Li Yaogang ', Gao Lian ², Zhu Meifang 1
(1. School of Materials Science and Engineering, Donghua University, Shanghai, 200,051;
2. State Key Laboratory of High Performance Structural Ceramics and Ultrastructure, Shanghai Institute of Silicate, Chinese Academy of Sciences, Shanghai, 200050)
With amorphous Nb2Os with high specific surface area prepared by precipitation method as raw material, niobium nitride nano powder was prepared by ammonolysis at 600~800 ℃ for 3~8h. The NbN powders synthesized at different ammonolysis temperatures and times were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption specific surface area (BET), thermal analysis (TG/DTA), nitrogen analysis, etc. The effects of ammonolysis temperature and time on the properties of NbN powders were studied. The results show that this method requires simple equipment, low nitriding temperature, short reaction time and high purity of product. The pure cubic phase NbN nano powder with particle size of 15-40 nm can be obtained by nitriding at 650-800 ℃ for 3-8 h.
Keywords: niobium pentoxide, ammonolysis, niobium nitride, nano powder
CLC No.: TF 123
Transition metal nitrides have high melting point, high hardness, corrosion resistance, electrical and magnetic properties similar to metals, and are widely used in cutting tools, wear-resistant parts, high-temperature structural materials, electromagnetic components and superconducting materials . Niobium nitride has high relative density (8.47), high melting point (2300 ℃), high hardness (Mohs hardness of 8, microhardness of 14.3GPa) and superconductivity. There are many nitrides in the Nb-N system , including: β- Nb₂N (hexagonal),Y-Nb₄N±,(tetragonal), δ- NbN-
(fcc), η- NbN(hexagonal),8'- NbN(hexagonal), NbsN₆,Nb₄N。 In addition, nitrogen dissolved in metals can form α - Nb (N) solid solution. Y-Nb ₄ N3 in transition metal nitrides, δ- NbN has the highest superconducting transition temperature. Many studies show that with the change of stoichiometric ratio, δ- The critical temperature of NbN is in the range of 15~16.5K, but some reach 17.3K. Due to the relatively high critical temperature, γ and δ The phase is expected to be used in the preparation of superconducting devices, especially in the replacement of traditional metal materials (Pb, Nb) in superconducting radio frequency resonators. As a new type of nitride material, NbN has a broad application prospect.
In industry, niobium nitride is usually prepared by direct method with metal niobium and nitrogen as raw materials and reduction method with niobium pentoxide added with carbon powder and nitrogen as raw materials . The biggest disadvantage of the reaction is that it is difficult to prepare ultrafine powder, and the reduction method requires secondary carbon removal, the process is complex, and the product contains a certain amount of impurities.
In recent years, more and more attention has been paid to the research of niobium nitride materials [3~11]. Niobium nitride materials have been prepared by different methods, mainly including combustion synthesis, carbothermal reduction, ball milling, and different precursor methods of ammonolysis. Buscaglia et al.  studied the influence of nitrogen pressure on the composition and superconducting critical temperature of synthetic NbN in combustion synthesis. Zhang et al.  prepared NbN powder by combustion method, and studied the influence of initial porosity, nitrogen pressure and amount of diluent on combustion process and product performance. Agrafiotis et al.  reported that niobium nitride powder was prepared by combustion synthesis, but the particle size of the synthesized product was only in the micrometer range. Ohyanogi et al.  prepared superconducting niobium nitride plates and niobium nitride wires by SHS. Ma Qingzhu et al [6 Niobium nitride was prepared by carbothermal reduction method using Nb2Os powder and NbO2 powder as raw materials under the same process conditions, and the relationship between raw materials and niobium nitride products was discussed. Wu Xuemei et al. 7 prepared niobium nitride ultrafine powders at room temperature by using a specially designed ball milling device, filling nitrogen under certain pressure after vacuuming, and ball milling pure niobium powder The thermal stability was analyzed, and the influence of different ball milling conditions on the structure of NbN ultrafine powder was studied. Brown et al.  synthesized niobium nitride using the dialkyl compounds of niobium as the niobium source and liquid ammonia as the nitrogen source. Chen et al.  prepared niobium nitride with nano metal niobium powder as raw material and ammonia as nitrogen source. NbNo6Oo3 powder was obtained by nitriding at 700 ℃ for 4h; Nitriding at 860 ℃ for 8h, a mixed phase of niobium nitride was obtained, including NbNo95 and NbNo 98 and NbN phases. Tessier et al.  prepared nitrides and oxynitrides of niobium using alkali metal niobate as raw material and ammonia as nitriding agent. Nb4Ns were synthesized by ammonolysis of NaNb3O ₈ K or Nb ₈ Og at 800 ℃. In addition, Angelkort et al. (11) prepared niobium nitride films on silicon substrates using rapid thermal processing (RTP). The thickness of the film is 200~500 nm, and the reaction temperature is controlled at 500~1100 ℃. It can be seen that the niobium nitride materials prepared at present include films, plates, wires and powder materials, but there are few reports on the preparation of NbN nano powders. The preparation of corresponding nitride nano powders by directly nitriding transition metal oxide nano powders with ammonia is a new method for the synthesis of transition metal nitride nano powders. Recently, we have synthesized TiN and CrN nano powders [12~14] from nano-TiO2 and Cr2O3 respectively. In this paper, the amorphous niobium pentoxide powder prepared by precipitation method was used as raw material for the first time, and the nano niobium nitride powder was prepared by ammonia direct nitridation method. The method has the following advantages: (1) high-purity nano niobium nitride powder can be obtained; (2) The required temperature for nitriding reaction is low; (3) The equipment used is simple; (4) Easy for mass production. In this paper, the influence of nitriding temperature and time on the properties of the powder was systematically studied, and the preparation process was optimized.
1.1 Preparation of powder
Take 2.5g of niobium hydroxide, add 100mL of concentrated sulfuric acid solution with a ratio of acid to water of 3:1, and heat it to dissolve it. Stir the solution at a speed of 400 ～ 800 r/min and add it drop by drop to 2500 mI distilled water to produce precipitation. Filter the precipitated product, wash it deeply with distilled water to remove anions, wash it twice with absolute ethanol, dry the filter cake at 100 ℃ for 12 hours, grind it, pass it through a 200 mesh sieve, and then calcine it at 300 ℃ for 2 hours to obtain amorphous niobium pentoxide powder with high specific surface area. Put the prepared amorphous niobium pentoxide and niobium dioxide powder with high specific surface area into a quartz boat, put it into a tubular atmosphere furnace, and inject ammonia gas. The ammonia gas flow is 1L/min, and the temperature rises to 600~800 ℃ at a rate of 10 ℃/min. At this temperature, keep the temperature for 3~8 h, and then naturally cool to room temperature under flowing ammonia gas to obtain nano niobium nitride powder.
1.2 Characterization of powder
D/MAX-2550V X-ray diffractometer (Cu Ka, 40kV) was used to analyze the phase of precursor and synthetic nitride powder, and determine the phase composition of the powder. The particle morphology and particle size of the powder were observed with JEM-200CX analytical electron microscope manufactured by JEOL Company of Japan. The specific surface area of the powder at liquid nitrogen temperature was measured with ASAP 2010 automatic adsorption specific surface meter of Micromeritics Company of the United States. The NETZSCH STA449C comprehensive thermal analyzer produced by German NET company was used for differential thermal analysis. The thermal analysis of precursor and synthetic nitride powder was carried out, with heating rate of 10 ℃/min and air atmosphere. The nitrogen content of synthetic niobium nitride powder was determined by the Elemental Varioel carbon, hydrogen and nitrogen analyzer.
2 Results and discussion
2.1 Characterization of precursors
In Fig. 1, (a) and (b) are the X-ray diffraction patterns of Nb (OH) s powder calcined at 300 and 500 ℃ for 2h, respectively. There is no obvious diffraction peak, indicating that the powder is amorphous. (c) The middle diffraction peak is completely consistent with the diffraction peak of Nb ∈ Os in JCPDS card 27-1003, indicating that crystalline Nb ∈ O powder can be obtained by calcination under this condition. Thermogravimetric analysis shows that the sediment has weight loss in the range of 75~300 ℃, which is caused by the removal of adsorbed water and bound water from the sediment, and there is basically no weight loss above 300 ℃. The differential thermal analysis shows that there is an endothermic peak corresponding to the precipitation dehydration near 107.8 ℃ and an exothermic peak corresponding to the crystallization of Nb ∈ Os at 579.9 ℃. This is consistent with the XRD analysis that the powder calcined at 500 ℃ for 2h is amorphous phase, and the powder calcined at 600 ℃ for 2h is crystalline Nb2Os. In this paper, Nb ∈ O powder calcined at 300 ℃ for 2h is selected as the precursor. BET analysis shows that its specific surface area is up to 198.1m ²/ g。 On the one hand, the precursor does not contain water; on the other hand, the anhydrous amorphous Nb ∈ O powder has a large specific surface area and high reactivity, which is conducive to the nitridation reaction.
Fig. 1 XRD of Nb2Os powder calcined at different temperatures
2.2 Characterization of synthetic nitride powder
2.2.1 XRD analysis
Fig. 2 is the X-ray diffraction pattern of the powder prepared under different nitriding conditions. (a) And (b) are the spectra of the powder obtained after nitriding at 600 ℃ and 650 ℃ for 5h respectively. It can be seen that there are diffraction peaks of Nb2Os and cubic phase NbN at the same time, indicating that the powder has started nitriding at 600 ℃, but there is still Nb ∈ O that is not nitrided after nitriding at 650 ℃ for 5h. Exists. In Figure 2, (b) is compared with (a), the diffraction peak of NbN is enhanced, and the diffraction peak of Nb ∈ Os is weakened, indicating that the nitridation rate increases with the increase of temperature. The diffraction peak of Nb2Os in Fig. 2 (c) disappears, indicating that X-ray diffraction pure NbN powder can be obtained after nitriding at 650 ℃ for 8h, and the nitriding time is prolonged, which is conducive to complete nitriding.
Fig. 2 XRD Diagram of Powders Prepared under Different Ammoniation Conditions
In Fig. 3, (a)~(c) are the XRD spectrums of the powders obtained by nitriding at 700, 750 and 800 ℃ for 3h, respectively. It can be seen that only the diffraction peaks corresponding to cubic phase NbN exist, indicating that pure cubic phase NbN powders can be obtained by X-ray diffraction after nitriding at 700~800 ℃ for 3h. From (a) to (c), the diffraction peak of NbN gradually increases, indicating that the higher the nitriding temperature is, the higher the crystallization degree is, and the larger the particle size is.
Fig. 3 XRD Diagram of Powder Prepared by Ammoniation for 3h at Different Temperatures
In conclusion, the direct nitridation of Nb ∈ Os to prepare NbN is a gas-solid reaction process, which requires a certain temperature and time from the perspective of reaction kinetics. The lower the temperature, the slower the reaction speed. Increasing the temperature will speed up the reaction speed, but at the same time, the grain will grow. The time is short, and the nitriding is incomplete. Prolonging the time will also make the grains grow. Therefore, nitriding temperature and nitriding time are two important factors that affect the nitriding rate and particle size of the powder. Reasonable selection of these two parameters can obtain cubic phase nano NbN powder with a certain particle size on the premise of ensuring complete nitridation. In this paper, amorphous oxide powder with high specific surface area was used as precursor to prepare nitride powder instead of crystalline oxide for the first time. The results show that the nitriding temperature and time can be reduced to obtain nitride nano powder.
2.2.2 TEM observation
Fig. 4 is the TEM photo of NbN powder synthesized under different nitriding conditions. It can be seen from Fig. 4 (a) and (b) that the particle size of NbN nano powder synthesized by nitriding at 650 ℃ for 8h and 700 ℃ for 3h is basically the same, and the particle size is in the range of 15-20 nm. Fig. 4 (c) shows that NbN powder with particle size of 30-40 nm can be obtained by nitriding at 800 ℃ for 3 h. The higher the nitriding temperature, the larger the particle size of the synthesized NbN powder. When the temperature increases from 700 ℃ to 800 ℃, the particle size of NbN increases obviously. From the above analysis, it can be seen that the optimized preparation condition is nitriding at 650~800 ℃ for 3~8h.
Fig. 4 TEM Diagram of Niobium Nitride Powder Synthesized under Different Conditions
2.2.3 Nitrogen content analysis
Table 1 shows the analysis results of nitrogen mass fraction in the synthesis of niobium nitride nano powder. It can be seen that the nitrogen mass fraction is close to the theoretical value of 13.10%. The higher the nitriding temperature and the longer the nitriding time, the higher the nitrogen mass fraction. It should be pointed out that the mass fraction of nitrogen in individual powders is greater than the theoretical value, but still within the error range.
2.2.4 TG/DTA analysis of niobium nitride nano powder
The thermal stability of NbN nano powder in air was studied by TG and DTA, and its TG/DTA curve is shown in Fig. 5. When NbN powder is heated in air, it will react with oxygen in the air to generate NbO ± at a certain temperature. It can be seen from Fig. 5 that the NbN powder starts to oxidize at 320 ℃ in the air and reacts to produce NbO2 at 450 ℃. The TG curve shows about 17% weight gain, which is basically consistent with the theoretical weight gain of 16.64%. The exothermic peak at 428.2 ℃ in the DTA curve also corresponds to the oxidation reaction. There is a weight loss at 540~570 ℃, which is caused by the conversion of NbO2 generated by the reaction into NbO. There is an exothermic peak corresponding to it at 563.5 ℃ in the TG curve. Further increase in temperature will eventually form Nb2Os . Generally, the thermal stability of nitrides in air is relatively low, so the sintering process should be carried out under the protection of nitrogen atmosphere.
Fig. 5 TG/DTA curve of synthetic NbN nano powder
(1) Cubic NbN ultrafine powders can be prepared by direct nitridation of amorphous Nb ∈ Os powders with high specific surface area by ammonia gas. The content of Nb and N in the powders is close to 1:1 stoichiometry.
(2) Nitriding temperature and time are two important factors affecting the performance of NbN powder. The optimized process conditions are 650~
Nitriding at 800 ℃ for 3-8h.
(3) The cubic phase NbN nano powder with particle size of 15~40 nm can be synthesized under the optimized process conditions.
(4) The synthesized nano NbN powder starts to oxidize at 320 ℃ in the air to produce NbO2540 ℃ and decompose into NbO.
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Synthesis of Nanosized NbN Powder with Cubic Structure by
(1.College of Materials Science and Engineering,Donghua University,Shanghai,200051,
2.State Key Laboratory of High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramic,Chinese Academy of Sciences, Shanghai,200050)
Abstract The nanosized niobium oxide powder was synthesized by precipitation method. Then it was nitrided by NH₃ at 600～800℃ for 3～8 h and obtained nanosized niobium nitride powder. X-ray powder diffraction(XRD),transmission electron microscopy(TEM),BET surface area techniques,thermal analysis(TG-DTA),and nitrogen content analysis were adopted for the characterization of the powders obtained under different ammonolysis conditions. The influences of the ammonolysis temperature and the holding time on the powder properties were investigated. It is concluded that this process need shorter time,lower temperature,and simple equipments. The 15～40 nm nanosized XRD pure cubic NbN powder
can be prepared by nitrided at 650～800℃ for 3～8h.
Keywords: Nb₂O,ammonolysis,niobium nitride,nanocrystalline powders