Initial Sintering Research Articles

Manufacture and performance of lightweight aggregate from waste drill cuttings

This research investigated the technical feasibility of transforming waste drill cuttings into lightweight aggregate. Drill cuttings produced from the North Sea oil field were dried, ball milled, formed into pellets and fired at temperatures between 1160 and 1190 °C. Physical properties of the manufactured lightweight aggregate, including particle density, water absorption and compressive strength, were determined. The drill cuttings had a typical evaporite composition containing high concentrations of chloride salts. This limits the potential for using the as-received drill cutting samples in lightweight aggregate production as the products formed show high levels of leaching. The addition of a washing pre-treatment to reduce the leaching of chloride ions was necessary. Washing also reduced the initial sintering temperature and improved lightweight aggregate properties. Sintering at 1180 °C produced lightweight aggregate with particle density of 1.29 g/cm³, water absorption of 3.6% and compressive strengths of 4.4 MPa. The research showed that lightweight aggregate manufacturing represents a resource efficient option for the reuse of waste drill cuttings and provides significant material saving and landfill diversion.

Spark Plasma Sintering of fine-grained SrWO4 and NaNd(WO4)2 tungstates ceramics with the scheelite structure for nuclear waste immobilization

Spark Plasma Sintering has been used to obtain first ever fine-grained SrWO4 and NaNd(WO4)2 ceramics with the scheelite structure and high density (95.8–99%) that are promising as materials for nuclear waste immobilization. It has been demonstrated that the densification intensity in tungstates at the initial sintering stage is determined by the agglomeration degree of input powders. The activation energy of fine-grained ceramics with the scheelite structure has been established to correlate at high temperatures with the activation energy of grain-boundary diffusion. Rapid sintering of fine-grained NaNd(WO4)2 ceramic results in internal tensile stress fields that lead to lowered hardness and fracture toughness of the ceramic.

Nonlinear absorption and refraction studies of truncated CuNb3O8 with high-repetition rate femtosecond pulses

Copper niobate with CuNb3O8 composition was prepared via one-step solid state reaction (750 °C, 12 h). Powder X-ray diffraction (XRD) confirmed the formation of parental CuNb2O6 phase with traces of CuNb3O8 on initial sintering (till 9 h), which was then transformed into pure CuNb3O8 on higher sintering (12 h). Field emission scanning electron microscope studies (FESEM) illustrates the formation of truncated cubes due to unification of layered structure upon prolonged heating. Third-order nonlinear optical properties of truncated CuNb3O8 was studied by Z-scan technique using Ti: Sapphire laser (800 nm, 150 fs, 80 MHz). Pure CuNb3O8 possessed strong nonlinear absorption (two-photon absorption coefficient of 5.3 × 10−10 m/W) and nonlinear refractive index n2 (2.1 × 10−16 m2/W) compared to mixed copper niobate (CuNb2O6 - CuNb3O8). The third-order nonlinear optical susceptibility of CuNb3O8 was ∼10−11 esu and was higher than the known nonlinear optical (NLO) systems such as lithium niobate and sodium niobate measured at similar pulse duration. Enhancement in nonlinearity arises due to the influence of morphology and here truncated cubes with the platonic solids offer high polarizability as the charges are concentrated in the edges of the system. The truncated cube CuNb3O8 exhibited strong optical limiting action with a low limiting threshold (34.6 μJ/cm2), thus making it appealing for ultrafast optical limiter devices towards photosensitive component protection from Infrared (IR) laser damage.

Research on the Densification Process of the Mechanically-Alloyed Amorphous 2Si-B-3C-N Powder

A mixture of the commercially available cubic silicon powder, hexagonal boron nitride powder and graphite powder was mechanically alloyed to prepare amorphous 2Si-B-3C-N composite powder. The amorphous powder was heated up to 1900°C in nitrogen, with a heating rate of 20°C/min and under a pressure of 80 MPa. Careful investigation was carried out on the densification curve, the microstructure and the mechanical properties of the prepared ceramics. Results show that the amorphous 2Si-B-3C-N powder mainly consists of near-spherical agglomerates, with an average size of 3.5±2.4 micrometers. When the amorphous powder was hot pressed, the densification process mainly included three stages, the denser packing of powder particles with the help of axial pressure, the initial sintering at about 1500-1800°C, and the rapid sintering at temperatures approximately higher than 1830°C. When the 2Si-B-3C-N ceramic was hot pressed at 1900°C for 10-30 mins, it exhibited large volume shrinkage, noticeable reduction of pores, and significantly improvement of density and mechanical properties. The applied high temperature and large pressure may give rise to severe plastic deformation, viscous flow and creep of powder particles, which greatly contribute to the rapid densification of the amorphous 2Si-B-3C-N powder.

Effects of fabrication method on initial powder characteristics and liquid phase sintering behaviour of tungsten

Direct electrochemical production of tungsten involves removal of oxygen in solid state from an important tungsten compound, calcium tungstate, in molten salts. A detailed experimental study was performed to characterize the liquid phase sintering behaviour of the tungsten powder (WE), which was produced by the direct electrochemical reduction technique. All of the tungsten heavy alloy (WHA) compositions covered in this study, contained four parts nickel, one part cobalt and over 90 wt% tungsten. The pellets were sintered at 1500 and 1550 °C under continuous flow of an equimolar hydrogen-argon gas mixture. A commercially available tungsten powder (WC) was also used to prepare the same tungsten heavy alloy compositions as those prepared by the electrochemically produced tungsten powder to act as a control group. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF) and hardness measurements were employed for characterization. The analyses provided that; the tungsten powder as obtained from the direct electrochemical reduction process contained some unreduced CaWO4 particles and they limited the grain growth and mass transport in the liquid phase. Although calculated relative densities were a bit lower than desired, the tungsten heavy alloys produced by the electrochemical reduction technique was promising for use when average grain size, dissolved tungsten contents and general/matrix hardness values are considered.

Comparative analyses of the IV group oxides additives influence on the sintering kinetics of zirconia nanopowders

Ceramics materials based on yttria stabilised tetragonal zirconia nanopowders (3Y-TZP) are widely used in the production fuel cells, oxygen sensors, refractories, etc. and intensively studied due to their outstanding mechanical and electrical properties. To obtain ceramics with specific properties different additives can be used. This allow for control over the features of a nanopowder’ structure and its consolidation during the sintering process. It is important examine in detail the initial sintering stage of tetragonal zirconia based nanopowders modified by third element additives. The present paper shows the impact of SiO2 and SnO2 additives and the influence of different methods of dopant addition (co-precipitation and mechanical mixing) on the kinetics of the initial sintering stage of tetragonal zirconia nanopowders. It demonstrates that sintering mechanism is changed by the addition of a small amount SiO2 and SnO2 by volume to the grain boundary diffusion in 3Y-TZP nanopowders obtained by the co-precipitation method. This change led to increased activation energy. This paper explores the reasons for the acceleration in the sintering process of nanopowders 3Y-TZP with SiO2 and SnO2 additives obtained by the mixing method. It shows that the sintering mechanism is the same as that of the initial 3Y-TZP powder obtained by the co-precipitation technique. The volume diffusion mechanism was the predominant mechanism at the initial sintering stage and was accompanied by a decrease in the activation energy of the sintering process.

Acetabular Cup with a Trabecular Coating: A Novel Approach to a Monolithic Cup Made of One High‐Strength Ceramic Material

An advanced monolithic cup comprising a thin liner and a trabecular coating is a promising alternative to the current modular ceramic cups for hip replacements because it can be anchored directly to the human bone and features larger femoral heads allowing a greater range of motion. In this study, the authors propose a fabrication route for an advanced monolithic cup of only one high‐strength ceramic material by joining a partially sintered liner and a coating with a ceramic paste as an intermediate layer and subsequent sintering. The challenge here is to avoid the formation of defects at the coating/intermediate layer interfaces during classical sintering which is due to compressive tension constrained coating sintering in the initial sintering stages. In this study, the defect formation is minimized by sintering at constant low shrinkage rates, as this results in favorable compressive stresses in the coating in the initial sintering stages and a decrease in the differences in the densification rates. For the first time, by using the proposed route, the authors prepare an advanced monolithic cup of only alumina‐toughened zirconia (ATZ) that contains a coating with a porosity of 50 vol% and interconnected pores between 150 and 450 μm, as required for bone‐ingrowth.

Synthesis and characterization of alumina-CNT membrane for cadmium removal from aqueous solution

We report a simple approach for synthesizing an alumina-carbon nanotube (Al2O3-CNT) composite membrane through a powder metallurgical method. The membrane was fabricated via uniaxial pressing of the composite powder mixture and subsequent solid-state, pressure-less sintering. Homogeneous dispersion of the CNTs within the alumina matrix was achieved by using gum arabic and sodium dodecyl sulfate as dispersants. The phase composition and microstructure of the synthesized membrane were characterized using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. The effect of process parameters (i.e., initial compaction load and sintering temperature) on the porosity, strength, and water flux of the membrane was investigated, and the results highlighted a strong influence of the process parameters on these properties. In particular, when the compaction load and sintering temperature were increased from 50 to 200 kN and 1200 to 1500 °C, the porosity of the membrane decreased from 65% to 31% and its strength increased from 0.76 to 15.64 MPa respectively. Finally, batch adsorption experiments were used to determine the cadmium removal efficiency of the alumina and CNTs adsorbents, as well as that an Al2O3-CNT powder mixture, whereas the efficiency of the membrane based on the above mixture was assessed using a flow loop system. The membrane removed 93% of the Cd present in a water solution containing 1 ppm Cd at pH 6.

In situ observation of deformation behavior of chain aggregate inclusions: a case study for Al2O3 at a liquid steel/argon interface

Particle aggregation is a widespread phenomenon spontaneously occurring in nature, and it is also widely explored in industry, for instance, metallurgical engineering and colloid chemical engineering. In this work, deformation behavior of Al2O3 aggregates in liquid steel is investigated as a case study. Using high-temperature confocal laser scanning microscopy, the morphologies of Al2O3 chain aggregates during their deformation are observed in situ at liquid steel/Ar interface. Thereafter, the change of morphology of aggregates over time is quantitatively investigated, and the results show that circularity of aggregates increases during deformation. This fact indicates that the morphology of aggregates tends towards spherical with time. The bending forces between the particles in an aggregate during the deformation are calculated based on the experimental data obtained from in situ observations. The force exerted to cause bending is predicted by the capillary force model established for attractive capillary force of inclusions. It is found that the bending force is slightly lower than the capillary force, and this indicates that the capillary force plays a significant role in the bending of aggregates during deformation. Moreover, the difference between the two forces is due to a resistance force existed from inflection part of the aggregate during deformation. The residence is quantitatively evaluated, and there is a clear tendency showing that the resistance force increases with decreasing distance between two particles during bending. In addition, initial sintering between contacted particles after bending is reported, and the radius of neck formed between sintered particles is evaluated. Finally, the effects of inclusion size and steel flow velocity in actual steelmaking process on the radius of neck and shear stress exerted on the sintered neck during deformation are discussed.

Scale-progressive healing mechanism dominating the ultrafast initial sintering kinetics of plasma-sprayed thermal barrier coatings

The thermal insulating performance of plasma-sprayed thermal barrier coatings (PS-TBCs) depends dominantly on inter-splat pores, which would be inevitably healed during high temperature exposure. The sintering kinetics of TBCs appears to be highly stage-sensitive. However, the ultrafast sintering kinetics during the initial sintering stage is not yet well understood. In this study, the sintering behavior of PS-TBCs was investigated in a scale-progressive (from nano- to micro-scale) way. Moreover, a novel healing mechanism suitable for lamellar TBCs was proposed based on a combined-effect of material and pore structure. Regarding the changing behavior of material, nano-scale roughening can be found at the as-deposited smooth pore surface after thermal exposure. Regarding the 2D featured inter-splat pores, the roughening behavior facilitates multiple contacts between the counter-surfaces of inter-splat pores. As a result, micro-scale bridge-connection can be observed at the healed parts. This multiple contacts mechanism caused by scale-progressive healing behavior significantly accelerated the matter transfer, resulting in ultrafast sintering kinetics at the initial sintering stage.

Effect of biomass ash addition on coal ash fusion process under CO2 atmosphere

The ash fusion process of Jincheng anthracite ash, rice straw ash and coal-biomass ashes with different rice straw ash addition ratios during co-gasification under CO2 atmosphere was elucidated by the ash fusion test, chemical analyses, optical heating stage microscopy (OHSM), X-ray diffraction and scanning electron microscopy equipped with energy-dispersive X-ray analyzer. Results showed that the initial sintering and initial melting temperatures of coal-biomass ashes observed by OHSM decreased with the addition of rice straw ash. The deformation temperatures determined by the ash fusion test were close to the initial melting temperatures. The biomass ash addition to coal ash introduced excessive amounts of alkaline compounds in the ash mixtures that led to lower initial temperatures of liquid phase formation. The ash fusion behavior of coal-biomass ashes with a rice straw ash addition ≤3 wt% was similar to that of coal ash and fits to the “softening-melting” ash fusion mechanism. In contrast, the ash fusion behavior of coal-biomass ashes with a greater addition of rice straw ash (>3 wt%) experienced dramatic changes at lower temperature and fits to the ‘‘melting-dissolution’’ ash fusion mechanism.

Compaction and solid-state sintering of tungsten powders: MPFEM simulation and experimental verification

The uniaxial die compaction and solid-state sintering of different sized tungsten powders in powder metallurgy (PM) process was numerically reproduced in multi-particle FEM (MPFEM) modeling from particulate scale. The effects of particle size, initial packing structure, compaction pressure and sintering temperature on the relative density of the tungsten powder component were systematically studied and discussed. Various macroscopic and microscopic properties of the powder mass with different initial packing structures during compaction and sintering were characterized and analyzed. These properties include the overall relative density, local stress distributions, force structure and transmission, particle rearrangement and deformation, void filling behavior, as well as the densification dynamics and mechanism. The results show that by properly controlling the operating conditions such as initial packing structure, compaction pressure and sintering temperature, high performance PM component with high relative density, uniform density, stress, and void distributions can be obtained. Meanwhile, physical experiments were implemented to verify the simulation results. It is indicated that MPFEM modeling used in current work can provide the researchers with an effective method to simulate the whole PM process for refractory tungsten powders from particulate scale, especially the coupling of different stages can reduce the assumptions in the modeling and make the simulation results more accurate and much closer to the actual process.

Binder jetting additive manufacturing with a particle-free metal ink as a binder precursor

Metal-Organic-Decomposition (MOD) ink is a particle-free metal ink that can precipitate metal nanoparticles upon heating. This ink-jettable metal nanoparticle ink can be used as a binder precursor to replace commonly used polymer adhesives in the Additive Manufacturing of metals components using binder jetting technology. After being deposited into the powder bed interstices and exposed to a heat, the jetted ink decomposes and form metal nanoparticles that are then fused by sintering to provide bonding strength to the printed parts. Regions of the powder bed that do not receive the jetted nanoparticles remain as loose powder, as the heat supplied to sinter nanoparticles is maintained below the initial sintering temperature of larger powder bed particles. The proof of concept is demonstrated in the context of printing copper by jetting copper MOD ink as a precursor to the binder, with the capability of producing satisfactory geometries when using sufficiently high binder saturation ratio. After sintering, the MOD ink printed parts produce a denser core section but more porous shell in the sintered parts, compared to the polymer binder printed counterparts.

Synthesis of fast sinterable oxygen deficient nano α−alumina & its optical properties

Nanocrystalline α-alumina powder was prepared successfully following gel combustion process using for the first time the combination of aluminum nitrate as oxidant and urea-formaldehyde as fuel. The combustion reaction was explained through thermal analyses (differential thermal analysis, DTA & thermo-gravimetric analysis, TGA). The synthesized powder was characterized by X-ray diffraction (XRD), infrared spectroscopy (Fourier Transformed Infrared Spectroscopy, FTIR), electron microscopy (Field Emission Scanning Electron Microscopy, FESEM & Transmission Electron Microscopy, TEM), optical spectra (absorption spectroscopy & photoluminescence, PL), etc. As evidenced from XRD pattern the as- synthesized powder was found to be amorphous. Full crystallinity of the as synthesized powder was obtained upon calcination above around 1100 °C. Synthesized particles are nanoporous, held in loosely agglomerated mass of nano sized spherical forms with size ranging in general between 50 and 70 nm. The photo luminescent spectrum shows a broad emission band that was deconvoluted into Gaussian sub-components. The resolved emission peaks imply emission from oxygen vacancy related radiative color centers. With heat treatment temperature, the PL intensity diminishes due to reduced defect (oxygen deficiency) concentration in nano-crystals. The presence of large concentration of anion deficiency, related to defects, are reflected in the intense emission of PL spectra and in lowering the band gap value with respect to the bulk counterpart; both phenomena being explained in this text. An initial sintering trial under spark plasma sintering shows the powders are quickly responsive to densification facilitated by the presence of defects.

Bonding quality and fracture analysis of polyamide 12 parts fabricated by fused deposition modeling

PurposeThis work aims to evaluate the influence of rheological properties of building materials on the bonding quality and ultimate tensile strength in the fused deposition modeling (FDM) process, through the investigation of parts printed by semi-crystalline and amorphous resins. Little information is currently available about the influence of the crystalline nature on FDM-printed part quality.Design/methodology/approachSemi-crystalline polyamide 12 and amorphous acrylonitrile butadiene styrene (ABS) were used to assess the influence of rheological properties on bonding quality and the tensile strength, by varying three important process parameters: materials, liquefier temperature and raster orientation. A fractography of both tensile and freeze-fractured samples was also investigated.FindingsThe rheological properties, mainly the melt viscosity, were found to have a significant influence on the bonding quality of fused filaments. Better bonding quality and higher tensile strength of FDM parts printed with semi-crystalline PA12, as compared with amorphous ABS, are suggested to be a result of higher initial sintering rates owing to the lower melt viscosity of PA12 at low shear rates. Near-full dense PA12 parts were obtained by FDM.Originality/valueThis project provides a variety of data and insight regarding the effect of materials properties on the mechanical performance of FDM-printed parts. The results showed that FDM technique allows the production of PA12 parts with adequate mechanical performance, overcoming the greatest limitation of a dependence on amorphous thermoplastics as a feedstock for the production of prototypes.

The agglomeration, coalescence and sliding of nanoparticles, leading to the rapid sintering of zirconia nanoceramics

Conventional sintering is a time- and energy-consuming process used for the densification of consolidated particles facilitated by atomic diffusion at high temperatures. Nanoparticles, with their increased surface free energy, can promote sintering; however, size reduction also promotes agglomeration, so hampering particle packing and complete densification. Here we show how the ordered agglomeration of zirconia primary crystallites into secondary particle assemblies ensures their homogeneous packing, while also preserving the high surface energy to higher temperatures, increasing the sintering activity. When exposed to intense electromagnetic radiation, providing rapid heating, the assembled crystallites are subjected to further agglomeration, coalescence and sliding, leading to rapid densification in the absence of extensive diffusional processes, cancelling out the grain growth during the initial sintering stages and providing a zirconia nanoceramic in only 2 minutes at 1300 °C.

Time-Dependent Diffusion Coefficient of Fe in MgB2 Superconductors

The iron (Fe) diffusion in superconducting MgB2 bulk samples has been studied for sintering time durations of 15 min, 30 min, 1 h, 2 h, and 4 h at 900∘C. Fe coating bulk polycrstalline superconducting MgB2 samples for Fe coating were prepared by pelletizing and used in the diffusion experiments with initial sintering at 800∘C for 1 h. A thin layer of Fe was coated on MgB2 pellets by evaporation in vacuum. Effects of Fe diffusion on the structural, electrical, and superconducting properties of MgB2 have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), energy-dispersive X-ray spectroscopy (EDS), and resistivity measurements. Fe diffused samples have slightly increased critical transition temperatures and have larger lattice parameter c values, in comparison with bare samples. Fe diffusion coefficients were calculated from depth profiles of c parameter and room temperature resistivity values. Depth profiles were obtained by successive removal of thin layers from Fe diffused surfaces of the samples. Our results have shown that the Fe diffusion coefficient decreases with increasing sintering time and resistivity measurements can be utilized for determination of diffusion coefficient.

Dehydrogenation and Sintering of TiH2: An In Situ Study

This first-ever study investigated dehydrogenation and microstructural evolution of TiH2 during sintering under vacuum using in situ neutron diffraction, in situ transmission electron microscopy, and ex situ neutron tomography. The densification behavior, microstructure, hydrogen concentration, and in situ phase transformation were reported. The shrinkage, weight loss percentage, and densification of the TiH2 powder compact monotonically increase with sintering temperature, while the open porosity behaves differently; porosity first increases at the initial sintering stage and then decreases during further sintering. The in situ phase transformation observations reveal that dehydrogenation starts from the outer area of either a particle or a powder compact and progressively carries forward into the interior of the particle or the compact. A shrinking core model was proposed to elucidate the dehydrogenation process for a single particle and a powder compact.

Determination of the initial ash sintering temperature by cold compression strength tests with regard to mineral transitions

Ash deposition, fouling, and slagging are commonly undesirable occurrences in coal utility boilers and gasifiers. Ash deposit formation is usually initiated by sintering of the particles. The sintering characteristics of the ash have to be evaluated to understand the process of deposit formation. Cold compression strength (CCS) tests on heat treated ash pellets produced under defined conditions can be applied for the determination of the initial sintering temperature (IST). Scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDX), high temperature X-ray diffraction analysis (HT-XRD), and thermogravimetric differential scanning calorimetry (TG-DSC) are analytical methods to characterize the mineral transitions responsible for sintering.CCS tests were done on ashes of three different coals to determine their IST. For this purpose ash pellets were pretreated at different temperatures. The influence of oxidizing, inert, and reducing conditions at ambient pressure was investigated. The IST were compared with the characteristic temperatures from ash fusibility tests (AFT). HT-XRD, TG-DSC, and SEM/EDX were applied for a more detailed characterization of sintering.The IST determined by CCS tests are considerably lower than the respective initial deformation temperature (IDT) obtained by AFT. Thus, the AFT is not suitable to detect IST. Major mineral transitions were found which are responsible for initial sintering in the case of the investigated ashes. The formation of silicates or the crystallization of iron-aluminum-oxides were found to induce sintering at oxidizing and inert conditions. Sintering at reducing conditions was initiated by carbonation of calcium compounds, silicate formation, or melting of iron sulfide. These phases can be understood as the relevant phases for initial strengthening of fouling deposits in coal utility boilers and gasifiers in the case of the investigated ashes.

The corrosion of kaolinite by iron minerals and the effects on geopolymerization

Iron-rich aluminosilicates with disordered structure (laterites) due to the corrosion of kaolinite by iron minerals were investigated as solid precursors for geopolymerization. The particle size distribution, B.E.T surface area, thermal activation, and chemical and mineralogical compositions were used to evaluate the reactivity of iron-rich laterites (35wt.% of Fe2O3-FeO). The raw materials in the temperature range between 25 and 500°C showed geopolymerization behaviour similar to that of metakaolin. At temperatures higher than 500°C, the coarsening of particles and the decrease of B.E.T surface area correspond to an initial sintering of laterites explaining the poor polycondensation/geopolymerization and the decrease of strength of the final products. The increase of the temperature of calcination of raw laterites between 25 and 500°C corresponds to a reduction of the setting time of geopolymer products. However, this variation of temperature did not significantly affect the flexural strength that remained between ~4 and ~6MPa, confirming the possibility to produce sustainable matrices, with more energy saving, using highly corroded laterites.