa, as well as electrical conductivity of 3.39E+6S/m.

Excess Conductivity Studies in Zn0.95Mn0.05O and ZnO Added YBa2Cu3O y Superconductors

We report electrical conductivity fluctuation analyses on YBa2Cu3Oy (denoted as YBCO) granular samples added with nanosize ZnMnO (ZnMnO for brevity) and ZnO (30 nm) particles. Nanoparticles are added to the precursor powders during the final sintering cycle of a two-step preparation process. Phase analysis by X-ray diffraction and granular structure examination by transmission electron microscopy (TEM) were carried out. When ZnMnO and ZnO are added to the YBCO, the orthorhombic structure is maintained. TEM and energy dispersive X-ray spectroscopy analysis show the presence of inhomogeneities embedded in the superconducting matrix. The temperature dependence of electrical resistivity in zero magnetic field has been measured on free, 1 wt.% ZnMnO and 1 wt.% ZnO added samples and the effect of microscopic inhomogeneities in the paraconductivity region has been reported. Data about the dimensionality of the thermodynamic fluctuation are obtained by analyzing the excess of conductivity Δσ as a function of the reduced temperature \(\varepsilon =\ln(\frac{T}{T_{c}^{mf}}-1)\) on the basis of the Aslamazov–Larkin theory. In the mean-field region a crossover from 3D to 2D was observed for each sample. 1D behavior of fluctuation conductivity was found at high temperatures (above the 2D regime) for nanoparticle added samples. The zero-temperature coherence length, the effective layer thickness of the two-dimensional system, the wire cross-sectional area for one-dimensional system, critical magnetic fields and critical current density are estimated. Superconducting parameters are affected by the nanoparticle additions.

Preparation and thermomechanical characterisation of aluminum titanate flexible ceramics

Flexible ceramics may be useful, for example to process refractory materials with an improved resistance to thermal shocks. A natural flexible sandstone, itacolumite, is mainly constituted of interlocked quartz grains and contains microcracks. Its microstructure allows some free motion between grains that induces its flexibility. The aim of this study is to prepare and characterize flexible aluminum titanate ceramics by mimicking the microstructure of itacolumite. Aluminum titanate (AT) has a high thermal expansion anisotropy that induces grain boundary microcracking leading to flexibility. Here, the flexibility is the capacity of the material to endorse large strain-to-rupture level. This concept is also closely related to a low value of the stiffness induced by damage mechanisms. In this study, the flexibility has been estimated by the measurement of the deflection at fracture on three-point bending test. By preparing AT samples sintered according to different heating cycles, the correlations between the sintering cycle, the microstructure and the flexibility have been studied. Grain size and microcrack width have been observed by scanning electron microscopy. A major parameter for flexibility is the microcrack volume fraction within the sample. Three types of AT materials have been processed: non flexible (NF), flexible (F), and very flexible (VF). Their thermal and mechanical behaviors have been investigated and showed that NF has a brittle behavior while F and VF have a nonlinear ductile one. This was found to be due to grain boundary microcracks network and to the interlocking of grains. VF is more flexible than F because its microcracks are wider. Flexibility improves the thermal shock resistance: F and VF have a higher thermal shock resistance than NF. Moreover, thermal expansion measurements during thermal cycles showed anomalous effects induced by crack closure when heating and crack opening when cooling.

Characterization of Yb:YAG ceramics as laser media

We report the preparation and characterization of 9.8 at.% Yb 3+ doped YAG polycrystalline ceramics for laser applications. Reactive sintering of commercial powders in a clean atmosphere and under high vacuum has been used to achieve the YAG phase. The selected experimental conditions for the powder treatment, shaping and sintering are described and their influence on the optical quality of the obtained samples has been discussed. Microstructural, optical and laser characterization of the ceramics have been performed. In particular the influence of the pre-sintering (calcination) and sintering cycles has been investigated by laser characterization allowing to find unexpected loss mechanisms that cannot be revealed by standard optical characterization.

Proliferation and mineralization of bone marrow cells cultured on macroporous hydroxyapatite scaffolds functionalized with collagen type I for bone tissue regeneration

This study concerns the preparation and in vitro characterization of functionalized hydroxyapatite (HA) porous scaffolds, which are intended to be used as drug-delivery systems and bone-regeneration matrices. Hydroxyapatite scaffolds were prepared using the polymer replication method, and, after being submitted to a specific sintering cycle, collagen Type I was incorporated on the surface. After the coating procedure, collagen was crosslinked using the N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) conjugation method. In this study, hydroxyapatite scaffolds with uncrosslinked and crosslinked Type I collagen were evaluated. Cell morphology and deposition of extracellular matrix were assessed by scanning electron microscopy, whereas cell distribution was visualized by means of methylene blue staining. MTS and total DNA quantification assays were used to evaluate the viability and proliferation of human bone marrow cells cultured on all the materials for 28 days. Results showed that the cells were able to adhere, proliferate, and form a mineralized matrix on the surface of all the materials. Furthermore, the cells were able to spread from one pore to another and form cell clusters. The results show that these scaffolds are good candidates to serve as drug delivery vehicles and for tissue engineering purposes.

Two-step sintering of ZnO varistors

In this research, effects of two-step sintering parameters, such as first and second step sintering temperatures (T 1 and T 2, respectively) and soaking time in the second step, on microstructure and electrical properties of ZnO varistors have been studied. In addition, slow rate of grain growth in the second step of sintering has also been investigated. Near fully dense (higher than 99%) and sub-micron grain size (1–0.75 μm) ZnO varistors have been fabricated by different two-step sintering cycles. Decreasing T 1 and T 2 result in longer necessary time in the second step to reach near full density with smaller final grain size. The superior electrical properties of varistors (V B: 2050–1022 V/mm and α: 96–60) are attributed to very fine microstructure and high concentration of ZnO–ZnO grains direct contacts. Furthermore, lower T 1 and T 2 with shorter soaking time improve electrical properties of the ZnO varistors.

Microwave Sintering of Titanium

Effective sintering of titanium requires the use of a high sintering temperature (≥1200 °C), preferably in high vacuum (< 10-2 Pa). This confines the heating and cooling rates to ~ 4 °C/min because of the limited thermal shock resistance of ceramic tube furnaces. Consequently, it leads to lengthy sintering cycles (10-12 hr). This work presents an assessment of microwave (MW) sintering of titanium. Titanium powders in the size ranges of <20 µm, 45-63 µm, and 100-150 µm were used to make green samples with compaction pressures ranging from 200-800 MPa. Sintering was carried out at 1200 °C for 2 hr in a 3 kW MW furnace with a 2.45 GHz multimode cavity under a vacuum of 2-6×10-3 Pa. The characteristics of MW heating of green titanium samples in vacuum are described in terms of the heating rate, vacuum fluctuations, and sparking discharge. The actual MW heating rate achieved from 350 °C to 1200 °C was 34 °C/min. The attendant densities are comparable to those attained by conventional vacuum sintering. Cross-sectional examinations revealed a fairly uniform pore distribution in MW-sintered samples made from either the coarse or fine titanium powder.

Zirconia-MWCNT nanocomposites for biomedical applications obtained by colloidal processing

Zirconia ceramics are widely used as femoral heads, but case studies show that delayed failure can occur in vivo due to crack propagation. The addition of carbon nanotubes (CNT) is aimed to avoid the slow crack propagation and to enhance the toughness of the ceramic material used for prostheses. However, to really enhance the mechanical properties of the material it is necessary to achieve a uniform distribution of the CNT in the zirconia matrix. Colloidal processing has demonstrated to be suitable for obtaining ceramic-based composites with homogeneous distribution of the phases and high green density. This work compares the colloidal behavior of the as-received multi wall carbon nanotubes (ar-MWCNT) and the partially coated MWCNT (pc-MWCNT) when immersed in a nanozirconia matrix. With pc-MWCNT an improvement in the dispersion is proved. Moreover, the sintered samples that contain pc-MWCNT show higher density, lower grain size, improved toughness and enhanced hardness under the same sintering cycle when compared to the samples with ar-MWCNT.

Microwave Sintering – A Novel Approach to Powder Technology

The present paper focus on preliminary work carried out at INETI concerning the use of microwave radiation applied to sintering of both ceramic and metal powders. Due to the characteristics of materials-radiation interaction, microwaves can become an interesting power source in powder technology and other processing routes, since it is possible to lower the sintering temperature and shorten the sintering cycles, leading to time and energy savings. Alumina, hydroxyapatite, titanium and stainless steel powder compacts were sintered in a modified commercial oven of 2.45GHz and 1000W nominal power. Microwave susceptors were used to enable temperature rise during the initial stage of the sintering cycles. Results on densification and microstructural evaluation of microwave sintered samples are reported and compared to conventionally sintered ones, when available. For similar porosity levels upon sintering, microwave radiation generally reduces sintering times from several hours to minutes. The results obtained so far are quite encouraging since in the case of alumina and stainless steel compacts, a decrease of about 200°C in the sintering temperature was achieved. It was also found that the green density plays a key role in the densification of both metallic and ceramic powders.

Optimising grey iron powder compacts

The grey iron microstructure Fe–2C–2Si powder based compact is tailored by different kinds of in situ and post sintering processing. This has been achieved by combining thermodynamic and kinetics modelling of microstructure development with sintering and controlled heat treatment experiments of tensile test specimens die compacted at 600 MPa. Applying optimised sintering conditions led to a grey iron like microstructure with 95% relative sintered density. Sinter hardening the compacts led to 500 MPa in yield strength and 600 MPa in ultimate tensile strength in combination with ductile fracture. Quenched and tempered condition showed the same strength values, but combined with brittle fracture due to martensitic structure. Pore rounding and partial pore filling by graphite were obtained by austenising isothermal hold during the cooling of the sintering cycle.