Preparation of Neodymium-Doped Yttrium Aluminum Garnet Transparent Ceramics by Homogeneous Precipitation Method

Neodymium-doped yttriumaluminumgarnet(Nd∶YAG)transparent polycrystalline ceramics,laser ce-ramic matrix materials,are gradually becomingsubsti-tutes for single crystals because they are provided withpredominantly chemical stability,highly doped ionconcentration,excellent optical performance and hightemperature stability[1,2].Presently,the conventionallyused methods to produce Nd∶YAG powders are solid-state processes[3],chemical precipitation methods[4,5]andlowtemperature combustiontechniqu…     

Sintering kinetics of YAG ceramics

Solid state reactive （SSR） sintering kinetics was observed for YAG ceramics. There were two densification stages in sin- tering process due to its reaction. After the first stage, samples began to expand, then, the second densification stage began. At a heat- ing rate of 10 ℃/min, the sample warped down and warped back to straight. The apparent activation energy of the first densification process was about 522 kJ/mol for the initial shrinkage of A1203 and Y203 mixed powder green-body, which increased in the follow- ing process due to the solid state reaction. In the second densification stage, synthesis reaction of YAG still worked. Green-bodies processed with higher heating rate got more shrinkage at the same temperature than lower heating rate green bodies. And its kinetic field diagram was abnormal, compared with that of other reported ceramics, such as Al203. It was found that the reaction of YAG provided positive effect to the sintering driving force. The apparent activation energy for densification of SSR YAG sintered in ArH5 atmosphere was 855 kJ/mol at temperature holding sintering. And the apparent activation energy for grain growth was 1053 kJ/mol.     

Effect of Additive on Microstructure of Transparent YAG Ceramics

Crystallineyttriumaluminumgarnet (Y3 Al5O12 )isanimportantlasermaterialduetoitssuperioropti calandmechanicalproperties[1,2 ] .NeodymiumdopedYAGisusedwidelyaslaserhostmaterialinvarioussolidstatelasers .SingleYAGcrystalsaregenerallysynthesizedbyCzochralskim…     

Application of Stereology on Neodymium-Doped Yttrium Aluminum Garnet （Nd： YAG） Transparent Ceramics

Nd ： YAG precursor powders were synthesized by homogeneous precipitation, and Nd ： YAG transparent ceramics were prepared by vacuum sintering at 1700 ℃ for 5 h. The ceramic materials were characterized by light transmittance and field emission gun-environment scanning microscope. Using statistics and stereology theory, study was carried out on the quantitative relationships between light transmittance and stereological parameters in three-dimensional Euclidean space. It is found that the transmittance of Nd：YAG with 1 mm in thickness is about 45% and 58% in visible and near-infrared wavelength, respectively. The transmittance linearly increases with increasing equivalent sphere diameter and reaches the theoretical value of single crystal when the equivalent sphere diameter is 20μm. The transmittance decreases with the increasing of mean specific area per unit volume of grain and discrete grains, and the transmittance decreases with increasing mean free distance of grains in Nd：YAG ceramics.     

Sintering effect on electrical properties and pulse aging behavior of （V2O5-Mn3O4-Er2O3）-doped zinc oxide varistor ceramics

The effect of sintering temperature on microstructure, electrical properties, and pulse aging behavior of(V2 O5-Mn3 O4-Er2 O3)-doped zinc oxide varistor ceramics was systematically studied. When the sintering temperature increased, the average grain size increased from 6.1 to 8.7 m and the sintered density decreased from 5.52 to 5.43 g/cm3. The breakdown field decreased from 3856 to 922 V/cm with an increase in the sintering temperature up to 900 C, whereas a further increase to 2352 V/cm at 925 C. The nonlinear coefficient increased pronouncedly from 4.6 to 30.0 with an increase in the sintering temperature. The varistor ceramics sintered at 850 C exhibited the best clamping characteristics, with the clamp voltage ratio of the range of 2.22–2.88 for pulse current of 1–25 A. The varistor ceramics sintered at 925 C exhibited the strongest stability, with %ΔE 1 mA/cm2=–8.8% after applying the multi-pulse current of 25 A.     

Influence of ammonium sulfate on YAG nanopowders and Yb:YAG ceramics synthesized by a novel homogeneous co-precipitation method

Homogeneous and dispersed Y_3 Al_5 O_(12)(yttrium aluminum garnet,YAG) nanopowders were synthesized via a homogeneous co-precipitation method from the mixed solutions of yttrium nitrate,aluminum nitrate and a small amount of ammonium sulfate using hot urea as the precipitant.The method has the superiorities that co-precipitation of cations is ensured and continuous decomposition of the hot urea is achieved to obtain the narrow size distribution particles.The addition of small amount of ammonium sulfate surfactant,although has no influence on YAG garnet phase formation,has significant effect on dispersion,particles distribution and sinterability of the resultant YAG and Yb:YAG powders.Compared with the undoped sample,the green body of Yb:YAG doped with ammonium sulfate has higher total shrinkage,linear shrinkage rate and relative density through sintering at 1600 ℃.The resultant Yb:YAG powders can be sintered into transparent ceramics at 1700 ℃ through vacuum sintering.The influence of the sulfate ions on characteristics of the resultant powders was thoroughly studied.     

Application of Stereology on Neodymium-Doped Yttrium Aluminum Garnet (Nd: YAG) Transparent Ceramics

Nd: YAG precursor powders were synthesized by homogeneous precipitation, and Nd: YAG transparent ceramics were prepared by vacuum sintering at 1700 ℃ for 5 h. The ceramic materials were characterized by light transmittance and field emission gun-environment scanning microscope. Using statistics and stereology theory, study was carried out on the quantitative relationships between light transmittance and stereological parameters in three-dimensional Euclidean space. It is found that the transmittance of Nd:YAG with 1 mm in thickness is about 45% and 58% in visible and near-infrared wavelength, respectively. The transmittance linearly increases with increasing equivalent sphere diameter and reaches the theoretical value of single crystal when the equivalent sphere diameter is 20μm. The transmittance decreases with the increasing of mean specific area per unit volume of grain and discrete grains, and the transmittance decreases with increasing mean free distance of grains in Nd:YAG ceramics.     

A Study of Grain Growth Kinetics in Sintered NdFeB Magnets

ＵｐｔｏｎｏｗｈｉｇｈｐｅｒｆｏｒｍａｎｃｅａｎｉｓｏｔｒｏｐｉｃＮｄＦｅＢ ｔｙｐｅｍａｇｎｅｔｓａｒｅｍａｉｎｌｙｍａｎｕｆａｃｔｕｒｅｄｂｙｔｈｅｐｏｗｄｅｒｍｅｔａｌｌｕｒｇｙｔｅｃｈｎｉｑｕｅ[1] .Ｔｈｅｆｉ ｎａｌｍｉｃｒｏｓｔｒｕｃｔｕｒｅｏｆｓｉｎｔｅｒｅｄＮｄＦｅＢｍａｇｎｅｔｓｉｓｄｅｔｅｒｍｉｎｅｄｂｙｔｈｅｐｒｅ ａｌｌｏｙｐｏｗｄｅｒｐａｒａｍｅ ｔｅｒｓａｎｄｓｉｎｔｅｒｉｎｇｃｏｎｄｉｔｉｏｎｓ .Ｔｈｅｔｅｃｈｎｏｌｏｇｉ ｃａｌｉｍｐｏｒｔａｎｃｅｏｆＮｄ2 Ｆｅ14 Ｂｇ…     

掺杂氧化铝对纳米钨粉烧结过程的影响

以碳热预还原和氢气深还原两步制备的纳米钨粉作为烧结原料,即先通过碳黑还原脱除三氧化钨中的大部分氧,再以氢还原脱除残留的氧。该方法制备的钨粉颗粒呈球形形貌,平均晶粒度可达90 nm。同时,向钨粉中掺杂质量分数为1%和2%的氧化铝,探究了氧化铝对钨粉烧结行为的影响。通过烧结样品的断口形貌和晶粒的平均尺寸分析发现,氧化铝对烧结后期的晶粒长大有明显的抑制作用,相同的烧结温度下晶粒的尺寸随着氧化铝含量的上升而减小。在1600 ℃时,纯钨粉烧结坯的晶粒平均尺寸为2.75 μm,但添加质量分数为1%和2%氧化铝的烧结样品晶粒平均尺寸约为1.5 μm,这是由于氧化铝能有效地抑制烧结后期的钨粉晶粒长大。纯钨粉和掺杂氧化铝钨粉的烧结坯的硬度随温度升高具有不同的趋势。掺杂钨粉烧结坯的硬度随着温度的升高而升高,且其最大值高于800 HV。但是,纯钨粉烧结坯的硬度随烧结温度增加而先增加后降低,在1400 ℃时取得最大值（473.6 HV）,这是由纯钨粉烧结坯的晶粒在高温下急剧长大所导致。在烧结温度为1600 ℃时,纯钨粉、掺杂质量分数1%和2%的氧化铝掺杂的钨粉的烧结坯的相对密度依次为98.52%、95.43%和93.5%。      

Roughening of the interface between grains and liquid matrix in sintered TaC-20Ni

The grains in TaC-20 wt pct Ni alloy prepared by liquidphase sintering at 1480°C have orthorhombic shapes with flat {100} faces and sharp edges. These grains show abnormal growth. When sintered at 2020°C, the grains develop round edges and corners, and the grain growth is nearly normal. Such a grain shape change indicates that the interface between the grains and the liquid matrix undergoes roughening transition with temperature increase. The interface roughening transition is reversed when a specimen initially sintered at 2020°C is heat treated again at 1400°C.     

SINTERING KINETICS IN IRON-COPPER ALLOYS WITH AND WITHOUT A LIQUID PHASE

Abstract

Sintering of iron-copper alloys has been studied in the temperature range 950–1250°C. The factors involved include compacting pressure, sintering temperature, sintering time, and atmosphere. The results are interpreted as a decrease in pore volume due to the filling of voids between particles by a diffusion mechanism. An empirical equation of the Arrhenius type, based upon volume change as a function of sintering time, has been derived in order to evaluate the rate constant of the sintering process.

Volume diffusion is considered to be the primary mechanism of material transport in alloys containing 0·5–2·0% copper, when sintered in the range 950–1250°C, and in alloys containing 5·0–10·0% copper, when sintered in the range 950–1050°C. The activation energy derived for the sintering process is 53·4 kcal/mole. Surface diffusion appears to be the operative mechanism of material transport in alloys containing 5·0–10·0% copper, when sintered above the melting point of copper. The activation energy for this sintering process is 32·6 kcal/mole.     

Structure and properties of WC-Co alloys in solid-phase sintering. I. Geometrical evolution

The paper examines the geometrical evolution and particulate interaction in WC-Co alloys produced by solid-phase sintering. It is shown that solid-phase sintering actively modifies the geometric structure: carbide particles grow, become facetted and grow together, and the cobalt phase is redistributed. The WC particles grow irregularly over different temperature ranges. The low rate of growth is characteristic of temperature ranges between 1050 and 1200°C. When alloys are sintered at 1200°C and higher temperatures, the growth of tungsten carbide particles intensifies substantially (by four times). Hence, the temperature 1200°C separates two structurization areas in solid-phase sintering. At this temperature, there is a bend on the specific resistivity curve, which is evidence of higher-quality grain and phase boundaries.     

Hard Alloy WC – 24% Ni Obtained in the Solid Phase from Ultrafine WC, NiO, and C Powders. Part 1. Density and Structure of Specimens

We have studied the density and structure of specimens of the alloy WC – 24 mass% Ni, obtained by combining into one step the processes of synthesis of the metallic phase and compaction of the ultrafine mixture of WC – Ni powders by high-energy pressing and sintering. We have established that reduction of nickel monoxide by carbon occurs at temperatures of 650-750°C and does not affect the shrinkage process which in the case of sintering begins only at a temperature of 1050°C. High-energy pressing of briquettes sintered at the indicated temperature reduces their porosity from 30-25% down to 8-4%. Specimens of porosity <1% can be obtained by pressing at 1150°C or 1050°C in the case of triple pressing. Raising the temperature at which the briquettes are heated is accompanied by enlargement of the pores together with a decrease in the total porosity, but at temperatures of 1300°C (sintering) and 1250°C (pressing), the pore dimensions are sharply reduced. The high density of the specimens pressed at low temperature does not provide low electrical resistance, which suggests the presence of weakly connected boundaries. When the specimens are sintered and pressed in the solid phase, we observe the growth of tungsten carbide particles. It is most rapid at 1150-1250°C, while at 1050°C the particle growth process slows down. Reduction of the metal oxide when the powders are heated promotes formation of structure in the higher temperature range.     

Grain Growth Behavior in Sintered Nd-Fe-B Magnets

The Nd2Fe14B grain growth behavior in sintered Nd-Fe-B magnets was quantitatively described.The effects of sintering temperature and time,and alloy powder size and its distribution on grain growth process were analyzed.Hence,possible grain growth mechanisms in these magnets were qualitatively discussed.The Nd2Fe14B grain growth proceeded at quite a high rate in the initial 0~1 h of sintering and from then onwards the grain growth rate decreased.A large average particle size or a wide particle size distribution of initial alloy powders was found to remarkably accelerate the grain growth process and even result in the occurrence of abnormal grain growth.On the basis of experimental results,two grain growth mechanisms were considered to operate during sintering of Nd-Fe-B magnets,that is,dissolution and re-precipitation of Nd2Fe14B particles,and Nd2Fe14B particle growth by coalescence.It was believed that Nd2Fe14B particle growth by coalescence not only produced a large average grain size and a wide grain size distribution,but also was the fundamental reason for the formation of abnormally large grains in the microstructure of sintered Nd-Fe-B magnets.     

Synthesis and Spectral Properties of Nd3+:Gd3Ga5O12 Nanopowder for Transparent Laser Ceramics

Nd3 :Gd3Ga5O12 (Nd:GGG) nanopowder for transparent laser ceramics was synthesized using sol-gel method. XRD, SEM, and fluorescence spectrum were used to study the properties of Nd3 :Gd3Ga5O12 nanopowder. XRD patterns of samples show that it has a cubic structure. Meanwhile, pure Nd:GGG crystals were obtained at 1000 ℃ for 12 h. SEM photographs show that dispersed, uniform, ball-like Nd:GGG nanopowder is obtained. Both XRD and SEM results show that the crystallization degree and the grain size increase with the increase in calcining temperature. Analysis of fluorescence spectrum shows that fluorescence emission occurs at 1062.7 nm, which is the result of Nd3 (4F3/2→4I11/2) transition. Homogenous Nd:GGG nanopowder with a small grain size synthesized using the sol-gel method is favorable for sintering the transparent ceramic, which proves that the nanopowder obtained is suitable as a precursor for preparing GGG transparent ceramics.     

Sintering effect on ageing behavior of rare earths （Pr6O11-Er2O3-Y2O3 ）-doped ZnO varistor ceramics

The electrical properties and ageing behavior of the rare earths (Pr6O11-Er2O3-Y2O3)-doped ZnO varistor ceramics were systematically investigated at sintering temperature range of 1335-1350°C. With an increase in the sintering temperature, the sintered density increased from 5.41 to 5.64g/cm3 and the average grain size increased from 5.8 to 7.9μm. The varistor properties and ageing behavior were significantly affected by small sintering temperature range of 1335-1350°C. The breakdown field noticeably decreased from 5767 to 3628V/cm with an increase in the sintering temperature. The varistor ceramics exhibited the highest nonlinear coefficient (43.2) at the sintering temperature of 1340°C. The varistor ceramics sintered at 1350°C exhibited a surprisingly excellent stability by exhibiting 0.3% in the variation rate of the breakdown field and 0.3% in the variation rate of the nonlinear coefficient for ageing stress of 0.95 E1mA/150°C/24 h.     

Effects of heating rate on densification and grain growth during field-assisted sintering of α-Al2O3 and MoSi2 powders

Two types of powders, electrically conductive MoSi₂ and insulating α-Al₂O₃, were sintered by a field-assisted sintering technique (FAST) using heating rates from 50 °C to 700 °C/min. The Al₂O₃ powders were sintered to 99 pct density at 1100 °C for 2 minutes under 45 Mpa pressure. For Al₂O₃, no exaggerated grain growth was observed and the final grain size inversely scaled with the heating rate. Such a grain growth behavior fits the literature models based on multiple transport mechanisms for constant-heating-rate sintering. The density reached by MoSi₂ under similar sintering conditions was 91 pct. The grain size was independent of the heating-rate value. Specific electrical field and pressure effects are shown to contribute to enhanced densification and minimal coarsening in each material.     

Effect of conventional and microwave sintering on the properties of yttria alumina garnet-dispersed austenitic stainless steel

The current study examines the effect of heating mode, temperature, and varying yttria alumina garnet (YAG) addition (5 and 10 wt pct) on the densification and properties of austenitic (316L) stainless steel. The straight 316L stainless steel and 316L-YAG composites were heated in a radiatively heated (conventional) and 2.45 GHz microwave sintering furnace. The compacts were consolidated through solid state as well as supersolidus sintering at 1200 °C and 1400 °C, respectively. Both 316L and 316L-YAG compacts couple with microwaves and heat to the sintering temperature rapidly (∼45 °C/min). The overall processing time was reduced by about 90 pct through microwave sintering. As compared to conventional sintering, compacts sintered in microwaves exhibit higher densification and finer microstructure but no corresponding improvement in mechanical properties and wear resistance. This has been correlated to elongated, irregular pore structure in microwave-sintered compacts.     

Adverse effect of high purity atmosphere on sintering of manganese steels

Abstract

The Fe–(2, 3)Mn and Fe–(2, 3)Mn–0·4C specimens were sintered in the dilatometer tests for 30 min at 1150°C in an H₂–H₂ O atmosphere with a dew point of –30 and –70°C. The Mn addition at sintering of the specimens with a dew point of –30°C resulted in higher swelling with a maximum at 1150°C and processing at a dew point of –70°C resulted in minimal differences in swelling of the specimens in dependence on the Mn addition. Processing in the high purity atmosphere (–70°C) causes a loss of Mn in the form of Mn vapour transported away by the flowing atmosphere needed for alloying of the Fe matrix. On the contrary, at sintering in a low purity atmosphere (–30°C) the reaction of Mn vapour with oxygen of high partial pressure occurs near the surface of the specimens. Sintering of Mn steels requires the low purity atmosphere in contrast to the thermodynamic requirements. The sintering of Mn steels occurred under the effect of Mn vapour formed by sublimation regardless the thermodynamics for Mn–O system.     

A study on the improvement of the sintered density of W-Ni-Mn heavy alloy

Liquid phase sintering behavior of 90W-6Ni-4Mn heavy alloy has been studied. The present work takes into account the thermodynamic oxidation/reduction reactions of the constituent elements W, Ni, and Mn. The sintering cycle consists of heating under high purity nitrogen gas, holding at reduction temperatures after the atmosphere is changed to dry hydrogen, and sintering at 1260 °C for 1 hour. As the reduction temperature increases from 1050 °C to 1200 °C, the relative sintered density increases from 92 pct, reaching 100 pct at temperatures above 1150 °C. The relative density increases with increasing holding time at the reduction temperature and remains unchanged with heating rate. The sintered microstructure has been analyzed by a scanning electron microscope and energy dispersive X-ray spectroscopy. The relative density was compared with those obtained from other investigators. It is found that the formation of manganese oxide due to reducing reactions of W and Ni powders should be avoided in order to obtain a W-Ni-Mn heavy alloy without pores.