Near-full Density Research Articles

La-doped SrTiO3 interconnect materials for anode-supported flat-tubular solid oxide fuel cells

An interconnect layer in an anode-supported flat-tubular solid oxide fuel cell connects electrically unit cells and separates fuel from oxidant in the adjoining cells. Nano-sized La-doped SrTiO3 for the interconnect is synthesized in this study by the Pechini method using citric acid. The materials with stoichiometric and Sr-deficient compositions are prepared and sintered in an oxidizing atmosphere. The synthesized fine powders exhibit high sinterability, leading to near-full densification. The Sr deficiency plays a crucial role in mechanical, electrical and thermal expansion properties. The interconnect is coated using the synthesized powder on a porous flat-tubular anode support by a screen printing process. The thin and dense layer is obtained after co-sintering in air, and the interconnect/anode interface remains intact upon reduction.

Interparticle Liquid Film Formation during Spark Plasma Sintering of Inconel 718 Superalloy

The use of powder metallurgy for near net shape sintering of superalloy could lead to major savings in machining time and material. The main challenge in sintering Inconel 718 is to avoid the formation of a prior particle boundary (PPB) network that is deleterious to the mechanical properties. Using the Spark Plasma Sintering (SPS) technique, it is believed that Inconel 718 powders could be sintered without forming a PPB network due to the fast heating rate achieved and the reported cleaning effect of particle surfaces by the interparticle arc discharges. In this study, Inconel 718 was consolidated to near-full density at 1200°C under 50 MPa of pressure with heating rates ranging from 20°C/min to 800°C/min. The densification behavior of the powder was studied through the analysis of the densification curves and observation of the microstructure evolution from interrupted tests. The fast densification of Inconel 718 in SPS was linked to the formation of a supersolidus liquid phase due to the nature of the heating in this technique.

Formation of Nanostructures in Ni-22Cr-11Fe-1<I>X</I> (<I>X</I> = Y<SUB>2</SUB>O<SUB>3</SUB>, TiO<SUB>2</SUB>) Alloys by High-Energy Ball-Milling

Powder mixtures of Ni, Cr, Fe and Y2O3 were high-energy ball-milled and subsequently sintered to fabricate Ni-based oxide-dispersion strengthened (ODS) alloys. Nano-sized Y2O3 and/or TiO2 seem to be dissolved in the Ni matrix forming a metastable solid solution during high-energy ball-milling or mechanical alloying (MA) process. The finely grained MA powders with high dislocation density facilitated the decomposition of oxides. The MA powders were consolidated to near-full density by spark plasma sintering at 1100 degrees C for 5 minutes in an Ar atmosphere. The Cr oxides as well as decomposed Y- and Ti-oxides thermally precipitated as oxide particles of several tens nanometers at this temperature, although sintering was carried out during a short time. The SPSed specimen showed a near full densification with almost pore-free microstructures. Examination of fractured surface showed a typical dimple rupture with fine and homogeneous distribution of dispersoids, indicating non-negligible room temperature ductility combined with high mechanical strength.

Processing and Characterization of Cu-Al-Ni Shape Memory Alloy Strips Prepared from Prealloyed Powder by Hot Densification Rolling of Powder Preforms

The present work deals with the preparation of near-full density Cu-Al-Ni shape memory alloy (SMA) strips from argon-atomized prealloyed powder via a powder metallurgy (PM) route comprising cold die compaction to prepare powder preforms, sintering, and hot densification rolling of unsheathed sintered powder preforms under protective atmosphere at 1273 K (1000 °C). It has been shown that argon-atomized spherical Cu-Al-Ni SMA powder consisted of very fine equiaxed grains and no appreciable grain growth occurred during sintering at 1273 K (1000 °C). It also has been shown that no appreciable densification occurred during sintering, and densification was primarily achieved by hot rolling. The densification behavior of the sintered powder preforms during hot rolling was discussed. The hot-rolled Cu-Al-Ni strips were heat-treated at 1223 K (950 °C) for 60 minutes and water quenched. The heat-treated strips consisted of equiaxed grains with average size approximately 90 μm. The heat-treated Cu-Al-Ni SMA strips consisted of self-accommodated \( \beta_{1}^{'} \) martensite primarily, and showed smooth \( \beta_{1} \Rightarrow \beta_{1}^{'} \) transformation behavior coupled with a very low hysteresis (≈25 K (25 °C)). The heat-treated strips exhibited an extremely good combination of mechanical properties with fracture strength of 530 MPa and 12.3 pct fracture strain. The mode of fracture in the finished strip was primarily void-coalescence-type ductile together with some brittle transgranular type. The shape memory tests showed almost 100 pct one-way shape recovery after 100 bending-unconstrained heating cycles at 4 pct applied prestrain, exhibiting good stability of Cu-Al-Ni strips under thermomechanical actuation cycling. The two-way shape memory strain was found approximately 0.45 pct after 15 training cycles at 4 pct training strain.

Spark Plasma Sintering of Mechanically Milled Pure Titanium with Stearic Acid Additive

Pure titanium powder was mechanically milled (MMed) using a vibrational ball mill with stearic acid. The MMed powders were consolidated by spark plasma sintering (SPS). Two sintering conditions were selected in order to understand the effect of the spark plasma sintering time on mechanical properties of bulk pure titanium. The mechanical properties and microstructures of SPS materials sintered at 1073 K for 180 s with the heating rate of 1.67 K/s and at 1073 K for 1.8 ks with the heating rate of 1 K/s were compared. The SPS materials produced by two different sintering conditions exhibited almost the same hardness and similar microstructure. Near-full density was achieved for the SPS materials for 180 s under an applied pressure of 49 MPa at a temperature of 1073 K. Formation of TiC by solid-state reaction was observed for the SPS materials consolidated from 4 and 8 h MMed powders. A maximum Vickers hardness of 933 HV obtained for the SPSed material fabricated from 4 h MMed powder with 0.50 g of PCA.

Sinter-ability of nanocrystalline tungsten powder

In this investigation, nanocrystalline tungsten powders prepared by either chemical method or high energy mechanical milling were sintered to study the size-dependence of the densification of tungsten powders. Results show that nanocrystalline tungsten powders produced via an ultra-high energy milling process have significantly enhanced sinterability compared to conventional submicron- or micron-sized powders. This study for the first time shows nanocrystalline tungsten powder can be sintered via a pressureless process to near-full densification at a temperature as low as 1100 °C.

Fabrication of high strength pure titanium by mechanical milling and spark plasma sintering and its properties

Pure titanium powder was mechanically milled (MMed) using a vibrational ball mill in the presence of stearic acid, which was added as a process control agent (PCA) . The MMed powders were subsequently consolidated into bulk materials by spark plasma sintering (SPS) to enhance the mechanical properties of the titanium. Changes in hardness, constituent phases and powder particle size resulting from the milling process were investigated by microhardness measurements, X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The hardness and microstructure of the SPSed materials was also studied by hardness measurements and optical microscopy, respectively. The Vickers microhardness of the titanium powders with 0.25 g of PCA was found to increase sharply from 189 to 513 HV after 8 h of mechanical milling (MM), and SEM observations revealed that the mean particle size changed from 24 to 10 μm. Following SPS for 0.5 h under an applied pressure of 49 MPa at a temperature of 1073 K, near-full density was achieved for the sintered material. Formation of TiC by solid-state reaction was observed for the SPS materials consolidated from 4 h and 8 h of MMed powders. In addition, formation of TiN was observed for the SPS materials consolidated from 8 h of MM with 0.50 g of PCA. A maximum Vickers hardness of 1253 HV exhibited for the SPSed material fabricated from 8 h MMed powder with 0.50 g of PCA.

Preparation of nanocrystalline Ni–Fe strip via mechanical alloying–compaction–sintering–hot rolling route

Nanocrystalline structures offer opportunity for the development of soft magnetic materials, such as 80 wt% Ni–20 wt% Fe, with superior properties. In recent years, nanocrystalline 80Ni–20Fe (wt%) alloy has been prepared by mechanical alloying of elemental powders. However, retention of nanocrystallinity during consolidation of powder is the key issue to take advantage of improved magnetic properties. In the present work, it has been shown that near-full density bulk nanocrystalline 80Ni–20Fe strip can be prepared via a route consisting of mechanical alloying, cold compaction, sintering, and multi-step unsheathed hot rolling. A crack-free strip of nanocrystalline 80Ni–20Fe, having 99% theoretical density and a grain size of approximately 55 nm, was successfully prepared by sintering and hot rolling of mechanically alloyed powder preforms at 1140 °C. The bulk nanocrystalline 80Ni–20Fe material resulted in a very narrow hysteresis loop indicating a very small hysteresis loss. The present study shows that mechanical alloying–sintering–hot rolling route can be a promising method for producing bulk nanocrystalline materials.

Fabrication and characterization of graded impedance impactors for gas gun experiments from tape cast metal powders

Fabrication of compositionally graded structures for use as light-gas gun impactors has been demonstrated using a tape casting technique. Mixtures of metal powders in the Mg–Cu system were cast into a series of tapes with uniform compositions ranging from 100% Mg to 100% Cu. The individual compositions were fabricated into monolithic pellets for characterization by laminating multiple layers together, thermally removing the organics, and hot-pressing to near-full density. The pellets were characterized by optical and scanning electron microscopy, X-ray diffraction, and measurement of density and sound wave velocity. The density and acoustic impedance were observed to vary monotonically (and nearly linearly) with composition. Graded structures were fabricated by stacking layers of different compositions in a sequence calculated to yield a desired acoustic impedance profile. The measured physical properties of the graded structures compare favorably with those predicted from the monolithic pellet characteristics. Fabrication of graded impactors by this technique is of significant interest for providing improved control of the pressure profile and impactor planarity in gas gun experiments.

Severe plastic deformation of nickel-coated aluminum precursor powders at elevated temperatures

Two nickel (Ni)-coated aluminum (AI) powders were consolidated to full or near-full density by using the equal-channel angular extrusion (ECAE) technique. Mixtures (in at. pct) of 78Al-22Ni (63Al-37Ni in wt pct), hereafter termed Al-22Ni, and of 39Al-61Ni (23Al-77Ni in wt pct), hereafter termed Al-61Ni, were placed in square-shaped copper (Cu) or Ni casings, sealed, and heated to a uniform temperature. Subsequently, the billets were dropped into the ECAE die and were single-pass extruded. Preheating temperatures ranged from ambient temperature to 700 °C. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and microhardness measurements were used to examine the resultant intermetallics. The onset and nature of the transformation from the precursors into the products were further studied by differential thermal analysis. It was found that the preheating temperature affected the transformation of the Ni-coated Al powder into a nickel-aluminide intermetallic. Specifically, samples of Al-22Ni and Al-61Ni, preheated and consolidated below the nominal reaction temperature of Ni and Al, consisted of a cellular Al structure interposed with a Ni boundary layer. The samples preheated and consolidated above this temperature consisted of Al3Ni and Al3Ni2, AlNi, and AlNi3, respectively. In this case, ECAE caused the multiphase structure of the intermetallics to homogenize and disperse; this was more so for 78Al-22Ni than for 39Al-61Ni. It was also found that the use of a Ni vs Cu casing material greatly improved the densification of the powders. Unlike that found with the Cu casing, full densification using a Ni casing was achieved in a single pass.

Hot explosive compaction of aluminum-nickelide composites

Two series of nickel-coated aluminum (Al-Ni) powder compositions were consolidated to full or near-full density by a hot-explosive-compaction (HEC) technique. Mixtures of 78Al-22Ni at. pct (63Al-37Ni wt pct) or 39Al-61Ni at. pct (23Al-77Ni wt pct) were placed in cylindrical containers, preheated to a range of temperatures from ambient to 1000 °C, and once at a uniform temperature, explosively compacted into a 150-mm-long and 15-mm-diameter rod-shaped billet using a cylindrical detonation arrangement. The resultant billets were sectioned and prepared for examination by optical microscopy and scanning electron microscopy (SEM). Energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and microhardness measurements were used to characterize the billet morphology, structure, and chemical composition. Analysis revealed that depending on the preheating temperature, the initial Al-Ni composition transformed into several aluminum nickelide (Al-Ni) intermetallics. The onset and nature of the shock-induced transformation from the precursors into the products are discussed.

Dehydrogenation of nanocrystalline TiH2 and consequent consolidation to form dense Ti

Dehydrogenation of nanocrystalline TiH2, produced by pulverization of commercially available powder, has been examined in detail by a combination of thermal analysis, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The dehydrogenation to form Ti occurs as a two-step process involving the formation of an intermediate phase, TiH. In-situ experiments on dehydrogenation inside a transmission electron microscope reveal the possibility of a powder-metallurgy process for consolidation of Ti components by vacuum annealing of nanocrystalline TiH2 at ∼0.5Tm, where Tm is the melting point of Ti. Near-full densification of Ti has been achieved by sintering nanocrystalline TiH2 under vacuum at ∼0.5Tm.

Friction behaviour of solid oxide lubricants as second phase in α-Al 2O 3 and stabilised ZrO 2 composites

The influence of metal oxide additives within alumina (α-Al 2O 3) and yttria-stabilised tetragonal zirconia (Y-TZP) matrices was studied with respect to the tribological behaviour of the composites. The solid lubricants CuO, ZnO, MgO, MnO 2 and B 2O 3 were added in sufficiently small quantities (1 or 5 wt.%) to preserve the mechanical properties of the matrix phase without increasing the specific wear rate. The additives selected were an attempt to provide the materials with a self-lubricating mechanism and therefore decrease the coefficient of dry-friction. The ceramics were prepared by means of wet-ball milling powders, compaction by isostatic pressing and densification by sintering. Near-full densities (98–99%) were obtained for all ceramics except that containing CuO (92%). Tribological behaviour of the various composite compositions was tested on a pin-on-disk assembly using both α-Al 2O 3 and ZrO 2 as counter surface. Friction reductions of almost 40% for the CuO composite was obtained while the influence of the other oxide additives were less pronounced.

Novel synthesis and characterization of an AB-type carbonate-substituted hydroxyapatite.

A novel synthesis route has been developed to produce a high-purity mixed AB-type carbonate-substituted hydroxyapatite (CHA) with a carbonate content that is comparable to the type and level observed in bone mineral. This method involves the aqueous precipitation in the presence of carbonate ions in solution of a calcium phosphate apatite with a Ca/P molar ratio greater than the stoichiometric value of 1.67 for hydroxyapatite (HA). The resulting calcium-rich carbonate-apatite is sintered/heat-treated in a carbon dioxide atmosphere to produce a single-phase, crystalline carbonate-substituted hydroxyapatite. In contrast to previous methods for producing B- or AB-type carbonate-substituted hydroxyapatites, no sodium or ammonium ions, which would be present in the reaction mixture from the sodium or ammonium carbonates commonly used as a source of carbonate ions, were present in the final product. The chemical and phase compositions of the carbonate-substituted hydroxyapatite was characterized by X-ray fluorescence and X-ray diffraction, respectively, and the level and nature of the carbonate substitution were studied using C-H-N analysis and Fourier transform infrared spectroscopy, respectively. The carbonate substitution improves the densification of hydroxyapatite and reduces the sintering temperature required to achieve near-full density by approximately 200 degrees C compared to stoichiometric HA. Initial studies have shown that these carbonate-substituted hydroxyapatites have improved mechanical and biologic properties compared to stoichiometric hydroxyapatite.

Densification of liquid-phase-sintered silicon carbide

There has been growing interest in pressureless liquidphase sintering (LPS) of SiC, with Al2O3 additions [1] or simultaneous Al2O3 and Y2O3 (or rare earth oxides) additions [2–6], as it allows sintering at lower temperatures (1850 to 1950 ◦C) relative to solid-state sintering. A particularly important aspect of this class of materials is that they can be significantly toughened through careful microstructure design, wherein elongated SiC grain-reinforcements are grown in situ during sintering [7–10]. The elongate nature of the SiC grains and the weakness at the grain boundaries promote crack-wake bridging in these ceramics, resulting in a significant increase in the toughness [8, 10, 11]. Although in situ reinforced LPS SiC ceramics represent a new class of potentially important structural materials, the mechanisms of densification and subsequent microstructural evolution in these ceramics are not well understood. Some advances have been made in the understanding of the microstructural evolution in LPS SiC [7, 12–16], however, reliable processing of these ceramics is still plagued with two important shortcomings, namely: (i) inadequate and irreproducible densification; and (ii) significant weight loss during sintering. In the context of pressureless sintering of LPS SiC with simultaneous additions of Al2O3 and Y2O3, prior studies have demonstrated that sintering in a contained crucible and embedding the specimens in a packing bed consisting of coarse Al2O3 and SiC powders dramatically improves the densification [4–7, 17]. However, these results could not be reproduced by other workers (see, e.g., Refs. [7, 9, 18, 19]). Thus, the broad objective of this study was to understand the role of the packing bed on the densification of LPS SiC, with the specific goal of being able to reproducibly sinter these materials to near-full densities and with minimum weight loss. A long elusive parameter that significantly influences the densification in this system, namely the composition of the packing-bed powder, was identified. In particular, it was found that an optimum concentration of Al2O3 in the packing-bed powder is essential for consistent, near-complete densification (1% porosity), accompanied with negligible weight loss. While the absence of Al2O3 in the packing bed has been known to result in incomplete densification, here it is shown that excess Al2O3 in the packingbed powder also adversely affects the densification.

Synthesis of Ti3SiC2 polycrystals by hot-isostatic pressing of the elemental powders

Synthesis of Ti3SiC2 polycrystals by hot-isostatic pressing of the elemental powders

Reaction phase-forming and mechanical properties of Fe3Al produced from elemental powders

The Fe[sub 3]Al-based intermetallics can be produced by several conventional processing routes. However, there are applications where the powder metallurgy (P/M) process offers a better or the only route for producing these materials. A refined microstructure of the P/M products is beneficial for improving mechanical properties of the Fe[sub 3]Al-based intermetallics. Conventional P/M processing routes utilize mostly the prealloyed powders and consolidation is conducted by sintering, hot isostatic pressing (HIP) or hot extrusion. These methods involve generally processing steps and are, therefore, rather expensive. Reactive sintering, as an alternative fabrication method, is one of the novel and attractive processes. It is a method to obtain dense intermetallic compounds and intermetallic matrix composites from elemental powders using a self-sustaining reaction. This process, also known as combustion process, offers advantages over conventional processing methods including the use of less expensive, readily available, and easily compacted elemental powder, lower processing temperatures and shorter processing times, in short, low cost and energy savings. On the other hand, the reaction process of elemental iron-aluminium mixtures has a particular problem, i.e., a high porosity of the products due to extensive swelling. In order to achieve near-full density, the reactive sintering process should be assisted by anmore » external pressure. In this case, reactive sintering is conducted in a HIP unit or a hot press. One should also appreciate that reactive hipping may provide near-net shape components which is important for reducing the cost because most of the intermetallics are hard-to-fabricate materials. This study describes the preparation of a binary Fe[sub 3]Al intermetallic compound by in-situ reaction phase-forming/consolidation from elemental powders, its mechanical properties, and a comparison of these properties with those of conventionally processed materials.« less

Synthesis of Air-Sinterable Lanthanum Chromite Powders

Lanthanum chromites, used as current interconnections in solid oxide fuel cells (SOFCs), have high electrical conductivity and are chemically stable. However, they are difficult to fabricate under conditions compatible with other SOFC components. A novel powder synthesis technique, the glycine/nitrate process, has been developed that produces chromite powders that can be sintered to near-full density in air at 1823 K. This paper describes the glycine/nitrate process for the synthesis and fabrication of lanthanum chromites. The sintering behavior and properties of a series of glycine/nitrate-produced lanthanum chromites are discussed. Some of the materials produced by this method match the thermal expansion of the electrolyte and maintain high electrical conductivity. 16 refs., 10 figs.