1144 Susumu Imashuku et al.


(CL) spectrometer (Imashuku et al., 2013a, 2014, 2016) using the phenomenon that an electron beam is generated by applying a temperature change to a pyroelectric crystal, such as cesium nitrate (CsNO3) or lithium tantalite (LiTaO3), in a vacuum (Brownridge, 1992). The portable EPMA and CL spectrometer mainly consisted of a pyroelectric crystal, a small rotary pump, a Peltier device, a 3Vbattery, detachable vacuum joints, an X-ray detector for EPMA, and optical spectrometer and digital camera as the CL spectrometer. By placing an electroconductive needle with a sharp tip and coating the side of the tip with an insulating material, the spatial resolution of the portable EPMA was improved to 40μm(Imashuku & Wagatsuma, 2017), which is sufficient for the analysis of the larger nonmetallic inclusions in steel. There has been research on CL analysis using a stationary CL microscope or CL spectro- meter equippedwith a scanning electronmicroscope (SEM) for the identification of nonmetallic inclusions in steel (Yin&Tsai, 2003; Kaushik et al., 2009; Imashuku et al., 2017a, 2017b). This analytical method can simultaneously identify the size, shape, and compositions of the inclusions by detecting or capturing their luminescence. However, the specification of the compo- sitions of the nonmetallic inclusionsiscomplicated anddifficult because the CL colors of some of the inclusions are changed by heat-treatment or the presence of impurities (Yin&Tsai, 2003). Thus, the portable CL spectrometer is expected to have the same difficulty for the on-site analysis. We consider that this difficulty can be solved by combining the portable EPMA and CL spectrometer (portable EMPA–CL analyzer); we first identify the size and shape of the inclusions by capturing a CL image and then determine the compositions using the portable EPMA. Therefore, in the present study, we investigated the potential of application of the portable EMPA–CL analyzer to on-siteanalysis. ThesamephenomenonasCLoccurs by using X-rays instead of an electron beam, which is termed X-ray excited optical luminescence (XEOL). XEOL analysis has potential in the on-line analysis of nonmetallic inclusions in steel because XEOL can be performed in air.We also examined the potential of the application of XEOL analysis to the on-line analysis. A model sample containing MgAl2O4 spinel and Al2O3 inclusions were used for both studies as the first step leading to the application of these analytical techniques to the practical use. The identification of MgAl2O4 spinel and Al2O3 inclusions is important in the steelmaking industry as well as the detection ofMnS and calcium aluminates (Atkinson & Shi, 2003). This is because both MgAl2O4 spinel and Al2O3 inclusions create surface flaws and cracking in steel and cause problems during steel production (Braun et al., 1979; Tozawa et al., 1999; Atkinson & Shi, 2003; Basu et al., 2004; Jin et al., 2010; Okuyama et al., 2010; Park & Todoroki, 2010; Harada et al., 2014; Zheng et al., 2016).


MATERIALS ANDMETHODS


A single model sample containing MgAl2O4 spinel and Al2O3 inclusions was used for identifying MgAl2O4 spinel and Al2O3 with a XEOL analyzer and a portable analyzer comprised of a CL spectrometer and EPMA. The model


Figure 1. a: Schematic illustration and (b) photograph of the portable X-ray excited optical luminescence analyzer.


sample was prepared by pressing a mixture of MgAl2O4 spinel, Al2O3, and copper powders into a pellet. Particle sizes from 20 to 53 μm of MgAl2O4 spinel and Al2O3 powder reagents were selected using a sieve before mixing. The sieved MgAl2O4 spinel and Al2O3 particles each with 0.2 mass% and 99.6 mass% of copper powder were then mixed with an agate mortar. The mixed powder was pressed into a pellet at 500 MPa. The surface of the model sample was polished using 2000-grit abrasive paper. XEOL analysis of the model sample was carried out as


shown in Figure 1. An X-ray tube with a rhodium target (TUB00050-RH2, Moxtek Inc., Orem, UT, USA) was operated at 20kV and 150 μA during the analysis. The distance between the head of the X-ray tube and the model sample was 2mm. An XEOL image of the model sample was captured with a digital single-lens reflex camera (D7000, Nikon Co., Tokyo, Japan) equipped with two close-up lenses with a focusing distance of 50mm (AC CLOSE-UP NO.5, Kenko Tokina Co., Ltd., Tokyo, Japan). The wavelength range that the digital single-lens reflex camera can detect was from 420 to 680 nm. The exposure time was 30 s. Analysis of the model sample using the portable CL


spectrometer and EPMA was conducted as shown in Figures 2a, 2b. The +z plane of a LiTaO3 single crystal, with dimension of 6×6mm in the x–y plane and 5mm in the z-axis, was attached to a Peltier device using silver paste. The temperature of the LiTaO3 crystal was changed by connect- ing the Peltier device to a DC power supply and monitored by a thermocouple set on the Peltier device. The other face of the Peltier device was attached to a metal rod using silver paste. The model sample was placed on another metal rod using carbon tape. The distance between the LiTaO3 crystal and the model sample was maintained at ~5mm.


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