Initial Sintering Research Articles

Characterization of the Evolution with Temperature of the Structure and Properties of Geopolymer-Cordierite Composites

This work is part of a research project aimed at producing ceramic-like materials, without the need for an initial sintering, for potential applications in catalysis or filtration at temperatures up to 1000 °C. In that context, cordierite-derived materials were prepared from recycled cordierite powder (automotive industry waste) bonded with metakaolin-potassium silicate geopolymer. The principle is that these materials, prepared at temperatures below 100 °C, acquire their final properties during the high-temperature commissioning. The focus is on the influence of the K/Al ratio and cordierite fraction on the stability of the dimensions and porosity during heating at 1000 °C, and on the final Young’s modulus and coefficient of thermal expansion. Conventional and high-temperature XRD evidenced the absence of crystallization of the geopolymer binder and interaction with the cordierite filler during the heating stage when K/Al = 1 or 0.75. By contrast, crystallization of kalsilite and leucite, and diffusion of potassium ions in the structure of cordierite is evidenced for K/Al = 1.5 and 2.3. These differences strongly influence the shrinkage due to sintering and the final properties. It is shown that a K/Al ratio of 0.75 or 1 is favorable to the stability of the porosity, around 25 to 30%. Moreover, a low coefficient of thermal expansion of 4 to 4.5 × 10−6 K−1 and a Young’s modulus of 40 to 45 GPa is obtained.

In-situ ash sintering process analysis during the co-combustion of the coal gasification fine slag and corn straw

The co-combustion of coal gasification fine slag (CGFS) and biomass is proposed to enhance the poor combustion characteristic of CGFS and alleviate the ash-related issues of biomass. In this study, the sintering mechanism of CGFS ash (FA) and corn straw ash (SA) was revealed by in-situ image analysis and thermodynamic modelling. The sintering process of FA includes initial sintering stage (S1), holding stage (S2), and primary melting stage (S3). The S1 is initiated by the flow of the low-viscosity slag, whereas the subsequent S2 and S3 stages are predominantly governed by the crystallization and then the melting of anorthite with increasing temperatures, respectively. The AAEMs in SA loosens the structure of FA, thereby facilitating the onset of S1 at a low temperature. Concurrently, the anorthite is gradually converted to diopside, which possesses a lower melting point, and subsequently into sanidine, which exhibits a higher melting point. This transformation extends the temperature range across S1 and S2, leading to an initial decrease followed by an increase of ash fusion temperatures with the SA addition. The dry bottom boiler suits for the co-combustion of CGFS and corn straw and the boiler exit temperature is suggested to be higher than 968 °C.

Influence of stabilized zirconium dioxide and high hydrostatic pressure on the kinetics of sintering nanopowders of metastable aluminum oxide

The paper presents an analysis of the effect of processing powders with high hydrostatic pressure (HHP) (300, 500, 700 MPa) and stabilized zirconium dioxide (ZrO2 + 3 mol.% Y2O3, (YSZ)) doping on the compaction of metastable nanopowders mixture with γ+θ-Al2O3 + n% YSZ (n = 0, 1, 5, 10, 15 wt%) composition during sintering. The behavior of nanopowders at the initial stage of sintering was studied by dilatometry at a constant heating rate of 5 °C/min, in temperature range from 20 °C to 1500 °C. It has been determined that sintering of compacts occurs in two stages: compaction of γ + θ-Al2O3 (Ⅰ-st stage metastable phases), compaction of the stable α-Al2O3 (ⅠⅠ-nd stage), accompanied by inhibition between them. An “effect of mutual protection against crystallization” of powder mixtures was determined using the X-ray diffraction method. This is characteristic of systems obtained by co-precipitation. A connection between the above mentioned inhibition and an increase in the YSZ additive was found. That, also, correlates to the effect of “mutual protection against crystallization” of Al2O3 – YSZ powder mixtures. A study of the mechanisms and activation energies of γ+θ-Al2O3 + n% YSZ systems’ (n = 0, 1, 5, 10, 15 wt%) sintering depending on the YSZ additive amount and the HHP value showed that a dopant and pressure increase leads to activation energy decrease of the initial sintering stage, and the prevailing sintering mechanism is volumetric diffusion in corundum grains. Also, the work shows that for γ+θ-Al2O3 + ≥5 wt% YSZ systems, the optimal compaction pressure is HHP ≥500 MPa, which corresponds to the transition of the sintering mechanism from grain-boundary diffusion to volumetric diffusion.

Copper-perovskite interfacial engineering to boost deNOx activity

By adjusting the Cu doping level and calcination temperature during synthesis of a series of noble metal-free lanthanum copper manganite perovskites LaCuxMn1-xO3, we show the importance of tuning the redox chemistry for optimum steering of the catalytic activity and N2 selectivity in the catalytic reduction of NO by CO. A prerequisite is the in situ formation of an extended copper-perovskite phase boundary. The associated improvement of redox and surface properties by Cu addition leads to an optimized balance of the reduction of the catalyst in the NO+CO reaction mixture. The creation of oxygen vacancies improves the reaction kinetics and initiates the NO dissociation. Strongly dependent on the calcination temperature, the Cu/Mn ratio, the reducing agent, as well as the reaction temperature as parameters for Cu-perovskite interfacial control, surface reduction can induce partial copper exsolution, significant perovskite reduction, agglomeration of exsolved Cu particles and eventual decomposition of the catalyst structure. We show that increasing the amount of initial copper incorporated into the perovskite structure can beneficially extend the phase boundary, but exceeding the amount of copper leads to increased sintering of the copper particles at the expense of catalyst activity, especially in repeated catalytic cycles. Increasing the calcination temperature, in addition to the initial sintering of the catalyst and shifting the reaction onset temperature to higher temperatures in the first cycle, improves the reduction resistance of the catalyst. Consequently, in order to form a higher amount of phase boundary-bound active sites, stronger reducing conditions than provided by the NO+CO reaction mixture are needed. This effect can be counterbalanced by using stronger reduction agents prior to performing subsequent catalytic cycles by inducing the exsolution process.

Transparent Ce3+-doped fluorapatite (FAP) ceramics fabricated by spark plasma sintering (SPS)

Polycrystalline Ce3+-doped fluorapatite (Ce:FAP) transparent ceramics with fine microstructures were fabricated through liquid-phase synthesis for the initial powder and spark plasma sintering (SPS) for full densification. These ceramics were confirmed to have a single-phase crystal structure, and the average grain sizes were determined to be 139 and 135 nm for the 1 and 2 at.% Ce-doping concentrations, respectively. Their emission spectra revealed that the fabricated ceramics convert UV light to visible light emission due to the 5d→4f electronic transition of Ce3+. These ceramics are expected to be useful in photonic applications.

W–CeO2 Core–Shell Powders and Macroscopic Migration of the Shell via Viscous Flow during the Initial Sintering Stage

To retard the mutual contact of W grains to inhibit their growth, in this study, CeO2·2H2O was first coated on the surface of pure W (undoped) particles by a weight percentage of 4% using a wet chemical method to prepare CeO2·2H2O-doped W-based (doped) powders, with W particles as the core and CeO2·2H2O as the shell (W–CeO2·2H2O core–shell structure), without hydrogen reduction treatment. The undoped and doped powders were subsequently sintered using a spark plasma sintering (SPS) apparatus to fabricate bulk materials. The macroscopic migration of the CeO2 shell in the core–shell W–CeO2 system via viscous flow during the initial sintering stage was studied through simulations and experiments. The results showed that a core–shell structure with W particles as the core and CeO2·2H2O as the shell was successfully prepared. The doped powder contained approximately 3.97% CeO2, consistent with the designed content of 4%. The shell materials migrated among the selected four sintered powders, filling the pores and contributing to the improvement in the relative density of the sintered bulk.

Effect of additive content on texture evolution and mechanical properties of Si3N4 ceramics prepared by hot pressing

The purpose of this research is to investigate influence of additive content on sintering behavior, texture evolution and mechanical properties of Si3N4 ceramics systematically. Si3N4 ceramics with different additive contents are prepared by hot pressing at 1750 °C for 1h. XRD analysis confirms that the densification process is dominated by particle rearrangement instead of phase transformation. Grain morphology characterization demonstrates that additive content affects relative amount between textured β-Si3N4 grains and β-Si3N4 nuclei during initial sintering. During subsequent sintering, β-Si3N4 nuclei surrounded by residual liquid phase elongated randomly, and then weaken texture degree of Si3N4 ceramics. The texture degree of Si3N4 ceramics decreases first and then increases as additives increase steadily. Based on the CTE misfit model and the stochastic model of crack propagation, texture degree, residual stress and grain size have a synergistic effect on crack propagation behavior of β-Si3N4 ceramics. The outstanding flexural strength (1096 MPa) and fracture toughness (10.61 MPa ∙ m1/2) is achieved in hot pressed SN21 with a relatively high texture degree (fL = 0.213).

Improvement of mid-temperature ZT in a Bi-Se-Te via a two two-step sintering process

A wide range of thermoelectric materials are available for selection. The standard classification is based on the temperature range of the application. This study fabricated mid-temperature Bi-Se-Te using spark plasma sintering (SPS). This analysis focused on the impact of sintering conditions on thermoelectric properties. The structural analysis indicated that initial and secondary sintering processes effectively produced the low-temperature thermoelectric material Bi2Se0.45Te2.55. The sintering duration and temperature changes mainly influenced the grain boundary density, with elevated temperatures inducing defects that impacted the performance. Secondary sintering resulted in layered structures with elongated grains, which enhanced the phonon scattering effect. This configuration markedly decreased the thermal conductivity, increasing the ZT value by 60%.

Formation of grain boundaries in nanocrystalline SiC ceramics examined by powder diffraction supported by MD simulations

Atomic structure of nanocrystalline SiC powder sintered under high-pressure high-temperature conditions was examined with use of powder diffraction technique supported by molecular dynamics simulation. Shape and atomic structure of grain surfaces was identified based on real space PDF analysis of experimental diffraction data and theoretical calculations performed for atomic models of nanograins relaxed by MD simulation. It is shown that in presence of about 5 % of excess carbon in the initial 11 nm size SiC powder HPHT sintering leads to formation of (100) and (110) grain boundaries formed by single C planes and (111) boundaries formed by double C planes. Under pressure 3 GPa and 1600°C sphere-like grains tend to transform to dodecahedrons terminated by single C atomic planes. Under 8 GPa the grains adopt the shape of cube-octahedrons terminated by (100) single C planes and (111) planes terminated by single and double C planes.

Postprocessing of tungsten carbide‐nickel preforms fabricated via binder jetting of sintered‐agglomerated powder

AbstractThis study binder jets a tungsten carbide‐nickel (WC‐Ni) sintered‐agglomerated composite powder, and postprocesses the preforms using an initial sintering step followed by a hot isostatic pressing (HIP) step. The effects of sintering temperatures, sintering durations, and HIP temperatures on notable properties (e.g., porosity, microstructure, hardness, and oxidation behavior) are quantified. The highest average relative density produced in this study was 96.8%, and volumetric shrinkage of these coupons was about 64%. Microstructural characterization shows that the WC grains are homogenously distributed throughout the nickel matrix and grow to an average diameter of 1.6 (a 60% increase) during processing. X‐ray diffraction patterns indicate that no unwanted products were formed. Processed coupons achieved a maximum hardness of 54 Rockwell C, limited by their internal porosity. Oxidation tests result in the production of WO3 and NiWO4 at temperatures above 600°C. Methodologies and results from this study can be leveraged to additively manufacture highly dense, geometrically complex WC‐Ni parts with small carbide grains, low nickel content, desirable microstructure, and suitable functional properties.

Microwave sintering of Ti(C,N)-based cermets: Study of the magnetic effect on metal phase

For microwave heating of Ti(C,N)-based cermets, this paper presents a microwave heating simulation study of pure Ti(C,N) ceramics and Ti(C,N)-based cermets using Comsol software. S-parameter inversion is employed to determine the equivalent relative permittivity of Ti(C,N)-based cermets with varying metal contents. The pairwise Ni metal plate model is utilized to investigate the law of mutual coupling voltage between metal plates. It was found that vigorous electric field intensity is generated around the metal particles of Ti(C,N)-based cermets, and high electric field intensity is generated between the metal particles. Model slice analysis and mechanism analysis are combined to reveal the following phenomena: (1)The surface of metal particles in Ti(C,N)-based cermets excites induced electromagnetic fields; (2)High mutual coupling voltages are generated between metal particles; (3)The reflection effect of metal particles on microwaves can increase the electric field intensity between metal particles. By statistically analyzing the temperature data of pure Ti(C,N) ceramics and Ti(C,N)-based cermets during the initial 30-min microwave sintering period, the temperature difference changing law can be concluded: (1)During the initial 30 min of microwave sintering, the temperature difference between the two gradually increased with time; (2)At 30 min, the temperature difference between the two reached 111 °C.

Fabrication and volume loading studies for Mo30W matrix surrogate cermets

Ceramic-metallic composite (cermet) fuel forms are promising options for use in nuclear thermal propulsion (NTP) due to their high melting point, inherent hydrogen compatibility, retained strength at high temperatures, and high thermal conductivity. Spark plasma sintering was used to fabricate cermet materials, resulting in improved microstructural characteristics. An alloy of molybdenum with 30 wt% tungsten was used as the matrix material. Spherical yttria stabilized zirconia (YSZ) and both spherical and angular hafnium nitride (HfN) were used as surrogate fuel particles for the cermets. Utilizing a new lab-scale particle coating method, high density composites with excellent particle-matrix interfaces were fabricated. The effects of volume loading on the microstructure of the matrix were also examined, testing particle loadings up to 90 vol%. Initial stage sintering models were applied to understand the effects of the added particles on the sintering of the Mo30W matrix. Recommendations for fabrication of uranium bearing cermets using Mo30W are also included.

Differential densification in Ho:Y2O3 transparent ceramics

Differential densification has been observed in the sintering of Ho:Y2O3 ceramics at heating rates of ≤3 °C. The temperature gradients in compacts during the heating process were simulated using ANSYS software. At a heating rate of 3 °C/min, the simulation results indicate a small temperature discrepancy of about 2 °C between the surface and the center area of the samples. Compared with those in the near-center (NC) area of the compact, the grain coarsening and rearrangement in the near-surface (NS) area are more pronounced during the initial and intermediate sintering stages. As a consequence, both the pore distribution and microstructure in the NS area were more uniform, which accelerated the elimination of pores in the subsequent sintering stage. This observation suggests that particle rearrangement in the early sintering stage is one of the main reasons for the enhancement in the ultimate densification of the ceramics. The NC area could be fully densified by using hot isostatic pressing (HIP). After HIP, the Ho:Y2O3 ceramics have high in-line transmittance of 80% at the wavelength of 500 nm.

Experimental study on the sintering characteristics of biomass ash

The alkaline metal content in biomass is high, which leads to a low initial sintering temperature. Therefore, during the process of direct biomass combustion for power generation, there are serious problems such as ash deposition and slagging on the heating surface, which has become a technical bottleneck for the safe, stable and long-term operation of boilers. In industry, the melting characteristics of ash are commonly used to guide boiler design and operation. However, traditional measurement of ash melting temperature has a large subjective error, and the releasing of alkaline metals during the measurement process results bit higher value, which affects the accuracy of the measured characteristic temperature. Comparingly, the initial sintering temperature is lower than the deformation temperature of the ash. This article provides a device for measuring the sintering temperature of biomass ash by using the pressure drop method. A novel two ash chamber system was invented, in which the alkali element released from the first ash chamber was captured by the downstream second ash chamber to compensate the escaping of alkali element. The initial sintering temperature of the second ash chamber has more accuracy with lower value than that measured by the traditional single sample chamber. XRD was used to analyze the mineral composition and causes of sintering of the ash samples before and after sintering. The results show that the sintering mechanisms of different types of biomass are different. In addition, combined with SEM analysis of rice husk, fruit core, rapeseed straw, and corn straw, the results show that the initial sintering temperature of the ash samples measured by the double chamber measurement method is more accurate than that measured by the single chamber measurement method, further validating the reliability of the two ash column system.

Investigation of the composition, structure and magnetic properties of the Nd2Fe14B ceramics dependence on the initial powder characteristics and spark plasma sintering modes

High phase and structural sensitivity of magnetic properties strongly impacts the fabrication quality of the permanent magnets, therefore revealing effects of synthesis conditions on physico-chemical characteristics of magnetic materials is of paramount importance for optimization of magnet production especially in case when non-conventional techniques are involved. Here we report for the influence of pulse current frequency on structure, composition, and magnetic properties of an industrially-important Nd2Fe14B magnetic ceramics on the initial powder characteristics and the modes of its spark plasma sintering (SPS) in a vacuum environment. The influence of initial powder mechanical grinding as well as the number of generated electric current pulses (in the range from 3.3 to 326.7 ms) in SPS conditions on the structure (defects and pores formation, grain size), phase and element composition (formation of secondary nonmagnetic phases and oxides, impurities) of Nd2Fe14B alloy was studied by SEM, TEM, EDS and XRD methods. The regularity of changes in the magnetization saturation value (σ) and coercive force (Нс) depending on the morphology, structure and composition of the initial Nd2Fe14B powder and the alloy obtained by mechanical milling and subsequent sintering at different modes of pulse current has been established.

Structural characterisation of unique core-shell shaped Al3Ni2/Al3Ni in-situ intermetallic in Al–4Cu-xNi via powder metallurgy (P/M)

An in-situ powder metallurgy technique was used to create Ni–Al3Ni/Al3Ni2 core-shell-shaped aluminum-based intermetallic reinforced composites. The impact of Ni addition on the phase composition, microstructure, and mechanical characteristics of the Al–4Cu-xNi (x = 0, 2, 4, 6, 8, 10 wt%) with respect to various sintering temperatures were examined. Microstructure progression was extensively studied using XRD, and SEM-EDX, and TEM techniques. Initial sintering condition witnessed the proliferation of “Single Core-Shell” structures, comprising Ni as core with Al3Ni2 intermetallic, whereas sample sintered at 620 °C, “Single Core-Shell” and “Double Core-Shell” type of structures containing Al3Ni2, Al3Ni intermetallic were formed between the Al matrix and Ni reinforcements. The 10 wt% Ni addition sample results in substantially high hardness and strength, as compared to the literature, due to the presence of various intermetallics such as Al2Cu, Al3Ni2, and Al3Ni and strong adherence between the formed phase and α-Al matrix. High compressive yield strength of 198.13 MPa and ultimate strength of 410.68 MPa with 24% total elongation was achieved for 10 wt% Ni sample, whereas there was an increase in hardness value of 124.21HV which is 2.4 times higher than that of base Aluminium.

Slow sintering in garnet‐containing Y and Gd zirconate–aluminate mixtures for thermal barrier coatings

AbstractMixtures of rare‐earth zirconates and aluminates containing Y or Y + Gd that form a two‐phase garnet–fluorite mixture exhibit much slower sintering than pure fluorite at 1400°C. An equivalent Y‐free, Gd‐containing composition that forms a perovskite aluminate instead of garnet showed faster densification after the metastable garnet decomposes. At 1500°C, the Y‐free sample also showed the fastest initial sintering rate, whereas there was more divergence in the sintering rate for the samples containing Y + Gd. The zirconate–aluminate with equimolar Y + Gd shows the slowest densification at 1500°C and retains ∼25% porosity after 250 h. The results highlight possibilities for designing compliant thermal barrier coatings that can retain significant porosity at 1400°C or higher.

Application of factorial model for an optimization of partial replacement of feldspar by talc on the sintering process

AbstractThis research used a factorial model containing two levels and three variables to evaluate the partial substitution of sodium feldspar (albite) by a talc ore found in abundance in the region of Itaiacoca—Brazil. The model can also be used to verify the influence of initial talc particle size, proportion, and sintering threshold temperature on the following physical properties, such as linear shrinkage, water absorption, bulk density, total porosity, and firing color. In this study, the mechanical strength of the compositions was evaluated by the flexural strength test. The factorial model indicated the sintering temperature as the variable that most affects the samples’ densification and the proportion of talc as the variable that changes the firing color. The experiment that used a higher sintering temperature combined with a coarser talc granulometry presented the highest mechanical strength. When more refined granulometry was used, there was the beginning of an overfire process. Water absorption values in the range of .04% and modulus of rupture of 49 MPa were obtained, confirming the talc's effectiveness as a secondary flux agent suitable for the formulation of ceramic bodies.

Disentangling creep and isothermal metamorphism during snow settlement with X-ray tomography

AbstractOnce fallen, snow settles due to the combined effects of metamorphism and deformation of the ice matrix under gravity. To understand how these coupled processes affect snow evolution, we performed oedometric compression tests and continuously monitored the snow microstructure with X-ray tomography. Centimetric samples with an initial density between 200 and 300 kg m−3 were followed during an initial sintering phase and under two different loads of 2.1 and 4.7 kPa at $-8^\circ$C for ~1 week. The microstructure captured at a voxel size of 8.5 μm was characterized by density, specific surface area (SSA) and two metrics related to bond network, namely the Euler characteristic and the minimum cut surface. Load-induced creep of the ice matrix was observed only for sufficiently low values of initial density (<290 kg m−3 in our tests), and was shown to be associated to a significant increase of the number of bonds. Application of the load, however, did not affect the individual bond size nor the SSA, which appeared to be mainly controlled by isothermal metamorphism. The uniaxial compression did not induce any creation of anisotropy on the microstructural characteristics. Overall, our results show that, for the considered conditions, the deformation of the ice matrix mainly leads to a reduction of the pore space and an increase of the coordination number, while metamorphism mainly affects the grain and bond sizes.

Effects of bridging fibers on the evolution of lamellar architecture during H2/H2O redox cycling of Fe-foams

Fe/Fe3O4 redox cycling via cyclic H2/H2O exposure at 800 °C is studied in lamellar Fe foams with 15 vol% fibers, created by freeze-casting. Fibers were integrated in the foams to mitigate densification during cycling by mechanically supporting neighboring lamellae, thus preventing buckling and sintering at contact points. Three fiber types are examined: short (0.1 mm) and long (1–2 mm) stainless-steel fibers, and long zirconia fibers. Long fibers bridge lamellae and have a marked effect on the architecture by increasing the initial interlamellar porosity (from < 60 to > 85%), with a corresponding decrease in foam shrinkage during initial reduction and sintering (from > 80 to < 55% volumetric loss). Though performance improves as compared to fiber-free foams, fiber effectiveness against damage decreases with cycling: after 10 redox cycles, porosity falls from 85 to 50% for foams with long fibers. One novel degradation mechanism is identified: fiber engulfment. This mechanism occurs over successive redox cycles, as material from the lamellae cyclically engulfs (as Fe3O4) and withdraws (as Fe) from the fibers, with a net transport from lamellae to fibers after each cycle. This cyclic coarsening mechanism alters foam architecture from bridged-lamellar (with evenly distributed porosity) to mixed lamellar/fibrous (with unevenly distributed porosity).