Min Dwell Time Research Articles

Morphological Control of Anodic TiO2 Nanotubes by the Modulation of Applied Potential

Abstract Bamboo-type TiO2 nanotube (BT-TONT) arrays were developed by biasing the facile alternating voltage (AV, 80–20 V) and compared to smooth-walled TiO2 nanotube (SW-TONT) arrays grown by forcing a constant voltage (CV). The duration at 20 V in the AV mode had a considerable influence on the morphological properties such as the thickness, ridge spacing, and pore size for the same cycles. As the duration at 20 V was increased, the length and ridge spacing decreased, while the pore size increased. This is due to the formation of a thick blocking layer that blocks the growth of TONT. However, the repetitive formation of ridges in the BT-TONT arrays induced an enlargement of the surface area, resulting in an improvement of the short-circuit current (Jsc) in dye-sensitized solar cells (DSSCs) in comparison with that of the SW-TONT arrays. In particular, the 20 V (20 min dwell time) TONTs exhibited a photoconversion efficiency (η) of 0.20% per thickness, compared with 0.077% for the SW-TONTs. Therefore, the formation of BT-TONT arrays can give an increased surface area for dye uptake, leading to an improved photoconversion efficiency.

Mechanical properties of ZrB2–SiC ceramic composites: room temperature instantaneous behaviour

The mechanical properties of ZrB2–30 wt-%SiC ultra high temperature ceramic composites have been studied. The composite was processed by hot pressing at 2100°C, 30 MPa and 45 min dwell time to achieve a good densification. Young’s modulus, single edge V notch beam fracture toughness, hardness, stress–strain deformation, four-point bending strength and Weibull parameters were measured. Fractography and microstructure analyses of ZrB2–30 wt-%SiC ceramic composite were also performed.

Low Temperature Bleaching of Cotton with a Novel Cationic Activator

A new cationic bleach activator (CBA) N-[4-pyridmoniomethyl)benzoyl]caprolactam bromide (PBCB) was synthesized and evaluated in a hot peroxide bleaching process. The effects of time, temperature and the concentrations of hydrogen peroxide and activator on the bleaching of cotton fabric were investigated using an orthogonal experimental method. The optimal bleaching process of PBCB-activated peroxide system was 5g/L H2O2, 0.5g/L CBA, 40°Cand 45min dwell time under neutral pH, with which a whiteness equivalent to that of the conventional-bleaching system was achieved, but the PBCB system resulted in almost no fiber damage based on fabric strength.

Microstructural stability of Ag sinter joining in thermal cycling

As a heat-resistant die attach technology processed at low temperatures, three Ag filler-based sinter joining materials have been proposed. Among these, Ag flake pastes exhibited the greatest potential. Joining was carried out by sintering Ag nanoparticles/flakes in air at 200 °C for 60 min. All of the joined samples survived up to 1,000 thermal cycles in a temperature range from −40 to 180/250 °C with a 30 min dwell time. In particular, the joining strengths with the Ag micron and, Ag nano-thick flake pastes maintained excellent strength. Neither thermal fatigue cracks nor large voids were observed in the Ag sintered layers. Thus, low-temperature and low-pressure sinter joining with Ag flakes is expected to have an application in high power semiconductor devices for ultra-high temperature operation.

Microstructure, orientation and damage evolution in SnPb, SnAgCu, and mixed solder interconnects under thermomechanical stress

In this study, the microstructure, orientation evolution and failure modes of SnPb, SnAgCu, and mixed (SnAgCu bump+SnPb paste) solder interconnects were studied by thermal cycling between 0^oC and 100^oC (with 15min dwell times and 15min ramps). The cross-sectioned ball grid array (BGA) solder interconnects were characterized by scanning electron microscopy (SEM) equipped with an electron backscattered diffraction (EBSD) detector to observe the orientation, microstructure and damage evolution under thermal cycling. The results show that the as-reflowed SnPb solder interconnect is composed of a limited number of grains with similar crystallographic orientations, while the mixed and SnAgCu solder interconnects are composed of a few @b-Sn grains or even only one grain. Significant recrystallization behavior was observed in strain concentration regions in all these interconnects after thermal cycling. Small subgrains with low angle grain boundaries could finally evolve into recrystallized grains by rotation and coalescence as thermomechanical fatigue continues. Due to strain enhanced diffusion, dramatic coarsening of Pb phase was observed in the recrystallized regions of SnPb and mixed solder interconnects; similarly, the coarsening of Ag3Sn intermetallic compound (IMC) particulates occurred in the recrystallized microstructures of mixed and SnAgCu solder interconnects after thermal cycling. Cracks were initiated and propagated mainly within the recrystallized microstructures by intergranular cracking in the solder bulk. Interphase sliding was frequently observed along the crack path of SnPb solder interconnects. Vickers microhardness results show that SnAgCu and mixed solder interconnects degrade significantly as thermal cycling continues, and hardness correlates well with the accumulated damage in these solder interconnects.

Comparative evaluation of traditional and self-priming hydrophilic resin

Background:The purpose of this study was to compare the microleakage of traditional composite (Charisma/Gluma Comfort Bond) and self-priming resin (Embrace Wetbond).Materials and Methods:Standardized Class V cavities partly in enamel and cementum were prepared in 20 extracted human premolars. Teeth were divided into two groups. Group 1 was restored with Charisma/Gluma Comfort Bond and Group 2 with Embrace Wetbond. The specimens were stored in distilled water at room temperature for 24 h and then subjected to 200 thermocycles at 5°C and 55°C with a 1 min dwell time. After thermocycling teeth were immersed in a 0.2% solution of methylene blue dye for 24 h. Teeth were sectioned vertically approximately midway through the facial and lingual surfaces using a diamond saw blade. Microleakage was evaluated at enamel and cementum surfaces using 10 × stereomicroscope. The statistical analysis was performed using Wilcoxon signed-rank test.Results:Wetbond showed less microleakage at occlusal and gingival margins as compared with Charisma/Gluma Comfort Bond and the results were statistically significant (P < 0.05).Conclusion:Class V cavities restored with Embrace Wetbond with fewer steps and fewer materials offers greater protection against microleakage at the tooth restorative interface.

Isothermal Aging Effects on the Thermal reliability Performance of Lead-Free Solder Joints

Electronics assemblies containing solder joints are often exposed to elevated temperatures for prolonged periods of time. The time-at-temperature stress impacts the overall package reliability of the assembled circuitry due to evolving materials, microstructural, and mechanical properties. It is especially important to understand the impact of isothermal aging on the long term behavior of lead (Pb)-free solder joints which operate in harsh environments. In this study, we have explored the effects of elevated temperature isothermal aging on the reliability of Sn-Ag-Cu (SAC) assemblies on board level packages. As the isothermal aging temperature increases, the Weybull characteristic lifetime for SAC 105 and 305 solder joints is drastically reduced compared to Sn-37Pb.In parallel mechanical studies on bulk solder specimens, the creep rate for SAC105, 305 rapidly increases with aging. A full test matrix with varying aging temperatures and solder alloys was considered. Package sizes ranged from 19mm, 0.8mm pitch ball grid arrays (BGAs) to 5mm, 0.4mm pitch μBGAs. The test structures were built on three different board finishes (ImSn, ImAg and SnPb). Storage condition temperatures were 25°C, 55°C, 85°C and 125°C with aging over time periods of 0, 6, and 12 months. Subsequently, the specimens were thermally cycled from −40°C to 125°C with 15 min dwell times at the high temperature. It was found that the thermal performance of lead-free fine-pitch packages significantly degrades up to 55–60% after aging at elevated temperature. The dominant failure mode can be associated with the growth of Cu6Sn5 intermetallic compounds (IMC) during the aging, particularly on the pad side.

The different pathways of spore germination and inactivation of Bacillus subtilis under high pressure and elevated temperatures

High pressure combined with elevated temperatures could produce commercially sterile and shelf stable low acid foods. Depending on the applied temperature and pressure level, bacterial endospores pass through different physiological pathways, which can induce germination or a subsequent inactivation. To improve the understanding of spore inactivation Bacillus subtilis spores and isogenic strains, which lack the gens for encoding the germinant receptors, were pressure treated. Therefore, the strains FB114 and FB115 and its wild type strain (PS832) were processed from 1s up to 24h in a pressure-temperature range of 200 – 1000MPa and 30 - 80°C under isothermal and isobaric conditions during dwell time in pressure stable ACES buffer solution. Afterwards, aliquots of each sample were mild heat treated and the released amount of DPA was quantified by HPLC, to estimate the amount of germinated spores. Further, two model approaches were tested to derive a global model from the kinetic data. It was confirmed for both FB-strains, that spore germination above 500MPa is possible without germinant receptors and even at 300MPa germination of these spores was detected. Furthermore, no spore inactivation occurred for pressures below 700MPa and temperatures up to 50°C. Whereas the behavior of PS832 was contrary, here an inactivation of more than 3 log10-cycles was detected at 300MPa after 30min dwell time at 40°C and no inactivation was observed at 550MPa, 37°C, 120min. Possibly, spores with nutrient receptors could proceed through stage II of germination at low pressure levels, whereas this germination step is retarded at higher pressures. Moreover, above 300MPa both germination mechanisms interfere with each other, whereas above 500MPa the opening of the Ca2+-DPA-channels is dominant. For all strains a strong acceleration of germination with increasing pressure and temperature was detected, whereas above a certain threshold pressure (p > 600MPa) the temperature played the dominant role for spore inactivation.

Trace elemental analysis of airborne particulate matter using dynamic reaction cell inductively coupled plasma – mass spectrometry: Application to monitoring episodic industrial emission events

The elemental composition of airborne particles is being increasingly monitored since several metals have been implicated in adverse human health outcomes and environmental deterioration while simultaneously providing clues to the identity and strength of their emission sources. However, quantification of several elements and transition metals in ambient aerosols, which are typically present only at trace levels, is fraught with interferences using quadrupole inductively coupled plasma – mass spectrometry (q-ICP-MS). We report improved measurements of key aerosol elements including Al, V, Cr, Fe, Ni, Cu, and Zn in airborne coarse particulate matter (PM 10) by exploiting ion-molecule reactions in a dynamic reaction cell (DRC) with NH 3 as the cell gas. Numerous other elements (Na, Mg, Si, K, Ca, Sc, Ti, Mn, Co, Ga, As, Se, Rb, Sr, Zr, Mo, Cd, Sn, Sb, Cs, Ba, Pb, Th, and U) and lanthanoids (Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), which are important trace components used for source apportionment studies, were also measured. Inter-laboratory comparison using sector field ICP-MS demonstrated the accuracy and precision of DRC-q-ICP-MS. This technique was used to determine the elemental composition of over 150 PM 10 samples collected from an industrialized region in Houston, TX. Samples were first digested using a combination of HF, HNO 3, and H 3BO 3 in two stages in a microwave oven each with set points of 200 °C, 1.55 MPa (225 psig), and 20 min dwell time. Trace metals were used to identify an episodic release of particles from a local source and subsequently track the atmospheric transport of the released particles. This establishes the inherent value of such measurements to developing air quality management strategies since emission events can significantly worsen air quality over a large area. Based on our findings, we recommend continuous independent monitoring of emissions to augment existing industry self-reporting regulatory requirements. Such environmental measurements will assist in establishing industrial regulatory compliance while simultaneously providing data necessary to develop scientifically defensible air quality management strategies.

Crack Development in a Low-Stress PBGA Package due to Continuous Recrystallization Leading to Formation of Orientations with [001] Parallel to the Interface

Thermal cycling was imposed on plastic ball grid array (PBGA) packages with a small die, a package design that does not impose a large strain on solder joints. Less cracking was observed after 2500 cycles from 0°C to 100°C (with 10 min dwell times and 10 min ramps) than in a prior study with a higher-stress package design, so these samples were thermally cycled (TC) to 6400 cycles to investigate the relationship between cracks, microstructure, and grain crystal orientation. Cracked joint locations within the package were identified using the dye and pry method, indicating that cracks were most often found in joints near the perimeter of the die. Using orientation imaging microscopy (OIM), cracks were observed in many joints having a variety of dominant crystal orientations where the c-axis was between 0° and ∼50° from the package interface. Continuous recrystallization processes occurred and caused gradual rotations of initial orientations that reduced the angle between the c-axis and the package interface. While cracks were observed in joints with a variety of orientations, cracks were highly correlated with recrystallized grains having the [001] c-axis nearly parallel to the interface (“red” orientations) in those joints that did not initially have this orientation.

Impact of Isothermal Aging on Long-Term Reliability of Fine-Pitch Ball Grid Array Packages with Sn-Ag-Cu Solder Interconnects: Surface Finish Effects

The interaction between isothermal aging and the long-term reliability of fine-pitch ball grid array (BGA) packages with Sn-3.0Ag-0.5Cu (wt.%) solder ball interconnects was investigated. In this study, 0.4-mm fine-pitch packages with 300-μm-diameter Sn-Ag-Cu solder balls were used. Two different package substrate surface finishes were selected to compare their effects on the final solder composition, especially the effect of Ni, during thermal cycling. To study the impact on thermal performance and long-term reliability, samples were isothermally aged and thermally cycled from 0°C to 100°C with 10 min dwell time. Based on Weibull plots for each aging condition, package lifetime was reduced by approximately 44% by aging at 150°C. Aging at 100°C showed a smaller impact but similar trend. The microstructure evolution was observed during thermal aging and thermal cycling with different phase microstructure transformations between electrolytic Ni/Au and organic solderability preservative (OSP) surface finishes, focusing on the microstructure evolution near the package-side interface. Different mechanisms after aging at various conditions were observed, and their impacts on the fatigue lifetime of solder joints are discussed.

Adhesive bonding of zirconia with single-liquid acidic primers and a tri-n-butylborane initiated acrylic resin.

The purpose of the current study was to evaluate the effect of acidic primers on the bond strength and durability of an acrylic resin luting agent to zirconia. Disk specimens were fabricated from zirconia partially stabilized with yttrium oxide (Katana, Noritake Dental Supply). The disks were treated with one of the following acidic primers: Acryl Bond (Shofu), All Bond II Primer B (Bisco), Alloy Primer (Kuraray), Estenia Opaque Primer (Kuraray), Eye Sight Opaque Primer (Kanebo), M.L. Primer (Shofu), MR. Bond (Tokuyama Dental), or Super-Bond Liquid (Sun Medical). Unprimed specimens served as the control. The disks were bonded with a tri-n-butylborane (TBB)-initiated acrylic resin. Shear bond strengths were determined both before and after 10,000 thermocycles (5°C and 55°C, 1 min dwell time each). The pre-thermocycling bond strength ranged from 0.7 MPa to 30.8 MPa, whereas post-thermocycling bond strength varied from 0.3 MPa to 17.6 MPa. The significantly highest post-thermocycling bond strength was obtained when using the Alloy Primer and Estenia Opaque Primer agents. Within the limitations of the current experiment, it can be concluded that application of either the Alloy Primer or the Estenia Opaque Primer, both of which contain 10-methacryloyloxydecyl dihydrogen phosphate (MDP), is recommended for bonding the Katana zirconia material with TBB-initiated acrylic resin.

Very high temperature creep behavior of a single crystal Ni-based superalloy under complex thermal cycling conditions

Very high temperature thermal cycling has been performed on the single crystal superalloy MC2 to evaluate the effect of periodic overheating on creep behavior. The experiments consist of alternately performing 1 min dwell time at 1100°C and 1150°C for every 15 min during creep test at 1050°C/120 MPa. Both thermal cycling and prior γ′-rafting appear to be deleterious to the cyclic creep properties. The observed non-isothermal creep behavior is correlated with γ′-dissolution/coalescence processes, especially during overheatings where γ′ micro-rafts seem to play a significant role.

The role of creep and fatigue in determining the high-temperature behaviour of AISI H11 tempered steel for aluminium extrusion dies

The present study was aimed to analyze the effect of loading cycles on the behaviour of the AISI H11 tool steel commonly used for aluminium extrusion dies working at high temperatures and under high, cyclic stresses. A technological test method in which the specimen geometry resembled the mandrel of a hollow extrusion die was developed. Finite element analyses were performed to aid in determining specimen geometry and dimensions as well as the levels of stress to be applied to the specimen so as to replicate the conditions typically encountered by industrial hollow extrusion dies. Tests were performed on a Gleeble thermomechanical simulator by heating the specimen using Joule's effect and by applying loading for up to 6.30 h or till specimen failure. Displacements during the tests at 380, 490, 540 and 580 °C and under the average stresses of 400, 600 and 800 MPa were determined. The specimens were tested under creep (with the load held at a fixed value), fatigue (cyclic loading) and creep–fatigue (cyclic loading with a 3 min dwell-time) loading, thereby allowing a direct comparison between different deforming mechanisms. The results showed that the test could physically simulate the cyclic loading on the hollow die during aluminium extrusion and that the creep condition represented the most severe working condition. In addition, the tests could reveal the interaction between creep and fatigue mechanisms.

Effects of reflow profile and thermal conditioning on intermetallic compound thickness for SnAgCu soldered joints

PurposeThe purpose of this paper is to investigate the effects of reflow time, reflow peak temperature, thermal shock and thermal aging on the intermetallic compound (IMC) thickness for Sn3.0Ag0.5Cu (SAC305) soldered joints.Design/methodology/approachA four‐factor factorial design with three replications is selected in the experiment. The input variables are the peak temperature, the duration of time above solder liquidus temperature (TAL), solder alloy and thermal shock. The peak temperature has three levels, 12, 22 and 32°C above solder liquidus temperatures (or 230, 240 and 250°C for SAC305 and 195, 205, and 215°C for SnPb). The TAL has two levels, 30 and 90 s. The thermally shocked test vehicles are subjected to air‐to‐air thermal shock conditioning from −40 to 125°C with 30 min dwell times (or 1 h/cycle) for 500 cycles. Samples both from the initial time zero and after thermal shock are cross‐sectioned. The IMC thickness is measured using scanning electron microscopy. Statistical analyses are conducted to compare the difference in IMC thickness growth between SAC305 solder joints and SnPb solder joints, and the difference in IMC thickness growth between after thermal shock and after thermal aging.FindingsThe IMC thickness increases with higher reflow peak temperature and longer time above liquidus. The IMC layer of SAC305 soldered joints is statistically significantly thicker than that of SnPb soldered joints when reflowed at comparable peak temperatures above liquidus and the same time above liquidus. Thermal conditioning leads to a smoother and thicker IMC layer. Thermal shock contributes to IMC growth merely through high‐temperature conditioning. The IMC thickness increases in SAC305 soldered joints after thermal shock or thermal aging are generally in agreement with prediction models such as that proposed by Hwang.Research limitations/implicationsIt is still unknown which thickness of IMC layer could result in damage to the solder.Practical implicationsThe IMC thickness of all samples is below 3 μm for both SnPb and SAC305 solder joints reflowed at the peak temperature ranging from 12 to 32°C above liquidus temperature and at times above liquidus ranging from 30 to 90 s. The IMC thickness is below 4 μm after subjecting to air‐to‐air thermal shock from −40 to 125°C with 30 min dwell time for 500 cycles or thermal aging at 125°C for 250 h.Originality/valueThe paper reports experimental results of IMC thickness at different thermal conditions. The application is useful for understanding the thickness growth of the IMC layer at various thermal conditions.

An analysis of trip chaining among older London residents

This paper examines the trip chaining complexity of individuals in London. We adopt two definitions of trip chaining. One based on a 30 min dwell time rule and a second based on home-to-home tours. Our focus is on the complexity of the trip chains as measured by the number of stops on a given tour. The analysis uses the London Area Travel Survey and examines the factors associated with trip chaining for people aged over 65. A comparison with those aged under 65 reveals that older people on average make more complex tours when the 30 min dwell time rule is applied as opposed to when the home-to-home definition is applied. It is further shown that the anchor points of the tour are critical for determining tour complexity, suggesting the usefulness of the 30 min definition. Our analysis also suggests that older people reduce total home-to-home tours by combining different trips into single tours. Through descriptive analysis and ordered probit regression models we examine how reported levels of disability affect their trip chaining and we examine household demographic characteristics as well as proxies for accessibility, such as local population density. The analysis shows that disabilities do not necessarily lead to reduced tour complexity except when walking difficulties become so severe that independent travel is not possible. We suggest that tour complexity might further increase in the future, for example as the spread of mobile phone usage appears to have a further positive influence on tour complexity. Implications for land-use and transport planning are discussed.

Evaluation of substrates for 90° peel adhesion—A collaborative study. I. Medical tapes

As part of a method development for peel testing, an interlaboratory comparison among Food and Drug Administration-Center for Drug Evaluation and Research, Food and Drug Administration-Center for Devices and Radiological Health and Southwest Research Institute was conducted using medical tapes. The aim was to determine which readily available substrate [stainless steel (SS), high density polyethylene (HDPE) or Vitro-Skin(R)] would best distinguish among various medical tapes. Five medical tapes (3M 1523, 3M 1525L, 3M 1776, Mepiform(R) and Mediderm(R) 3505) were evaluated on four different substrates (SS, HDPE, Vitro-Skin, and human cadaver skin) using the following peel parameters: approximately 3 min dwell time, 90 degrees peel angle, and 300 mm/min peel rate. No substrate mimics cadaver skin for all five tapes. SS had the best ability to distinguish among the medical tapes. Overall, for quality control purposes (yielding good discrimination and precision), SS would be the optimal substrate.

Efficacy of Single-Session Percutaneous Drainage and 50% Acetic Acid Sclerotherapy for Treatment of Simple Renal Cysts

To evaluate the efficacy and long-term results of single-session 50% acetic acid sclerotherapy for the treatment of simple renal cysts, and to compare the therapeutic results of 5 and 20 min sclerosant dwell techniques. During the past 9 years, 50% acetic acid sclerotherapy was performed on 67 cysts in 66 patients. An acetic acid volume corresponding to a mean of 23% of the aspirated cyst volume was injected into the cysts. A 20 min dwell time with position changes was performed in 32 cysts (31 patients; group I) and 8% of volume for a 5 min dwell time in 35 cysts (35 patients; group II). Three- and 6-month sonographic or CT follow-up was performed for a minimum of 1 year. Complete regression was defined as no remaining cyst measurable on sonography with or without a scar at the renal cortex. Partial regression was defined as a decreased cyst volume compared with that before sclerotherapy. The Mann-Whitney U-test was used to compare the therapeutic results between the two groups. For 67 simple renal cysts, complete regression on follow-up was observed in 21 of 32 cysts (66%; group I) and 22 of 35 cysts (63%; group II); the remaining 24 cysts all showed partial regression. The partial reduction rate of the cyst's volume was 97.4% (91.3-99.4%) in group I and 96.9% (90.8-99.5 %) in group II. There were no procedure-related major complications, and no statistically significant differences in the complete regression and partial volume reduction rates between the two groups (p > 0.05). Fifty percent acetic acid is an effective and safe sclerosing agent for simple renal cysts. Fifty percent acetic acid sclerotherapy with a 5 min sclerosant dwell time, using a volume of about 10% of the aspirated volume, is sufficient for satisfactory results of simple renal cyst sclerotherapy.

Microwave-assisted extraction of rare earth elements from petroleum refining catalysts and ambient fine aerosols prior to inductively coupled plasma-mass spectrometry

A robust microwave-assisted acid digestion procedure followed by inductively coupled plasma-mass spectrometry (ICP-MS) was developed to quantify rare earth elements (REEs) in fluidized-bed catalytic cracking (FCC) catalysts and atmospheric fine particulate matter (PM 2.5). High temperature (200 °C), high pressure (200 psig), acid digestion (HNO 3, HF and H 3BO 3) with 20 min dwell time effectively solubilized REEs from six fresh catalysts, a spent catalyst and PM 2.5. This method was also employed to measure 27 non-REEs including Na, Mg, Al, Si, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Zr, Mo, Cd, Cs, Ba, Pb and U. Complete extraction of several REEs (Y, La, Ce, Pr, Nd, Tb, Dy and Er) required HF indicating that they were closely associated with the aluminosilicate structure of the zeolite FCC catalysts. Internal standardization using 115In quantitatively corrected non-spectral interferences in the catalyst digestate matrix. Inter-laboratory comparison using ICP-optical emission spectroscopy (ICP-OES) and instrumental neutron activation analysis (INAA) demonstrated the applicability of the newly developed analytical method for accurate analysis of REEs in FCC catalysts. The method developed for FCC catalysts was also successfully implemented to measure trace to ultra-trace concentrations of La, Ce, Pr, Nd, Sm, Gd, Eu and Dy in ambient PM 2.5 in an industrial area of Houston, TX.

Microstructure and mechanical properties of an HfB 2 + 30 vol.% SiC composite consolidated by spark plasma sintering

An ultra-high-temperature HfB 2–SiC composite was successfully consolidated by spark plasma sintering. The powder mixture of HfB 2 + 30 vol.% β-SiC was brought to full density without any deliberate addition of sintering aids, and applying the following conditions: 2100 °C peak temperature, 100 °C min −1 heating rate, 2 min dwell time, and 30 MPa applied pressure. The microstructure consisted of regular diboride grains (2 μm mean size) and SiC particulates evenly distributed intergranularly. The only secondary phase was monoclinic HfO 2. The incorporated SiC particulates played a key role in enhancing the sinterability of HfB 2. Flexural strength at 25 °C and 1500 °C in ambient air was 590 ± 50 and 600 ± 15 MPa, respectively. Fracture toughness at room temperature (RT) (3.9 ± 0.3 MPa √m) did not decrease at 1500 °C (4.0 ± 0.1 MPa √m). Grain boundaries depleted of secondary phases were fundamental for the retention of strength and fracture toughness at high temperature. The thermal shock resistance, evaluated through the water-quenching method, was 500 °C.