Synthesis of zirconium powders from a sponge by a combination of self-propagating high-temperature synthesis and dehydrogenation

Introduction

Zirconium has a high melting point, good ductility, excellent corrosion resistance, and a low neutron absorption cross-section [1], and therefore it is widely used in the nuclear industry as a cladding material for fuel rods [2–4]. The use of zirconium and its compounds in the nuclear industry requires high purity and fine particle size distribution of the starting powders. Such powders can be obtained using various methods, including: thermal reduction of metals, electrolysis of molten salts, electrode oxidation, and the hydrogenation-dehydrogenation (HDH) process [5].

The HDH method has become widespread due to its simple process flow, low production cost, and good productivity [6; 7]. The hydrogenation-dehydrogenation (HDH) process is a solid-phase method in which zirconium absorbs hydrogen at a specific temperature and pressure, followed by hydrogen removal during vacuum heat treatment of the zirconium hydride [8]. There are three main stages in this process: hydrogenation, grinding, and dehydrogenation. During hydrogenation, zirconium is saturated with hydrogen, which alters its crystal structure and mechanical properties, resulting in the formation of brittle zirconium hydride. the next step is to grind zirconium hydride – the resulting powders are sieved and distributed according to particle size. In the final stage, hydrogen is removed from the zirconium by vacuum annealing at an elevated temperature to obtain pure zirconium powder [9; 10].

It is possible to increase the efficiency of zirconium hydrogenation-dehydrogenation technology using self-propagating high-temperature synthesis (SHS) [11; 12]. A special feature of SHS is that the hydrogenation process proceeds without external energy supply, solely due to the heat released by the exothermic reaction: Zr + H₂ → ZrH₂ + Q (40 kcal/mol). Subsequently, the synthesized zirconium hydride undergoes the same operations as in the conventional process: grinding, sieving, and dehydrogenation [13; 14]. The SHS process for hydrogenation of metal powders, including zirconium [15], was used at the Institute of Chemical Physics of the Academy of Sciences of the Armenian SSR (Yerevan) and the Insyitute of Chemical of the USSR Academy of Sciences (Chernogolovka) [16]. Currently, the development of the scientific foundations of SHS hydrogenation and thermal dehydrogenation technology for large-scale zirconium raw materials is of great importance.

JSC Chepetsky Mechanical Plant (CMP) was the first in Russia to master the production of zirconium sponge from scratch [17]. Sponge production includes three major processing operations: chlorination of zirconium ore concentrate to produce technical zirconium tetrachloride contaminated with iron, silicon, titanium, carbon, and other impurities; purification of technical zirconium tetrachloride from impurities, including the most difficult stage – purification from hafnium by extractive rectification; and production of zirconium sponge by magnesiothermic reduction of purified zirconium tetrachloride. Compared with the fluoride-based technology for zirconium powder production, the sponge process involves half as many process steps, requires less electricity and raw materials, and significantly reduces waste. At the same time, high purity of zirconium is ensured with respect to hafnium content, which is fundamentally important for the nuclear industry. the rectification separation technology for zirconium and hafnium chlorides introduced at CMP made it possible to obtain high-purity zirconium tetrachloride with a hafnium content of less than 0.01 %.

Thus, Russia now has access to a relatively inexpensive, hafnium-free zirconium semi-finished product (sponge). the use of such powders with reduced hafnium content, combined with powder metallurgy and additive manufacturing methods [18–20], opens the possibility of manufacturing complex products for the nuclear industry – including parts with internal cavities and lightweight structures achieved through topology optimization [21; 22]. However, the first step is to convert the zirconium sponge into powder. This task is difficult or even impossible to accomplish by mechanical grinding due to the high strength and ductility of zirconium.

The purpose of this work was to develop a new energy-efficient method for producing zirconium powders from sponge using SHS hydrogenation and thermal dehydrogenation. to achieve this goal, it was necessary to develop a process for SHS hydrogenation of zirconium sponge that ensures hydride formation throughout the entire charge volume, and to characterize the resulting zirconium powders.

Research methodology

Zirconium sponge, titanium sponge and grade A hydrogen gas (99.99 %) were used as feedstock. The chemical composition (wt. %) of the initial zirconium sponge is presented below:

Zr + Hf . . . .    ≥99.2        C . . . . . . . . .    ≤0.05

Hf . . . . . . . .    ≤4.5        N₂ . . . . . . . .    ≤0.025

Fe + Cr . . . .    ≤0.2        O₂ . . . . . . . .    ≤0.16

H₂ . . . . . . . .    ≤0.025            

The use of a large zirconium sponge (fraction 20–40 mm) required the development of a special SHS hydrogenation scheme (Fig. 1) to ensure the stability of the process. In this scheme, a zirconium sponge is placed in the center of the reactor, and a more dispersed titanium sponge (5–20 mm fraction) is sprinkled on the periphery, which is a key feature of this system. In this case, the necessary thermal conductivity of the filling is ensured, and the reaction of layered combustion takes place throughout the entire volume.

According to the developed scheme, zirconium and titanium sponges were placed in a high-pressure reactor with a volume of 2000 cm³. After sealing the reactor, hydrogen was supplied to the system until a pressure of 2 MPa was reached. the reaction was initiated by heating a metal spiral that was in contact with the ignition powder. During combustion, hydrogen was absorbed, as a result of which the pressure in the reactor dropped, to maintain the stability of combustion, the pressure in the reactor was maintained at a level of at least 1 MPa.

The hydrogenation process proceeds in a solid-phase mode, so no strong bond is formed between titanium and zirconium. Due to this, after the completion of SHS hydrogenation and cooling, the resulting products are easily separated from each other. For further experiments, samples of zirconium hydride were selected, which were then ground into powder in a ceramic mortar.

At the next stage, the resulting zirconium hydride powder was subjected to dehydrogenation. to do this, it was placed on a molybdenum substrate in a vacuum furnace, after sealing which the air was pumped out by a vacuum pump. Vacuuming was carried out to a residual pressure of <10 Pa, after which heating was turned on to a maximum set temperature of 750 °C with exposure for 2 h. Cooling was carried out together with the furnace [23].

The phase composition of the obtained powders was determined by X-ray phase analysis (XRD) on a Dron-3 diffractometer using СuK_α-radiation. the particle size of the obtained powders was studied using a laser particle analyzer “MicroSizer 201”. The measurement error did not exceed 1.2 %. the morphology of the particles was studied using a Zeiss Ultra plus ultra-high resolution autoemission scanning electron microscope (SEM) based on Ultra 55.

Results and discussion

According to the proposed scheme (see Fig. 1), the SHS hydrogenation of zirconium sponge was carried out, followed by grinding, yielding zirconium hydride powder. X-ray diffraction (XRD) analysis of the initial zirconium sponge (Fig. 2, a) showed that it is single-phase and corresponds to α-Zr with an HCP lattice. The XRD pattern exhibits texture resulting from the surface preparation procedure (grinding/polishing). The phase composition of the synthesized zirconium hydride is mainly represented by the ε-ZrH₂ phase with a tetragonal (CaF₂-type) crystal lattice (Fig. 2, b). Based on the Zr–H phase diagram (Fig. 2, c), the formation of the ε-phase indicates a high hydrogen content, confirming the efficiency of the proposed hydrogenation scheme, which yields zirconium hydride close to the stoichiometric composition ZrH₂ [24].

Subsequently, the resulting zirconium hydride was subjected to grinding and thermal dehydrogenation under vacuum, yielding a fine zirconium powder. Its morphology and particle size distribution are presented in Fig. 3.

According to the results of the particle size distribution analysis, the powder exhibits an almost monomodal particle size distribution with a main peak in the range of 10–100 µm (approximately 80 wt. %) and a minor fraction of submicron particles. The overall particle size ranged from 0.5 to over 100 µm. The highest product yield was observed in the 10–80 µm range, making this powder suitable for the fabrication of zirconium products with the required performance characteristics using both powder metallurgy and additive manufacturing technologies.

SEM morphology analysis of the zirconium powder revealed a fragmented particle structure, confirmed by their irregular shape and complex surface morphology. This structure is directly related to the powder production method and indicates that brittle fracture occurred predominantly along grain boundaries and crystal lattice defects during grinding.

XRD analysis of the powder synthesized by thermal dehydrogenation (Fig. 4) revealed its multiphase composition. the equilibrium low-temperature α-Zr phase with an HCP lattice is predominant, which provides key properties for use in the nuclear industry, such as a low thermal neutron capture cross section, high corrosion resistance, and high mechanical strength. In addition, a small fraction of the δ-ZrH phase with a cubic lattice was found, characterized by a lower hydrogen content relative to the ε-ZrH₂ hydride [25].

The formation of a secondary hydride (ZrH) phase during the synthesis of zirconium powder indicates incomplete dehydrogenation and, consequently, the need to optimize the dehydrogenation conditions.

Conclusion

A new promising method for hydrogenation of a large fraction zirconium sponge is proposed. This approach enables efficient hydrogenation throughout the entire material thickness, yielding the ε-ZrH2 phase with a high hydrogen content, which confirms the effectiveness of the hydrogenation process. Subsequent thermal dehydrogenation of the resulting hydride in vacuum produced a fine zirconium powder with a predominantly fragmented particle morphology and a particle size distribution in which about 80 % of the particles fall within the 10–100 µm range. However, the formation of residual secondary hydride phases (e.g., δ-ZrH) during synthesis indicates the need for further research in this promising direction to optimize the process and improve powder quality based on inexpensive zirconium sponge feedstock using the energy efficient SHS hydrogenation and thermal dehydrogenation methods.