Translucent Alumina Research Articles

Translucent Polycrystalline Alumina: Influence of Roughness and Thickness on In-Line Transmittance

Translucent polycrystalline alumina ceramic has its main application as discharge arc tube in high pressure sodium lamp. Translucent polycrystalline alumina bodies with two different additive compositions were produced by a conventional pressureless sintering route (1850 °C for 3h under vacuum). Different surface conditions (roughness) and thickness were produced by grinding and polishing the samples. Roughness (Ra and Ry) and in-line transmittance of the samples were measured. A great influence of surface condition on the transmittance was observed. The influence of thickness (0.4 ∼ 1.6mm) of polished samples on in-line transmittance was evaluated. An exponential correlation between transmittance and sample thickness was observed. For comparison, the same analysis was made on single crystal alumina (sapphire) discs.

Thermal Behaviour of Cutting Grain in Grinding

Temperature at the interface between the cutting grain and a workpiece is measured using a two-color pyrometer with a fused fiber coupler. For the cutting grain, a conical tool of translucent alumina is used. The infrared rays radiated from the interface and transmitted through the conical tool are accepted by a fused fiber coupler and led to two infrared detectors of different spectral sensitivity. The temperature increases rapidly and reaches a saturated value after a short time (about 0.1 ms). The saturated temperature measured increases with the increase in the cutting speed up to the melting point of the work material.

Second‐order stereology for pores in translucent alumina studied by confocal scanning laser microscopy

SummaryThe three‐dimensional (3‐D) arrangement of pores in translucent alumina was investigated with a confocal scanning laser microscope (CSLM). By moving the focal plane of the CSLM down into the material, a stack of serial thin optical sections was obtained to produce a 3‐D image of the pores. Computer‐based image analysis was used to obtain the coordinates of the pore centroids. The distance distribution function G(r) and the second‐order functions K(r), L(r), H(r) and g(r) were used to analyse the spatial point pattern of the pore centroids. Estimates of the preceding functions obtained from eight stacks of sections were compared with the corresponding functions for a 3‐D stationary Poisson point process, which served as a reference model for complete spatial randomness. The analysis suggested that the pore centroids were arranged in an aggregated pattern within a range of about 10 μm.

Effect of processing conditions on the characteristics of pores in hot isostatically pressed alumina

Effect of processing conditions on the characteristics of residual pores was studied with an optical microscope in hot isostatically pressed translucent alumina ceramics. Green bodies formed by isostatic pressing were sintered at 1300, 1400 and 1600°C and then hot isostatically pressed at a temperature 50°C below the respective sintering temperature for 1 h at 100 MPa. All specimens were fully dense within experimental accuracy (±0.1%), and the grain size increased with increasing sintering/hot isostatic pressing temperatures. A variety of pores were found in all specimens. The distribution of pores was uniform at various locations within the specimen. The pore population decreased with increasing pore size, but was finite in the size range exceeding 84 μm. The pores in this range increased with increasing sintering/hot isostatic pressing temperature. Except for these large pores, the pore population was similar under all processing conditions.

Preparation of High‐Strength and Translucent Alumina by Hot Isostatic Pressing

A vacuum‐pressure slip‐casting technique and hot isostatic pressing (HIPing) were used to prepare high‐strength and translucent alumina ceramics. A low‐viscosity and high‐solids‐content slurry (46 vol% solids) was prepared, and a dense green compact was formed. The samples were sintered and subjected to capsule‐free HIPing. Extremely high‐density (99.9%) and fine‐grained (0.7 to 15 μm in diameter) alumina ceramics were obtained. The HIPed samples showed high bend strength and translucency with in‐line transmittance of 30% to 46% (1 mm thick).

Transmission Optical Properties of Polycrystalline Alumina with Submicron Grains

Translucent alumina ceramics with extremely high in-line optical transmittance to visible light and light mechanical strength were produced by applying HIP process to a sample which was densified at a relatively low temperature after injection molding. Measurements of bulk density and average grain size, X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) analysis, and SEM and TEM observations were conducted

Formation of sodium aluminates at high temperatures under the influence of Mg and Ca

The formation of sodium aluminates at high temperatures under the influence of Mg and Ca was studied to obtain insight into lifetime processes that occur in high pressure sodium (HPS) discharge lamps with translucent alumina envelopes. Powders with molar composition Na 2CO 3-5Al 2O 3, Na 2CO 3-9Al 2O 3, Na 2CO 3-MgO-5Al 2O 3 and 2Na 2CO 3-CaCO 3-5Al 2O 3 were thoroughly mixed and fired at temperatures up to 1600°C for top-level times up to 100 hrs. Quantitative X-ray diffraction was used to study the composition of the quenched mixtures. In the systems Na 2CO 3-5Al 2O 3 and Na 2CO 3-9Al 2O 3 the compound Na 2O.Al 2O 3 is formed first at low temperatures, coincident with the decomposition of carbonate. This compound subsequently reacts with Al 2O 3 to form a mixture of β- and β′−Al 2O 3 in a weight ratio of 1:6. At temperatures above 1400°C β′−Al 2O 3 transforms to β-Al 2O 3. Remarkably this transformation is most complete in the Na 2CO 3-5Al 2O 3 mixture. In the Na 2CO 3-MgO-5Al 2O 3 mixture the compound Na 2O.Al 2O 3 is also formed first, directly followed by MgO.Al 2O 3. β- and β′−Al 2O 3 are formed subsequently in a weight ratio of about 1:1.6. At temperatures above 1400°C stabilized β′−Al 2O 3 is then formed almost completely. The reactions in the system 2Na 2CO 3-CaCO 3-5Al 2O 3 are very complex. The compound Na 2O.Al 2O 3 is formed first, directly followed by the ternary compound Na 2O.CaO.2Al 2O 3 which forms solid solutions with Na 2O.Al 2O 3, starting at 1150°C. At about 1000°C the compound Na 2O.3CaO.8Al 2O 3 starts to form with a maximum around 1200°C. At higher temperatures it is consumed again, resulting in a complete disappearance at 1600°C. β/β′−Al 2O 3 also starts to form at 1000°C and is still present at 1600°C, together with a solid solution of Na 2O.CaO.2Al 2O 3 and Na 2O-Al 2O 3. Conclusions can be drawn from the present results that are relevant for an understanding of lifetime processes in HPS lamps.

Sodium corrosion resistance of translucent alumina: effect of additives and sintering conditions

The resistance of Al2O3 against corrosion by sodium in the temperature range 800 to 1000° C was studied by holding samples in liquid sodium in molybdenum containers for several hundred hours. Thermodynamic calculations indicating that Al2O3 is unstable in contact with sodium were confirmed by testing sapphire samples. A remarkable aspect was the large anisotropy in the corrosion resistance of sapphire. The effect of the sintering atmosphere and the amount of MgO and CaO dopant in polycrystalline alumina on the corrosion resistance was considered. Vacuum sintering yielded more resistant materials than hydrogen sintering. Low firing temperatures as well as low dopant levels also proved beneficial. In particular, the absence of CaO improves the corrosion resistance considerably. Consequently, the use of this additive should be avoided.

Fracture of translucent alumina: temperature dependence and influence of CaO dope

Fracture of translucent alumina: temperature dependence and influence of CaO dope

Mg Distributions and Microstructure of the Translucent Alumina Ceramic

Mg Distributions and Microstructure of the Translucent Alumina Ceramic

High-pressure sodium discharge lamps

The review article deals with the high-pressure sodium (HPS) discharge lamp, characterised by a sodium vapour pressure above 5 kPa and a power density of about 100 Wcm−3. In its short history – the sodium-resistant translucent alumina used as the discharge tube material was invented only in the late 1950s – continuous research and development has led to a good understanding of the discharge physics, and to an improvement of the lamp properties. The various physical aspects, like plasma temperature, emitted spectrum and energy balance, are discussed. Technical aspects like discharge-tube design, tube material and seal, and the ignition and stabilisation of the lamp are also covered. The HPS lamp is a compact light source emitting golden-white light with luminous efficacies of up to 140 l mW−1. It has already found widespread applications, mainly for outdoor lighting. Further developments in the field of design diversification, especially lamps with lower wattage and improved light quality, will make this light source also suitable for indoor applications.

Translucent ceramics as lamp envelopes

The properties of oxide ceramics which may be sintered to translucency are discussed and it is shown that only alumina is at present suitable to prepare lamp arc tubes. A brief description of the preparation of alumina powder is given with particular emphasis on the properties of powders required for the manufacture of ware of optical quality. The processes involved in the manufacture of sintered translucent alumina are described, and it is pointed out that faults in sintered ware may be connected with problems in powder rheology. The theoretical limit to translucency in ceramics is examined, showing that purity and high sintered density of ware are essential. A method of producing hermetic seals in ceramic lamp arc tubes is also described.

Radiation Characteristics of High-Pressure Sodium Lamps

The radiation characteristics of high-pressure sodium lamps were examined by changing the bore and input power of an arc tube:(a) Increase of the bore results in a better color rendition. This is the same effect as is observed with an increased sodium vapour pressure.(b) In case the input power is kept constant, the lamp efficacy drops with an increase of the bore. However, in case the wall loading is maintained constant, the efficacy scarcely depends on the bore, since it rises with an increase of the input power.The individual factor determining the lamp efficacy was studied through the theoretical analysis of the phenomena in an arc tube. Relationship between each factor and the three discharge parameters, viz., the bore, input power and vapour pressure, has been clarified as follows:(a) The radiant power Pr (W·cm-1) is determined only by the input power Pin (W·cm-1) through the equation Pr=0.78 (Pin-6.7) in a translucent alumina ceramic tube; where the above 0.78 and 6.7 correspond respectively to total transmittance of a light path and thermal conduction loss per unit arc length Pc (W·cm-1).(b) The visible component of the radiant power and the luminous efficacy of the radiation depend on the three discharge parameters; they drop with an increase the bore and the vapour pressure, and rise with increase of the input power.