Translucent Alumina Research Articles

Effect of different additives and sintering regimes on the optical properties of DLP printed translucent alumina

Additive manufacturing of components with enhanced characteristics to combine the geometrical flexibility of layer-by-layer manufacturing and the remarkable thermomechanical and optical properties of advanced materials has become a focus for research in the last decades. In this investigation, in-house developed Al2O3 slurries mixed with either MgO or MgAl2O4 were digitally light processed and the resulting green parts underwent slow thermal debinding at low heating rates to produce dense and stable brown samples, which were subsequently subjected to different sintering regimes to obtain translucent samples. Air sintering trials were conducted at 1700 °C and the effect of different heating rates and holding times at the sintering temperature were investigated. In addition, vacuum sintering at 1600 °C was conducted for the best conditions to assess the effect of an oxygen-reduced atmosphere. Geometrical dimensions and density data were collected at the brown and fired stages to observe their evolution through the process with the help of high-magnification optical microscopy. Scanning electron microscope (SEM) images of the sintered samples were collected to observe and characterise the obtained microstructure. In addition, X-ray diffraction (XRD) was used to identify the crystal structure and assess the insurgence of secondary phases. Finally, total transmittance and in-line transmittance were measured to evaluate the translucency level achieved by the produced sintered samples. Translucent alumina samples exhibiting a maximum total transmittance of 90.4% and a maximum real in-line transmittance (RIT) of 16.9% in the visible and near-infrared regions were fabricated.

Influence of particle size and aggregates on translucent alumina ceramics fabricated by gelcasting

Three types of alumina powders and a polymer electrolyte Isobam system were used to fabricate translucent alumina ceramics via gelcasting, and each powder had the highest attainable solid loading. The effects of alumina particle size on slurry dispersion, microstructures and properties of green bodies and ceramics were investigated. All of the high-solid loading slurries contained strong aggregates, which could bridge with each other to form weak agglomerates. The smaller the initial particle size was, the more the contained particle number per aggregate. In addition, the smaller the aggregate size was, the lower the sintering activation energy of the ceramic and, finally, the higher the relative density. The initial particle size of the powder may not be the critical factor determining ceramic sintering, but the aggregate size and effective volume fraction of the aggregates may be the critical factors. Finally, the in-line transmittance of the ceramic with a higher density reached 22% at a wavelength of 650 nm and a thickness of 0.8 mm.

Highly textured 3D-printed translucent alumina through pressure-assisted sintering

Templated grain growth is a promising method for the fabrication of translucent alumina ceramics and a possible way to circumvent light scattering due to birefringence. In the present paper the effect of temperature, heating rate and dwell time on grain size, orientation, hardness and in-line transmittance is investigated on 3D-printed textured alumina. The samples were prepared by lithography-based ceramic manufacturing (LCM) of suspensions with fine-grained alumina, seeded by high aspect ratio templates and sintered by spark plasma sintering (SPS). A fully developed textured microstructure was achieved at 1600 °C for 1 h, whereas sufficient in-line transmittance of 54.6 % at 550 nm was achieved at 1700 °C for 3 h, albeit at the cost of considerable grain growth and loss of microstructural texturing.

Impact of sintering thermal history on MgO‐doped alumina: Reduction of grain growth constant through high heating rates

AbstractThis work examines the impact of sintering thermal history on kinetics of grain growth for 1264 ppm (0.1 wt%) MgO‐doped alumina. Results suggest that sintering with a high heating rate in the order of ∼103°C min−1 retards grain growth rate. By contrast, a slow heating rate of 5°C min−1 resulted in an increase in grain growth constant by a factor of ∼4. The segregation of Mg to grain boundaries, favored by short sintering duration, is considered to be one of the possible factors contributing to the retarded grain growth rate for the rapidly heated compacts. Furthermore, it is proposed that the simultaneous combination of retarded grain boundary migration rate and limited surface‐diffusion‐driven grain coarsening, promoted by rapid heating, enables pressure‐less fabrication of translucent alumina without a need for vacuum or hydrogen atmosphere. In fact, it is demonstrated that weakly translucent alumina can be produced at relatively low temperatures of 1475–1550°C by sintering with a high heating rate (herein, ∼103°C min−1) in ambient atmosphere.

PRODUCTION OF TRANSLUCENT ALUMINA CERAMICS BY TAPE CASTING

Alumina powders are utilized in many industries like electronics, metallurgy optoelectronics, and fine ceramic composites. In this study, non-aqueous-based tape casting of alumina was carried out in a non-continuous single-blade tape casting machine. A slurry of alumina powder in a solvent was cast on a stationary surface with additives like dispersants, binders, and plasticizers. After the green tapes were dried, they were sintered to obtain the final desired shape. The hydraulic pressing method was used to increase the density of the alumina tapes. Layered alumina ceramics were produced at 5 different temperatures by pressureless sintering method (1450°C, 1500°C, 1550°C, 1600°C and 1650°C) for 5 h to observe the effect of sintering temperature. Results revealed the sintering conditions for obtaining translucent alumina, with the utilized slurry composition. Translucent alumina ceramics have gained importance as parts of semiconductor devices, substrates for electric parts, as heat/corrosion materials in reaction tubes and crucibles, and are used in medical equipment.

FOREWORD FROM THE EDITOR-IN-CHIEF

Welcome to the first issue for the year 2023, a significant milestone for us as we celebrate the centennial of the Republic of Türkiye, the quarter-millennial anniversary of the Turkish Naval Academy (TNA), and the vicennial celebration of the Journal of Naval Sciences and Engineering (JNSE). These historical connections symbolize the remarkable progress achieved by our country, academy, and journal.
 
 In this issue, we are delighted to present five research papers that contribute to the advancement of naval sciences and engineering. These papers cover a wide range of topics, including the production of translucent alumina ceramics by tape casting, spectroscopic investigation of argon glow discharge in a plasma medium, thermal analysis of XLPE insulated submarine cables for different loading conditions, scale effects on the linear hydrodynamic coefficients of DARPA SUBOFF, and numerical investigation of HVAC systems in a naval ship compartment.
 
 We would also like to remind you that starting from September 1, 2022, we have been providing Digital Object Identifiers (DOIs) for accepted papers, ensuring their persistent identification. Additionally, we have implemented a new publishing process where papers are made available online before the printed hardcopy format, allowing for broader accessibility and faster dissemination of research findings.
 
 Our November 2023 issue is dedicated to highlighting cutting-edge advancements and research in naval technology and engineering. We have published a call for papers online, encouraging submissions on various topics, including 3D modeling, digital twin technology, IoT applications, 3D printing, virtual and augmented reality, high-performance computing, automation with robots, drones, and autonomous vehicles, shipyard construction and maintenance cost analysis, lean production methodologies, corrosion protection, wearable technologies for personnel monitoring, and big data analytics in preventive maintenance. We eagerly invite researchers and scholars to submit their work, contributing to this issue. The submitted papers in these areas will be given special attention alongside the journal's regular coverage, offering a comprehensive exploration of the latest advancements in naval sciences and engineering.
 
 We would like to extend our heartfelt gratitude to the authors for their valuable contributions to this issue, as well as our dedicated editorial and advisory board members for their expertise and guidance. We are also immensely grateful to the diligent reviewers who have generously contributed their time and expertise to ensure the quality and rigor of the papers published in this issue. Your insightful feedback and constructive criticism have played a vital role in shaping the content of this journal. 
 
 Additionally, we would like to express our appreciation to our readers for their continued support and trust in JNSE. Thank you all for your invaluable contributions.
 
 Fatih ERDEN, Ph.D.
 Editor-in-Chief

DLP of Translucent Alumina: In-Depth Investigation on Slurry Development and Debinding Regimes

Exploring the feasibility of producing near-net-shape components with advanced properties and geometrical features via 3D printing has incrementally become the research focus of various studies. Digital light processing (DLP) technology can manufacture complex-structured components for various technical applications. The aims of this research were to investigate Al2O3 ceramic slurry preparation procedures to identify the ideal components to add to an in-house-developed ceramic slurry, to determine the optimal DLP printing parameters and conditions while understanding their effect on the green part properties and to evaluate the appropriate debinding regime to achieve fully dense crack-free fired parts capable of exhibiting translucent behaviours. The slurry obtained from the ball-milled powder at 800 rpm for 1 h, together with 2 wt.% BYK-145 as a dispersant and the highest achievable solid loading of 85 wt.%, showed the desired rheological and photopolymerisation properties. Full-factorial design of experiments (DOE) was employed to study the impact of the printing parameters on the density and the dimensions of the samples. Different debinding regimes were investigated and it was proven that the lowest debinding heat rate (0.2 °C/min) and longer holding times helped to reduce defects and promote densification (>99.0%), providing optimal grounds to obtain translucent fired parts.

Prefabricated crack propagation in translucent alumina ceramic sheets during flame thermal shock

The prefabricated cracks propagation of ceramic under flame thermal shock was studied by real-time observation. The experimental results show that wing cracks are generated from both tips of the prefabricated crack, and the complete failure of the samples is a high-speed process. The crack propagation speed is fast at first and then gradually slows down. The maximum propagation speed of the near flame tip increases with the prefabricated crack distance to the heated surface, while an inverse trend is observed for the far flame tip. The effect of prefabricated crack angle on thermal shock crack propagation was also considered. In addition, numerical simulations based on dynamic fracture mechanics were carried out to reproduce the entire cracking process. To this end, the relation between the dynamic stress intensity factors and the crack growth speed in the speed range of 100 m/s to 2600 m/s was identified by combing experiments and numerical simulations. The simulation results and the above-mentioned relation are consistent with the experimental observations. The numerical model can faithfully reproduce the crack evolution process of the thermal shock, which is difficult to observe in an experiment due to the high speed of the cracking process.

Ultra-fast High-temperature Sintering (UHS) of translucent alumina

Pressure-less consolidation of transparent alumina requires several hours dwelling at high temperature (1600 °C) under a strongly reducing atmosphere (hydrogen). Here, we report on the ultrafast consolidation of translucent alumina ceramics at 1360 °C. Transparency was promoted by synergistic effects of high initial green density (62.7%) and rapid sintering using Ultra-fast High-temperature Sintering (UHS) technique. The proposed approach, using a heating rate of 430 °C/min and dwelling time of 15 min, resulted in ultra-fine-grained translucent alumina ceramics with a grain size of 0.39 μm, and an in-line transmittance of 28.7% at a wavelength of 700 nm. For comparison, conventionally fired counterparts were opaque due to their incomplete densification and pore coalescence.

Water‐assisted cold isostatic pressing to enhance sinterability of alumina ceramics

AbstractWater‐Assisted cold isostatic Pressing (WAP) enabled pressure‐less sintering of fine grained (0.6 μm) and nearly full dense (99.3%) weakly translucent alumina ceramics. As a term of comparison, Dry Pressed (DP) samples, prepared under identical processing conditions, maintained an opaque appearance, and their relative density did not exceed 97.7%. The enhanced compaction of WAP allowed to lower the sintering temperature down to 1350°C, which is approximately 100–150°C lower than that of DP powder. WAP samples resulted in an apparent activation energy for sintering (640 kJ/mol) matching literature values, confirming that the enhanced sinterability was attributed to an improved green density rather than a change in sintering mechanisms.

Cracking in the translucent alumina ceramic during flame thermal shock

Crack measurement after thermal shock is usually considered as a replacement because real-time observation of thermal shock experiments is difficult to achieve. This paper presents an experimental approach for real-time displaying thermal shock cracking using oxygen-acetylene flame and high-speed imaging of translucent ceramic. We capture the crack propagation process, calculate the crack propagation speed, discuss the effect of sample size and flame heat flux on the crack propagation, and analyze the difference between the crack propagation under cold shock and hot shock. This paper further improves the mechanism of thermal shock damage of ceramic materials.

High‐Quality Translucent Alumina Ceramic Through Digital Light Processing Stereolithography Method

Herein, translucent alumina ceramics with fine complex geometric shapes are successfully fabricated by a digital light processing (DLP) process followed by vacuum sintering. The interlayer bonding condition is the key for preparing high‐quality translucent alumina for this layer‐by‐layer additive manufacturing (AM) method. The curing characteristics of alumina suspensions are carefully investigated by curing depth and polymerization conversion behavior. It is clear that the residual stress resulted from the polymerization process is more critical than the curing depth for selecting optimum curing time. Finally, high‐quality translucent alumina ceramic and complex structures are realized using this novel AM technique. It reveals that DLP 3D printing method can be a promising technology for the fabrication of transparent ceramics.

Light scattering models for describing the transmittance of transparent and translucent alumina and zirconia ceramics

The original Apetz-van-Bruggen model for predicting the transmittance of dense polycrystalline ceramics with randomly oriented birefringent crystallites is shown to be incorrect. In a correct Apetz-van-Bruggen equivalent composite model (ECM) the volume fraction must be 1/3 instead of 1/2 and the factor 2/3 in front of the maximum birefringence must be abandoned. This ECM is in good agreement with the simplified version of Pecharromán’s dense polycrystalline model (DPM) and can be improved by replacing the Jobst approximation (i.e. the large-size limit of the Rayleigh-Gans approximation) by the van-de-Hulst approximation. Similarly, a new DPM is proposed that combines the van-de-Hulst approximation with Pecharromán’s texture function. However, all these models fail for small grains. Therefore two new models (infimum-supremum-based ECM and DPM) are proposed, which combine the full Rayleigh-Gans approximation (for small grains) with the van-de-Hulst approximation (for large grains) and provide (for all grain sizes) predictions almost identical to Mie theory.

Optical and mechanical properties of mn-doped transparent alumina and their comparison with selected rare earth and transient metal doped aluminas

Mn-doped translucent alumina prepared in this work was characterised in terms of densification process, microstructure, optical and mechanical properties. The manganese effect was compared with the influence of chromium, erbium and europium in previously prepared doped transparent alumina. Contrary to them, manganese accelerates both densification and grain growth. Real in line transmittance (RIT) at 632.8 nm and transmission/diffuse reflectance spectra in UV-VIS-NIR range of Mn-doped alumina were measured. The lower RIT values and rapidly decreased transmittance/reflectance below 800 nm are caused by relatively high absorption of light by Mn species and by increased grain size. The deep red emission of Mn4+ centres that almost fade at higher manganese concentration was indicated in photoluminescence spectra, with a decay time of ∼670 μs. The dopant introduction in all cases resulted in an increase in hardness of doped alumina up to 20 % compared to undoped alumina and a decrease in related fracture toughness.

Multi-Wire Sawing of Translucent Alumina Ceramics

Multi-wire sawing has emerged as the leading technology in wafer production for a variety of semiconductor materials. This study investigates the process stability and efficiency of conventional semiconductor multi-wire slurry saws in routinely machining translucent, high-density alumina ceramics. The brittle and fine-grained translucent alumina ceramics with extreme hardness and wear resistance represents a major challenge for the process. The alumina ceramic substrates are used for sensor applications, energy storage technology and applications in power electronics. An ideal adaptation of the sawing process parameters to the workpiece properties guarantees the efficiency of the slurry sawing process and the quality of the ceramic wafers. An indicator of the efficiency and cutting ability of the sawing process is the size of the bow of the wire web. The first time was shown that the wire bow can be used for the characterization of the sawing processes for hard and brittle technical ceramics. It was found that a longer workpiece length, a higher number of wafers and stronger abrasive wear lead to an increased size of the bow. The rocking frequency has no measurable influence on the size of the bow. Knowledge of these relations is an extremely valuable tool in the sawing process development for hard and brittle technical ceramics.

Strengthening of alumina ceramics under cold compression

High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al2O3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure.

Effect of ZrO2 and MgO doping on microstructure and properties of translucent alumina fabricated by rapid vacuum sintering

Translucent alumina doped with ZrO2 and MgO was fabricated by two-step vacuum sintering (1475[Formula: see text]C/30[Formula: see text]min followed by 1320[Formula: see text]C/20[Formula: see text]min) with a high heating rate (80[Formula: see text]C/min). Densification, grain size, phase composition, mechanical properties and translucency of the alumina were investigated. The results indicated that co-doping of ZrO2 and MgO showed a synergetic effect on grain refinement and densification process. On one hand, the solubility of MgO in alumina was increased by the ZrO2 additive, which was favorable for the densification. On the other hand, more favorable ZrO2 accommodation sites existed at the alumina grain boundaries created by the MgO, effectively decreasing the grain size. Finally, the sample with porosity of 0.5% and mean grain size of 1.21[Formula: see text][Formula: see text]m was obtained at a co-doped content of 0.35[Formula: see text]wt.% of ZrO2 and MgO (mass ratio of 1:1). The co-dopants led to the enhanced hardness and bending strength as well as a favorable fracture toughness of the translucent alumina, when compared with the MgO single-doped and doping free samples. Moreover, total transmission of as high as 51% and good translucency was also obtained. The improved properties widened the prospect of the translucent alumina used as dental restorative materials.

Pressure filtration assisted gel casting in translucent alumina ceramics fabrication

High density and homogeneous green body is important for fabricating transparent ceramics. Inspired by the improved samples’ homogeneity in slip casting, we introduced low pressure filtration to gel casting to further increase the relative density and homogeneity of the green body and to shorten the drying time. The effect of pressure filtration on drying characteristics, bulk density and porosity of the pre-sintered bodies, and the microstructure and optical transmittance of the resultant ceramics were intensively investigated. The results showed that pressure filtration can reduce the drying time by 27.3%, improve the density of the pre-sintered bodies and reduce the residual pores of the sintered ceramics. The in-line transmittance of the translucent alumina prepared by the pressure filtration assisted gel casting was 32.7% at 600 nm with a thickness of 1 mm, which is about 6% higher than that without pressure filtration. In principle, this process can be applied to fabricate any other high-performance ceramic.

Effects of Sintering Aids (Y3+, La3+ and Mg2+) on the Optical Transmittance of Translucent Alumina Ceramics

Effects of Sintering Aids (Y³⁺, La³⁺ and Mg²⁺) on the Optical Transmittance of Translucent Alumina Ceramics

Surface modification of alumina powder particles through stearic acid for the fabrication of translucent alumina ceramics by injection molding

In this paper, we present an improved injection molding technique for the fabrication of translucent high-quality alumina ceramics. In contrast to the traditional approach, the stearic acid (SA) was already introduced during a ball milling of the ceramic powders, prior to the blending process, to modify the surface of the alumina powders. According to the analysis based on an alumina-paraffin oil model system, contact angle measurements using polar solvents, as well as high-resolution transmission electron microscopy investigations, 2wt% of SA was determined as the optimal amount of SA to be added to the powder. Furthermore, the surface modification mechanism was found to be based on an esterification reaction. The results demonstrate that the surface modification could effectively inhibit the agglomeration of the particles during the injection molding process, allowing for the fabrication of dense translucent pore-free alumina ceramics. The surface modification through SA has found its way into bio-medical applications and ceramic crafts.