Structural Tolerance of Zirconium Diboride under Electron Irradiation through in-situ Convergent Beam Electron Diffraction and Energy-dispersive X-ray Spectroscopy

Zirconium diboride (ZrB2) is a ultra-high temperature ceramic with high melting temperature (>3000 K), super strength and hardness, good thermal conductivity, and excellent resistance to plasma erosion and chemical erosion, being widely applied in hypersonic flight, arc-electrodes, refractory materials, and plasma-facing materials in fusion reactors. Therefore, the structural stability of the material is essential for the applications. Up to now, the material was investigated under neutron irradiation [1] and high-energy ion irradiations [2]. It was reported that the neutron irradiation caused swelling and catastrophic cracking [1]. High-energy ions (30 keV He+) changed the surface morphology and caused the bright blue and brown colors on ZrB2 surfaces between 920 K and 1120 K [2]. The electron irradiation tolerance of the material has not been reported yet. Here we used convergent beam electron diffraction (CBED) and energy-dispersive X-ray spectroscopy (EDS) to investigate structural tolerances of ZrB2 crystals under high-energy electrons at room temperature under vacuum. The investigation would benefit the understanding of its structural tolerance at microscales. The ZrB2 powders were synthesized by the arc-plasma method. X-ray diffraction indicated the synthesized ZrB2 powders were single-phase. The powders were mechanically ground in ethanol and deposited on TEM lacey-carbon grids. The ZrB2 crystals on the grids were then characterized on a transmission electron microscope (JEM 1400Flash) with an acceleration of 120 kV. The TEM was equipped with an Oxford energy-dispersive X-ray spectrometer with a Be thin-window. Several ZrB2 microsized crystals were chosen and irradiated under the TEM electron beam with a current density of 500 pA/cm2 for 10, 20, 30, 40, and 60 min at room temperature in the TEM column. Their crystalline structures were characterized by CBED, selected-area electron diffraction (CBED), and EDS during the irradiation procedures. The characterizations were carried out below 20 pA/cm2 in order to avoid potential irradiation damages to the crystals. The EDS data were collected by the AZtec 5.0 SP1 software platform and the acquisition time was long enough to ensure the measurement error of the calculated element ratio is less than 0.2 wt%. Fig. 1 shows CBED patterns of one typical ZrB2 crystalline particle under electron irradiations. The diffraction patterns indicated that the crystallographic structure of the compound is totally tolerant to high-energy (120 keV) electron irradiation up to 60 min under 500 pA/cm2. SAED patterns and TEM images (insets in Fig. 1) confirmed the structure stability under the irradiations. However, more detailed analyses showed that the CBED Kikuchi bands are slightly blurred after irradiations. Additionally, a faint diffraction ring was naked-eye observed in the SAED patterns of the irradiated ZrB2 crystals. The faint diffraction ring and blurred Kikuchi bands indicated that some kinds of disorders were produced in the crystals under the irradiations. In order to explore the original of the possible disorders, EDS spectra of the ZrB2 crystals were in-situ recorded during the irradiations. Fig. 2a shows element ratio of the crystal under irradiation. Boron content continuously decreased with irradiation time. Because of the EDS window absorption, it is hard to measure the accurate boron content of the ZrB2 crystals. However, the decreasing boron content indicated that the boron atoms lost during the electron irradiation. With the lost of boron atoms, the Zr content increased with irradiation time. Most possibly, the boron atoms were kicked off the original lattice sites, some surface boron atoms left the crystal and some inside boron atoms occupied disordered positions, resulting in faint or blurred diffraction rings / bands. At the same time, the electron diffraction patterns were slightly affected by the irradiation because Zr atoms (not kicked off under irradiation) were frame of the crystalline structure and contributed most to the diffraction patterns. CBED patterns under various irradiation time. Left corner: dark-field TEM images (scale bar: 1 um). Right corner: SAED patterns. (a) Element content under various irradiation time. (b) Boron lost under various electron flux. In order to quantize the lost of boron atoms under irradiation, we normalized the boron content of the crystal and calculated relative boron lost from the element content plotted in Fig. 2a. Fig. 2b shows the relative boron lost under various electron flux. More than 10% boron atoms were lost during the irradiation. This phenomenon, light atoms were kicked off, was also observed on MoS2 2D materials [3, 4]. As a summary, ZrB2 crystals were irradiated under 120 keV electrons in vacuum at room temperature and in-situ characterized by convergent beam electron diffraction and energy-dispersive X-ray spectroscopy. The in-situ investigation indicated that the compound is structural stable under 120 keV electron irradiations with 500 pA/cm2 up to 60 min. Boron atoms should be kicked off from the lattice and induced in structural disorder. The disorder of boron atoms will be confirmed from high-resolution TEM and Z-contrast annular dark-field imaging in STEM. More work will be carried out under 200 keV and 80 keV electron irradiations [5].