Advanced Engineering MaterialsVolume 16, Issue 6 p. 755-759 Communication Mechanical Properties of Hot Isostatically Pressed Li0.35La0.55TiO3 Christopher Cooper, Christopher Cooper U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorAnthony C. Sutorik, Anthony C. Sutorik U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorJared Wright, Jared Wright U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorE. Arthur Luoto III, E. Arthur Luoto III U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorGary Gilde, Gary Gilde U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorJeff Wolfenstine, Corresponding Author Jeff Wolfenstine jeffrey.b.wolfenstine.civ@mail.mil U.S. Army Research Laboratory, RDRL SED C, 2800 Powder Mill Road, Adelphi, MD, 20783 USASearch for more papers by this author Christopher Cooper, Christopher Cooper U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorAnthony C. Sutorik, Anthony C. Sutorik U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorJared Wright, Jared Wright U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorE. Arthur Luoto III, E. Arthur Luoto III U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorGary Gilde, Gary Gilde U.S. Army Research Laboratory, RDRL WMM E, Building 4600, Aberdeen Proving Ground, Aberdeen, MD, 21005 USASearch for more papers by this authorJeff Wolfenstine, Corresponding Author Jeff Wolfenstine jeffrey.b.wolfenstine.civ@mail.mil U.S. Army Research Laboratory, RDRL SED C, 2800 Powder Mill Road, Adelphi, MD, 20783 USASearch for more papers by this author First published: 28 March 2014 https://doi.org/10.1002/adem.201400071Citations: 11 The manuscripts was modified in September 2014 after initial online publication. Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract The mechanical properties of polycrystalline Li0.35La0.55TiO3 (LLTO) are investigated after sintering followed by hot isostatic pressing (HIP). The elastic modulus (223 GPa), hardness (Knoop hardness 7.2 GPa), and flexure strength (252 MPa) values for HIP LLTO are in general higher than those for other solid Li-ion conductors thus, making it an excellent choice as a Li-ion conducting solid electrolyte based on mechanical properties. Citing Literature Volume16, Issue6Special Issue: Advanced Ceramics and Coating ProcessingJune 2014Pages 755-759 RelatedInformation

The strength of two coarse‐grained (grain size > 200 μm) cubic ceramics, a magnesium aluminate spinel (MgAl2O4) and an AlON , along with a fine‐grained (1.5 μm) MgAl2O4, was determined by conducting a series of four‐point and equibiaxial flexure tests on specimens of different sizes. Weibull strength size scaling revealed a linear relationship on a log–log plot between average flexure strength and effective specimen area for the fine‐grained spinel, but a nonlinear relationship for both coarse‐grained materials. Initial fractography showed that each material had a single flaw population limiting the strength over the entire specimen size range, which does not account for the nonlinear size scaling relationship in the two coarse‐grained materials. However, further fractography revealed that in both materials there was an initial flaw and a critical flaw. The former appears to be machining/polishing damage that started the fracture process while the latter was a cleaved grain in AlON or a cracked grain boundary in the HP/HIP spinel that lead to fracture of the specimen. The difference between the initial and critical flaw size coupled with a detailed analysis of the strength as a function of test specimen thickness accounted for the nonlinear strength size scaling relationship. As a result, strength values obtained using thin test specimens can lead to an erroneous strength prediction for large components made of these ceramics. The implication of these findings is that strength tests must be conducted using appropriately thick specimens to obtain a representative strength value. If appropriately thick specimens cannot be tested, then fractography must be conducted to determine the flaw size. If the flaw size is sufficiently large, compared with the specimen thickness, then the strength must be adjusted according to a stress field correction factor to obtain a more accurate strength value.

This chapter contains sections titled: Introduction Experimental Results and Discussion Conclusion Acknowledgement

Magnesium aluminate spinel solid solutions with the alumina‐rich compositions MgO·2Al2O3 and MgO·2.5Al2O3 have been prepared as polycrystalline ceramics with average in‐line transmissions at 550 nm of 85.5 ± 0.3% and 80.9 ± 0.4%, respectively. Starting powders are prepared from combinations of high purity Mg(OH)2 and γ‐Al2O3 thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The optimum amount of LiF added varies with the alumina composition of the spinel solid solution. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa in Ar. Both compositions exhibit exaggerated grain growth with average sizes well over 500 μm. Knoop hardness measurements are 11.2 ± 0.3 GPa for MgO·2Al2O3 and 11.0 ± 0.4 GPa for MgO·2.5Al2O3.

We report what we believe to be the first preparation and optical spectroscopy of Er3+ doped into bulk AlN ceramic. The material was prepared via hot press sintering of AlN with Er2O3 and [NH4][ErF4], which yielded fully dense, translucent, hexagonal AlN. The Er3+ concentration is a small fraction of a percent, and resides in multiple sites, with one type of center dominant. A number of the energy levels of Er3+ are identified for this center. The temperature dependent fluorescence lifetime is probably radiative, and on that basis the stimulated emission and absorption cross section spectra are inferred for the 4I13/2 ↔ 4I15/2 transitions.

Magnesium aluminate spinel with the alumina rich composition MgO · 1.5 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in‐line transmission at 550 nm of 83.3 ± 0.9% and >80% throughout the visible spectrum. This finding significantly increases the compositional range over which polycrystalline magnesium aluminates can be prepared as fully dense ceramics with high transparency to visible light. Starting powders are prepared from combinations of high purity Mg(OH)2 and γ‐Al2O3 thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load, followed by hot isostatic pressing at 1850°C under 200 MPa Ar. The crucial parameter for forming highly transparent MgO · 1.5 Al2O3 ceramic from this procedure is to hold the amount of LiF to 0.25 wt%.

Solid solution magnesium aluminate spinel with the alumina-rich composition MgO center dot 1.2 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in-line transmission at 550 nm of 84.8 +/- 2.7% and >82% throughout the visible spectrum. Starting powders are prepared from mixtures of high purity Mg(OH)(2) and gamma-Al2O3 thoroughly mixed in an aqueous slurry. Water is removed by rotary evaporation. The solids are collected, dried, calcined, mixed with LiF (as a sintering aid), and sieved. The powders are sintered into dense ceramics by hot pressing at 1600 degrees C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850 degrees C under 200 MPa Ar. Final grain sizes ranged between 300 and 1000 mu m. Samples exhibited flexural strength of 176.8 +/- 46.2 MPa; hardness of 12.3 +/- 0.2 GPa; and elastic modulus of 292.9 +/- 7.5 GPa. Control samples of stoichiometric magnesium aluminate spinel (MgO center dot Al2O3) were prepared with the same procedure and exhibited comparable values for transmission and physical properties.

The lattice and total Li+-ionic conductivity of Li0.29La0.57TiO3 ceramic (LLTO) sintered at 1200 °C were determined as functions of powder calcination temperature and sintering duration, and these results were correlated with the relative degrees of Li+-ordering, Li-content, grain size, and bulk density to assess the relative impact of these parameters on material performance. Under all conditions, LLTO formed with a high degree of tetragonal superstructure to its perovskite related framework, and the lattice conductivity closely followed the relative amounts of the superstructure, as evaluated via determination of the sample ordering parameter from X-ray diffraction data. LLTO powders that were calcined at 900 °C for 1 h and sintered at 1200 °C for 6 h gave lattice conductivity values (~1.14 × 10−3 S cm−1) comparable within the highest ranges reported in the literature. This coincided with the lowest degree of tetragonal superstructure formation, and it was also found to be largely independent of the values of Li-content measured on sintered ceramic despite significant Li2O volatilization at longer sintering times (up to 23 % after 12 h at 1200 °C). Samples of LLTO powder that were calcined at 1100 °C and sintered at 1200 °C for 12 h resulted in the highest total Li-ion conductivity value ~6.30 × 10−5 S cm−1. The total conductivity of LLTO varied inversely with grain size when the grains were <20 μm but was insensitive to that parameter above that size threshold. The strongest influence on total conductivity was primarily the bulk ceramic density. It was estimated from measured values that as the bulk ceramic density approached the full theoretical value for LLTO the total conductivity could near the lattice conductivity of ~1.2 × 10−3 S cm−1.

Magnesium hydroxide, Mg ( OH ) 2 (both 99% and 99.99% purity grades), γ‐ Al 2 O 3 , and AlOOH have been investigated for the direct production of transparent MgAl 2 O 4 . The highest average in‐line transmittance through 3.5–4 mm thick samples is 84.2 ± 1.0% at 550 nm, attained by using mixtures of 99.99% pure Mg ( OH ) 2 and γ‐ Al 2 O 3 . Other formulations exhibited 77–80% visible transmission, suggesting that further optimization can improve sample consistency. All samples exhibited a Knoop hardness of ~12 GPa, and elastic modulus of ~280 GPa. Biaxial flexural strength measurements ranged from 85 to 136 MPa depending on starting materials.

With the rapid advancement of the computer power and the recent advances in the numerical techniques and materials model have allowed the accurate simulation of the ballistic impact into multilayer transparent armor configurations. The development of next generation ceramics is a key to providing the enhanced protection and extended service performance for future U.S. military systems. A significant development program is underway to improve processing and reduce costs associated with magnesium aluminate spinet. Pre-stressing of the ceramic materials has been shown to improve its ballistic characteristics. The purpose of this effort is to apply the existing modeling took in advancing ceramic transparent armor materials to fielded applications. This paper reports on the effect of various compressive stress levels on the depth of penetration (DOP) in the polycarbonate of a spinel/polycarbonate laminate impacted by a projectile. The ballistic behavior of the pre-stressed spinel was studied by ANSYS/AUTODYN commercial software and the results will be discussed in detail.

Polycrystalline spinel serves as an alternative to materials such as sapphire and magnesium fluoride that are currently being used in electromagnetic window applications such as missile domes, where high strength, high hardness and high transmittance in the visible and infrared spectra are required. The cubic crystal lattice of spinel imparts an isotropy to the bulk optical property, which eliminates optical distortion due to birefringence that occurs in sapphire and other non-cubic materials. The current study is to find a reliable manufacturing process to produce large magnesium aluminate spinel domes from powder consolidation efficiently. A binder-less dry ball milling process was used to deflocculate the spinel powder to increase its fluidity in an effort to ease the shape-forming. Dry ball milling time trials were conducted at several intervals to determine the appropriate level of time required to break up both the hard and soft agglomerates associated with the virgin spinel powder. The common problems encountered in dry powder shape-forming are crack growth and delamination of the green body during cold isostatic pressing (CIPing). The cracking and the delamination are due to the buildup of stress gradients on the green body that are created by the frictional force between the powder and the die wall or mold wall. To understand the stresses during the CIPing process, a finite element analysis of stresses on the green body was conducted. The simulation was used to evaluate the effect of die tooling and process characteristics on the development of stress gradients in the green body dome. Additionally, the effect of friction between the die wall and powder was examined by the simulation. It was found that by mitigating the frictional forces, cracking and delamination on the green body could be eliminated. A stepped-pressure CIPing technique was developed to reduce stress gradient build-up during CIPing. Also, oleic acid lubricant was applied to the die wall to reduce the wall friction between the powder and the die itself. As a result of these two above-mentioned methods, it was demonstrated that it is possible to consolidate a binder-free powder into large defect-free domes.

ABSTRACT The dominant materials solution used for ballistic transparency protection of armored tactical platforms in commercial and military applications is low cost glass backed by polycarbonate. Due to the high cost of testing transparent ceramics, a modelling approach has been undertaken in parallel with ballistic testing to validate armor designs based on a transparent magnesium aluminate spinel, MgAl 2 O 4 , striking-ply backed by polycarbonate. Finite element modelling is used to predict unsuccessful designs and reduce number of laminate configurations in experimental testing. The purpose of this report is to demonstrate the importance of modeling tools in advancing ceramic transparent armor materials to fielded applications. The effect of various shape defects, located at various locations on the surface and in the interior of spinel hard face of the laminate target, on the failure of the transparent material will compared with relative available experimental data and they be discussed in detail.

The dominant materials technology used for ballistic transparency protection of armored tactical platforms in commercial and military applications is low cost glass. Currently, the development of next generation ceramics is a key to providing enhanced protection capability and extended service performance for future armored windows. A development program is underway to improve processing and reduce costs associated with transparent ceramic materials. such as magnesium aluminate spinel. MgAl(2)O(4). A critical element of this development is 10 understand the influence of defects on the failure performances. both statically and dynamically. Although commercial efforts continue to focus on critical cost reduction an processing parameters. interest continues to grow toward applying ceramics toward armor system design. Future ballistic designs are being assembled in a virtual environment to begin to extrapolate the importance of defects. including flaw density, crystal orientation, shape and size. position with respect to the ceramic surface, and ceramic plate dimensions on the performance of ceramic spinel under dynamic impacts. While additional developments remain to be completed, the successful development of process methods that result in consistent high quality transparent ceramics is allowing validation of models to increase the insertion potential for these transparent ceramics into next generation tactical military platforms. The purpose of this report is to demonstrate the importance of modeling tools in advancing ceramic transparent armor materials to fielded applications. The effect of various shape defects. located in the interior and on the surface of spinel, on the failure of the transparent material will be discussed in detail.

A significant challenge in the fielding of transparent MgAl2O4 (spinel) ceramic parts for a variety of military applications is the limited availability and fairly high cost of starting powder with consistent quality and performance. In addition, available powders often require additional processing (particularly the addition of a sintering aid such as LiF) prior to ceramic forming and sintering. Although the current sources of commercial spinel powder are limited, separate Mg and Al oxides or hydroxides are among the most widely produced ceramic powders on the market. If stoichiometric combinations of such powders could be substituted with modest effort into existing procedures for transparent spinel manufacture, significant gains could be made in cost, availability, and consistency of the resulting ceramic bodies. To this end we have studied the suitability of various commercial sources of MgO, Mg(OH)2, γ-Al2O3, and AlOOH for transparent MgAl2O4 production. Our methods have been kept simple to facilitate comparisons between trials and to maintain a focus on eventual manufacturing feasibility. Stoichiometric mixtures of Mg and Al powders are thoroughly mixed in an aqueous slurry. The solids are collected, dried, calcined, milled with LiF (as a sintering aid), and sieved. The powders are sintered into dense ceramics with standard hot pressing and hot isostatic pressing procedures. Resulting ceramic transmission is measured and correlated with the purity, surface area, and phase composition of the prepared powders.