Low Temperature Densification Research Articles

Rapid low-temperature densification of Ag2Se bulk material: mass transfer through the dissociative adsorption reaction of Ag protrusions and Se saturated vapor

In materials science, the impact of density on a material’s capabilities is profound. Conventional sintering requires high temperatures and is energy-demanding, propelling the pursuit of less intensive, low-temperature densification methods. Electric field-assisted sintering has recently gained attention for its simplicity and effectiveness, offering a new frontier in low-temperature densification. In this study, dense bulk materials were produced by subjecting monophasic Ag2Se powders to electric field-assisted sintering, where a direct current with an average value of 4 A was applied, achieving a peak temperature of 344 K. The novel low-temperature densification mechanism unfolds thus: nanoscale silver protrusions, stimulated by electrical current, engage in a dissociative adsorption reaction with the ambient saturated selenium vapor. This process swiftly engenders the formation of fresh silver selenide (Ag2Se) compounds, initiating nucleation and subsequent growth. Consecutively, these compounds seamlessly occupy and expand, perpetually bridging the interstices amidst the powders. In a scant 8 s, the density swiftly surpassed 99%, yielding a bulk material that exhibited a ZT value of 1.07 at 390 K. This investigation not only attains an unparalleled density at low temperatures but also charts a pioneering course for material densification in such conditions.

Cr-assisted low-temperature densification of (Ti,Zr,Nb,Ta,Mo)C high-entropy carbides with ultrafine grain and enhanced hardness

Cr-assisted low-temperature densification of (Ti,Zr,Nb,Ta,Mo)C high-entropy carbides with ultrafine grain and enhanced hardness

High-pressure low-temperature densification of NASICON-based LATP electrolytes for solid-state lithium batteries

Ceramic oxides are promising solid electrolytes for Lithium metal solid-state batteries (SSBs) because of their high ionic conductivity and stability under ambient conditions. Nonetheless, the need for high-temperature densification approaches challenges their processability towards upscaling and their implementation into practical SSB devices. Here, we investigate a High-Pressure Low-Temperature (HPLT) processing technique for the densification of NASICON-based Li1+xAlxTi2-x(PO4)3 (LATP) solid electrolyte, providing an understanding of several key parameters such as temperature, pressure and time. Our results show that nanometric LATP can be densified using the HPLT technique at 200°C within 2 min delivering an ionic conductivity of 6.15×10−5 Scm−1, which is the highest reported value at this temperature without the addition of solvents. On the other hand, by combining HPLT with a short post-heat treatment at 700 or 800 °C for 1 hour, highly dense pellets with an ionic conductivity of 10−4 Scm−1 can be obtained. This represents a reduction of the densification temperature of 400°C compared to conventional high-temperature sintering and shows the potential of HPLT to overcome current densification constraints derived from the use of very high temperatures.

Low-temperature sintering of ZnTiO3 using CaV2O6 as a liquid-forming additive for LTCC applications

CaV2O6 (CV) is investigated as a sintering aid for the low-temperature densification of ZnTiO3 (ZT). The incorporation of 5 vol% of CV drastically decreases the sintering temperature of ZT from 1175 to 825 °C and prevents its decomposition to Zn2TiO4 and TiO2. The densification temperature reaches as low as 690 °C when the concentration of CV approaches 30 vol%. The formation of a eutectic at 687 °C from unreacted V2O5 and TiO2 present in CV and ZT powder, respectively, and the lowering of the melting point of CV in the presence of ZT, as evidenced by DSC analysis, are responsible for the low-temperature densification by liquid phase sintering. The XRD analysis indicates no reaction between the ZT and CV phases in the composites. The SEM analysis shows the presence of CV in the boundaries of ZT grains and a decrease in average grain size from 5.9 to 1.24 μm when the CV concentration increases from zero to 30 vol%. The permittivity value decreases from 24.25 to 22.71, and the loss factor increases from 7.449 × 10−4 to 1.321 × 10−3 when the CV concentration in the composite increases from 5 to 30 vol%. The composite containing 5 vol% CV shows the highest Qu × f value of 9215, a temperature coefficient of the resonance frequency of −51 ppm/°C, a thermal expansion coefficient of 9.3 ppm/°C, and a thermal conductivity of 4.72 Wm−1K−1. The ZT-CV composites are compatible with silver at their sintering temperatures, showing their suitability for LTCC applications.

Effect of Sn addition on the low temperature shrinkage of Ni nanoparticles

In this study, in-situ formation of intermetallic compound (IMC) was proposed to reduce the sintering shrinkage mismatch between Ni electrode and dielectric material for multilayer ceramic capacitors (MLCCs). Tin (Sn) was selected as an alloying element and introduced in the form of SnCl2 into Ni nanopowder. The Sn precursor was decomposed at ∼450 °C and a cubic Ni3Sn intermetallic phase was formed at 500 °C, which was gradually transformed to a hexagonal Ni3Sn phase at 700 °C and thereafter thermally decomposed above 800 °C forming the Ni–Sn solid solution. The Sn addition shifted the shrinkage onset temperature of Ni electrode to a higher temperature by 300–400 °C and effectively retarded the low temperature densification through in-situ formation of Ni3Sn intermetallics by inhibiting the inter-diffusion between Ni nanoparticles. Furthermore, the sintered Sn added Ni (Ni–3Sn) electrode exhibited an electrical conductivity comparable to that of pure Ni electrode, demonstrating its suitability as an alternative electrode for MLCCs.

Fabricating exotic boron carbide ceramics by shaker milling and reactive spark plasma sintering of B and C: Application to the elusive B6C

A route based on the shaker milling of B and C powders followed by reactive spark plasma sintering (SPS) of the resulting B+C powder mixture is presented for the fabrication of exotic boron carbide monolithic ceramics with controlled stoichiometry, and applied to the elusive B6C ceramics. Firstly, it is shown that shaker milling for only 1 h refines down to the nanoscale, mechanically activates, and intimately mixes the B and C powders, making the resulting B+C powder mixture far more sinterable. Secondly, it is shown that B and C react during the heating ramp of the SPS cycles, resulting in a fully dense superhard (∼30 GPa) boron carbide monolith with the desired stoichiometry (B6C in this case) and fine twinned-grain microstructure at an SPS temperature (1550 °C in this case) at which typical commercial B4C powders barely densify. This low-temperature densification is possible mainly due to the enhanced solid-state sinterability promoted by both shaker milling and structural ordering, with the exothermic heat released during the formation of the boron carbide contributing little. And thirdly, it is shown that there is an optimal SPS temperature above which the hardness of these exotic boron carbide monoliths decreases because there is less twinning hardening.

Low temperature densification and dielectric characterization of CeO2 ceramics

A near full densification is essential to obtain high dielectric constant and quality factor together with minimum dielectric loss for microwave dielectrics. Limited densification of CeO2 ceramics was improved by using 5 wt% anorthite-based glass and a high densification between 1050 °C and 1150 °C range (e.g., relative density 97.5–99%) was successfully obtained. Phase analysis revealed a cubic CeO2 phase without any crystalline secondary phases formed. Dielectric properties followed the densification path such that maximum dielectric constant (minimum dielectric loss) was obtained at 1150 °C as 21.36 (0.0040) at 5 MHz. The unloaded quality factor (Q), resonance-frequency dependent quality factor (Qxf), and dielectric constant (loss) for a TEM mode dielectric resonator were calculated as 445, 1288 GHz, and 21 (0.0022), respectively. The CeO2-(anorthite-based) glass ceramic was studied for the first time in literature and the results are promising from the low temperature densification and microwave properties points of views.

Fast and low-temperature densification of highly conductive Li7La3Zr2O12 ceramic electrolytes for solid-state batteries

Garnet-type electrolyte like Li7La3Zr2O12 (LLZO) is a promising candidate for solid-state batteries (SSBs) due to its high Li + conductivity. Obtaining a dense microstructure for their practical use typically requires 1200 °C, which often lead to structural issues in addition to concerns on the scale up at industrial level.In this study, we propose a two-step process to fabricate LLZO electrolytes with a high relative density under low temperature. During the first step, a high quasi-isostatic pressure of 5.5 GPa was applied at temperatures below 500 °C (high pressure and low temperature, HPLT), reaching a density of over 97% with absence of grain growth. Then, a brief treatment at moderate temperature below 800 °C was applied to promote grain growth, leading to an ionic conductivity of 3·10−4 S cm−1. A relationship between the microstructure obtained under key parameters: temperature, pressure, and time; and the ionic transport of LLZO electrolytes has been established. Besides the high ionic conductivity obtained via HPLT, stripping and plating of lithium in a symmetric cell demonstrated the practical application.This work provides guidelines on the control of the microstructure of LLZO from processing parameters and contributes to the understanding of the correlations between microstructure and conductivity.

Phase composition, densification behavior and high-temperature strength of carbon-doped ZrB2–ZrSi2 ceramics

Sintering additive of ZrSi2 is beneficial to low-temperature densification of ZrB2-based ultra-high temperature ceramics (UHTCs), nevertheless, residual ZrSi2 would severely reduce the high-temperature strength of UHTCs. In the current work, to achieve low-temperature preparation of ZrB2-based UHTCs with good high-temperature strength, active carbon (C) was added into ZrB2–ZrSi2 ceramics to convert the sintering additive into refractory phases of SiC/ZrC. C-doped ZrB2–ZrSi2 powders were hot-pressed at different temperatures to investigate the phase composition, microstructure and densification behavior. TG-DSC, XRD and SEM results indicated that ZrSi2 was almost entirely converted into SiC and ZrC at 1600 °C for 2h, meanwhile, a fully-dense C-doped ZrB2–ZrSi2 ceramic was obtained at such sintering conditions. In addition, the high-temperature (1800 °C) strength of C-doped ZrB2–ZrSi2 ceramic was 390 ± 21 MPa, which was much higher than that of the C-undoped one. The excellent high-temperature mechanical properties could be attributed to the transformation of low-melting-point phases into refractory phases.

Hexavalent Ions Insertion in Garnet Li7 La3 Zr2 O12 Toward a Low Temperature Densification Reaction.

Nowadays, solid electrolytes are considered the main alternative to conventional liquid electrolytes in lithium batteries. The fabrication of these materials is however limited by the strict synthesis conditions, requiring high temperatures which can negatively impact the final performances. Here, it is shown that a modification of garnet-based Li7 La3 Zr2 O12 (LLZO) and the incorporation of tellurium can accelerate the synthesis process by lowering the formation temperature of cubic LLZO at temperatures below 700 °C. Optimized synthesis at 750 °C showed a decrease in particle size and cell parameter for samples with higher amounts of Te and the evaluation of electrochemical performances reported for LLZO Te0.25 a value of ionic conductivity of 5,15×10-5 S cm-1 after hot-pressing at 700 °C, two orders of magnitude higher than commercial Al-LLZO undergoing the same working conditions, and the highest value at this densification temperature. Partial segregation of Te-rich phases occurs for high-temperature densification. Our study shows the advantages of Te insertion on the sintering process of LLZO garnet and demonstrates the achievement of highly conductive LLZO with a low-temperature treatment.

Low Temperature Densification and Electrochemical Properties of Sm0.16Ce0.84O1.92electrolyte by Zn Doping

Low Temperature Densification and Electrochemical Properties of Sm0.16Ce0.84O1.92electrolyte by Zn Doping

Colloidal Processing and Sintering of Alumina and Zirconia Fine Particles Using New Microbial Dispersant

Application of new microbial dispersant to colloidal processing of fine powders of zirconia and alumina was conducted for obtaining fine-grained microstructures. Well-dispersed aqueous suspensions of the fine powders were prepared by adding appropriate amount of new microbial dispersant. The suspensions were characterized by measuring particle size, zeta-potential and rheological properties. Dense green bodies of zirconia and alumina were obtained by slip casting of the suspensions and further CIP treatment. Low-temperature densification was observed, and dense and fine-grained sintered bodies were obtained.

Metastable properties of a garnet type Li5La3Bi2O12 solid electrolyte towards low temperature pressure driven densification

Hot-pressing leads to low temperature densification of garnet type Li5La3Bi2O12 solid electrolyte and density functional theory calculations allow to explore its metastable properties under the formation of impurities.

Formation mechanisms of Ti 3(Si,Al)C 2/Al 2O 3 composites from Ti 3AlC 2 and SiO via low-temperature sintering

Ti₃SiC₂/Al₂O₃ composites have attracted attention due to their excellent mechanical and electromagnetic properties, but the high temperatures (≥ 1400 ℃) required for the densification of aluminum oxide (Al₂O₃) leads to the decomposition of Ti₃SiC₂. To address this issue, Ti₃(Si_<i>x</i>Al_1−<i>x</i>)C₂/Al₂O₃ (<i>x</i> represents the Si content) composites were synthesized for the first time via hot-pressing (HP) sintering and current-assisted sintering (CAS) of mixed Ti₃AlC₂ and silicon monoxide (SiO) powders at 1300 and 1200 ℃, respectively. Both approaches produced composites with <i>x</i> values greater than 0.9, indicating that the compositions of the prepared composites were similar to those of Ti₃SiC₂/Al₂O₃ composites. The synthetic mechanism involved substitution and continuous interdiffusion of Al and Si atoms. The composite prepared by CAS at 1200 ℃ was compacted, whereas the composite prepared by HP had a low density. The low-temperature densification mechanism is attributed to the combined effects of amorphous SiO, liquid Al, and the high heating rates for CAS. The flexural strength and hardness of the composite prepared by CAS were also comparable to those of compacted Ti₃SiC₂/Al₂O₃ composites.

Cold sintering of BaZr0.7Ce0.2Y0.1O3-δ ceramics by controlling the phase composition of the starting powders

Protonic ceramic conductors based on the solid solution BaZr0.7Ce0.2Y0.1O3-δ (BZCY72) have great potential as electrolytes in various energy conversion devices. However, their refractory nature requires very high sintering temperatures to achieve sufficient densification for gas tightness. In this study, we present a novel processing route using the cold sintering process (CSP) to consolidate BZCY72 powders at 350°C using 400 MPa and deionized water as a sintering aid. Through a systematic variation of the calcination pretreatment, we demonstrated that the cold sintering ability of BZCY can be tuned through the phase composition of the starting powder. The cold sintering process of BZCY bases on a decomposition reaction of a Ce-rich BZCY phase, which plays a major role for low temperature densification. A subsequent post thermal treatment (PTT) at 1300°C is sufficient to regain the BZCY phase resulting in total conductivities of 1*10−3 Scm−1 at 500°C in air.

Low-temperature densification of high entropy diboride based composites with fine grains and excellent mechanical properties

High entropy diborides require high temperature (>2000 °C) for densification, leading to grain coarsening and deteriorated mechanical properties. Herein, fully dense and fine-grained (Hf, Zr, Ta, Nb, Ti)B2 ceramics were successfully fabricated by spark plasma sintering at 1500 °C using metallic cobalt as additives. The result showed that the relative density of (Hf, Zr, Ta, Nb, Ti)B2 increased tremendously from 49.3% to 99.5% as 5 vol% cobalt (HEB5C-15) were introduced. The Co–B based liquid phase was reactively formed during sintering, accelerating the densification by wetting grain boundaries and resulting the presence of Co segregation. Due to the significantly reduced sintering temperature, sluggish diffusion characteristic of the high entropy matrix and the segregation drag effect, the resulting (Hf, Zr, Ta, Nb, Ti)B2 grain size was as small as 0.5 μm. Vickers hardness of HEB5C-15 densified at 1500 °C was 24.90 ± 1.39 GPa, at least 20% higher than the corresponding value in the cobalt free counterpart densified at 2000 °C.

Ultrafine-Grained Tungsten Heavy Alloy Prepared by High-Pressure Spark Plasma Sintering.

Tungsten heavy alloy (WHA) is an ideal material employed for kinetic energy penetrators due to its high density and excellent mechanical properties. However, it is difficult to obtain ultrafine-grained tungsten alloy with excellent properties by traditional powder metallurgy method because of severe grain growth at a high sintering temperature with a long soaking time. In this study, the sintering behavior of tungsten alloys was studied at 800 to 1300 °C, and highly dense 93W-5.6Ni-1.4Fe (wt.%) WHA was successfully fabricated at a low temperature of 950 °C with a high pressure of 150 MPa by spark plasma sintering. The as-sintered tungsten alloy possesses a high relative density (98.6%), ultrafine grain size (271 nm) and high dislocation density (2.6 × 1016 m−2), which results in excellent properties such as a high hardness (1079 HV1). The high sintering pressure is considered to support an additional driving force for the sintering and lead to a low-temperature densification, which effectively limits grain growth.

Electrophoretic deposition and low-temperature densification of Cu1.35Mn1.65O4 spinel for an interconnect protective coating in solid oxide fuel cells

As a protective coating of the interconnects in solid oxide fuel cells, spinel-structured Cu1.35Mn1.65O4 powder was coated onto 460FC stainless steel by using the electrophoretic deposition method. A suitable amount of iodine was added to ethanol to charge the spinel powder with a high zeta potential value. Stainless steel substrates were immersed in a slurry, and a DC voltage in the range of 20–60 V was applied for 30–120 s. Because a low-temperature densification of the coated film is crucial for minimizing Cr out-diffusion from the stainless steel substrate, the coated spinel was decomposed into Cu and MnO by applying a heat treatment at 800 °C in a 5% H2/95% N2 atmosphere. Then, it was oxidized at 700 °C in air, leading to appropriate densification. The area-specific resistance of the films was 15–29 mΩ cm2 after 1000 h at 700 °C in air.

Spark-plasma sintering of boron carbide–silicon carbide composites at 1400 °C from B4C+Si: Densification and sintering/reaction mechanisms

The feasibility of fabricating novel boron carbide–silicon carbide composites by spark-plasma sintering (SPS) of B4C+Si powder mixtures at only 1400 °C was investigated. First, it is shown that B4C can be fully densified at 1400 °C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide–SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B4C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B4C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.

Effect of synthesis route on optical properties of Cr:Al2O3 transparent ceramics sintered under high pressure

Transparent polycrystalline ceramics doped with active ions are suitable for many potential optical applications. Fabrication methods that utilize applied external pressure show promise as they allow reducing sintering temperatures and produce transparent non-cubic ceramics. However, not much attention has been given to the dissolution of doping elements while sintering under relatively high pressure and low temperature conditions. In this study, we employed high-pressure spark plasma sintering (HPSPS) for fabrication of Cr:Al 2 O 3 ceramics and investigated the effect of two different synthesis routes on densification and optical properties. Namely, HPSPS of Cr-doped alumina powders prepared by directly doping with 0.05, 0.2, and 0.5 Cr at.% via co-precipitation and alumina mixed with similar mol.% of Cr 2 O 3 nanoparticles towards doping via solid-state reactive sintering (so-called ‘synthesized’ and ‘mixed’, respectively). Remarkable differences in optical properties between samples obtained by each method were observed. The ceramics fabricated from synthesized powders exhibited pink shades of ruby, high transparency, and strong photoluminescence. In contrast, the mixed ceramics exhibited green color, low transparency, and weak photoluminescence. It was found that in the green samples, most of the Cr 2 O 3 nanoparticles remain as undissolved nano-inclusions. These inclusions are detrimental to optical properties and cause green pigmentation of the alumina. Post-sintering heat treatment dissolved the inclusions and the added Cr 3+ in alumina increased photoluminescence. Evidently, doping transparent alumina via reactive sintering is not viable for processes such as HPSPS. Nevertheless, rapid low temperature densification might enable the design of functional translucent ceramics with integrated second phase nanoparticles. • High-pressure spark plasma sintering was used to fabricate polycrystalline Cr:Al 2 O 3 . • Cr-doping via co-precipitation yielded highly transparent Cr 3+ :Al 2 O 3 ceramics. • Mixed Cr 2 O 3 -Al 2 O 3 yielded green translucent ceramics due to Cr 2 O 3 nano-inclusions. • Post-sintering heat treatment dissolved Cr 2 O 3 and increased photoluminescence. • Reactive sintering is not a viable synthesis route for doped transparent alumina.