Preparation Of LaMnO₃ Perovskite-type Oxide Nanofibers Via Electrospinning

LaMnO₃–based oxide becomes the hot topic of condensed Matter in recent decades because of its special structure and singular magnetic properties. People prepared LaMnO₃ nanomaterials and Nanofilms by using variety methods, however, the reports about preparation of one dimensional nanomaterials are less, this materials is very important significance to fields of catalytic and electrolyte materials.In 1934, Formhals designed the device and process conditions of electrospinning and applied patent. It was considered the earliest exploitation application. In fact, electrospinning was recognized at the end of the last century with the rapid development of nano-technology. Since 2002, more than 20 varieties of ceramic fibers have been produced by this method. Appling electrospinning technique and Sol-Gel process in this dissertation, to explore synthesis technology and best synthesis condiction of preparation of LaMnO₃ nanomaterials by electrospinning. The main results were as follows:1. 10 wt% concentration of the polyvinyl alcohol (PVA) agent made fromthe precursor solution for electrospinningSpinning the initial PVA/LaMnO₃ compound was fibrous structure, Fiber diameter is in the range of tens to hundreds Nanometer. Diameter distribution was not uniform, the thinnest reached dozens of Micron, the vast majority of fiber distribution was about 400 nm. fiber surface was smoothness and fiber length is also very long. After calcining at 600℃for 1 h, fiber change thin apparently, while fiber surface becomes rough, the vast majority of fiber diameter is about 100 nm. This is due to organic ingredients of acetic acid and PVA decomposed ,combused and reacted in the vicinity of temperature and released volatile gases (such as H2O, CO2, etc.), while LaMnO₃ Crystal started formation, fiber morphology change greatly. After sintering at 1100℃for 1 h, the sample had pearl chain structure, fiber surface was more rough and crystallized completely. The average diameter of fiber was about 300 nm, and fiber diameter became thicker compared to 600°C sintered fiber, This is due to that the sample of sintering at 600°C fibers was composed of small grain and until 1100℃sintering, small grains formed a large grain, while large grain growed toward the surrounding, fiber was composed of large grain and fiber became thicker.2. With polyvinylpyrrolidone (PVP) as a complexing agent (8wt% 10 wt% and 12wt% PVP) electrospinning.Precursor solutions made of complexing agent (8 wt% 10wt% and 12wt% PVP concentration respectively) proceeded electrospinning. Experimental results show that the diameter of 8wt% PVP/LaMnO₃ nano-composite fiber was very thin and the average diameter was approximately 150 nm, fibronectin phenomena was apparent. The diameter of 10wt% PVP/LaMnO₃ nano-composite fiber was coarse, diameter of most fiber was about 300 nm, but the fiber was uniform and had no adhesion phenomenon. The diameter of 12wt% PVP/LaMnO₃ nano-composite fiber was most crude and typical fiber diameter was about 500 nm. Three samples were compared that showed the fiber diameter was thicker gradually with the concentration increasing. The diameter of 10wt%PVP/LaMnO₃ nano-composite fiber was more uniform. So elected 10 wt% of the PVP/LaMnO₃ nanofiber composite samples and stayed for 2 h at 300℃, 400℃,500℃, 600℃,700℃, 800℃, 900℃, 1000℃and 1100℃,respectively.XRD analysis showed that after calcination at 300℃, the sample is amorphous structure. after calcination from 400℃to 600℃, there was characteristic peak of La2O₃, but the characteristic peaks of LaMnO₃ had not appeared. after calcination until 700℃, the characteristic peaks of the clearer integrity cubic perovskite LaMnO₃ appeared. With the calcination temperature continues to increased, LaMnO₃ diffraction peak of samples intensitied. Application Scherrer formula, after calcination at 700℃average diameter was 14.6 nm. After calcination at 800℃, average diameter was 15.2 nm.SEM observation showed that samples maintain fibrous structure when the sintering temperature was 300℃, and surface was smoothness, most fiber diameters were about 200 nm. At 400℃to 600℃calcination, the fiber diameter gradually changed thin and fiber surface was rough, which indicated that PVP and acetate and other organic ingredients among samples gradually combused and reacted with increasing temperature and released volatile gases (such as H2O, CO2, etc.). As temperature reached 700℃, samples shown a pearl chain structure that indicated that organic compounds fully volatilized below the temperature and samples crystallized, fiber diameter was about 80 nm. When temperature reached 800℃, fibers were also thicker and diameters of the majority of fibers were about 140 nm. With the temperature of calcination increased gradually, fiber diameter was thicker and fiber surface was extremely rough. With the rise of temperature, the samples appeared cracked and the crack is more serious at the higher temperature. This is due to that fiber composition at the beginning was composed of small grains, and with the rise of temperature, small grain growed up to a large grain and large grain growed again toward surround. When the temperature reached 1100℃, which observed that the long-grain completely growed together, the structure of fiber damage basically.For fiber samples( 700℃and 800℃for 2 h after), measurement of transmission electron microscopy (TEM) and electron diffraction (ED) Analysis showed that ,after calcinations(700℃), crystallization of fiber formed chainlike structure and surface was roughness, fiber diameter is about 100 nm. ED showed the samples at 700℃has become crystal particles connecting structure, grain size was about 15 nm. After sintering (800℃) , fiber surface was more rough, but fiber diameter had no change significantly. Photos from the ED can be seen that electron diffraction spots more obvious clear, fiber grains become larger and grain size was about 20 nm.The LaMnO₃ fiber samples (calcination of 800℃for 2 h) was through high-temperature conductivity tests, a number of silicon wafer were synthesized by solid-phase methord as reference samples.σ-T (℃) and lnσ-T (K) relations showed that with the rise of temperature, conductivity of the sample made of Nanofibers is gradually increasing, but changes of their conductivity with temperature was slower than the reference samples, and at the same temperature, conductivity of reference sample were higher than fiber samples. Measurement of density showed, density of the sample made of fibers (2.864×10³kg/m³) is less than that of the reference sample (3.301×10³kg/m³). Differences of electrical Properties may be due to different density of the sample structure.In summary, this paper uses electrospinning technology and sol-gel method, using polyvinylpyrrolidone (PVP) as a complexing agent (10 wt%), Though calcination (electrospinning and 700℃) , preparing the cubic structure LaMnO₃ nanofibers ,whose diameter is beween 50 and 100 nm. Fibril formed chainlike structure and its surface was roughness. Fiber internal is composed of crystal mark by connecting, grain size of about 15 nm.As a new practical technology to prepare inorganic materials Nanofibers, electrospinning with the sol-gel technique to prepare LaMnO₃ perovskite structure of the ceramic nanofibers, is a relatively simple and common preparation technology. In other rare earth transition metal oxides functional nano-fiber materials and the preparation of applications facts ,it will have very broad application prospects.