The Influence of Sintering Temperature on the Transport Properties of GdBa₂Cu₃O₇ Superconductor Prepared from Nano-Powders via the Co-Precipitation Method

Abstract

This study examines the influence of sintering temperature on the structural and transport properties of GdBa₂Cu₃O₇ (Gd123) superconductors prepared from nano-sized precursors via the co-precipitation method. The metal-oxalate precursor (average particle size < 50 nm) was calcined at 900 °C for 12 h, and then the prepared pellets were sintered under an oxygen atmosphere in the range of 920–950 °C for 15 h. All samples showed metallic properties and a sharp superconducting transition. Critical temperatures T_C(R=0) were 94–95 K, with higher sintering temperatures steadily boosting critical current density. X-ray diffraction confirmed orthorhombic Gd123 as the dominant phase, with its phase fraction increasing from 92% to 99.8% as the sintering temperature increased. SEM micrographs showed large, densely packed grains, with higher sintering temperatures promoting improved grain connectivity and reduced porosity. The sample sintered at 950 °C exhibited the most favorable transport performance, attributed to enhanced intergranular coupling and the presence of nanoscale secondary phases acting as effective flux-pinning centers. Overall, these results demonstrate that careful control of sintering temperature can significantly optimize the microstructure and superconducting properties of Gd123 materials, supporting their advancement for practical electrical and magnetic applications.

1. Introduction

The synthesis of REBa₂Cu₃O_7−δ (RE123) ceramic superconductors (RE includes La, Dy, Gd, Sm, Yb, Ho, Nd, and Y) has attracted considerable interest due to their promising electrical and magnetic properties, such as power transmission, magnetic levitation, and energy storage [1,2,3,4,5,6,7,8]. Several synthesis techniques have been used, such as the conventional solid-state reaction (SSR), which achieves a transition temperature (T_C) of 92 K [4,5]; pulsed laser deposition (PLD), typically yielding high-quality epitaxial thin films with a T_C between 91 and 93 K under optimal oxygenation and substrate conditions [9,10,11]; co-precipitation (COP), which uses nanoscale precursors for better homogeneity and results in a T_C of 94 K [12,13,14,15,16,17]; and chemically modified photosensitive methods, where the resulting thin film has a T_C of approximately 90 K [18,19]. The SSR method typically uses high-purity oxides and carbonates as precursors, involving multiple grinding steps and extended heat treatments to ensure complete reaction. However, this approach can introduce contamination during processing [20,21,22]. In contrast, the COP method offers advantages by employing nanoscale precursors, reducing the need for prolonged sintering and grinding [23,24,25,26].

Gadolinium-based GdBa₂Cu₃O_7−δ (Gd123), forms an orthorhombic crystal structure when oxygen deficiency is low (δ < 0.6), and its critical temperature (T_C) is above 90 K [7]. Secondary phases, such as Gd₂BaCuO₅ (Gd211), are known to boost transport properties by serving as flux-pinning centers [8].

Research has shown that sintering time influences the superconducting abilities of Gd123, with 15 h at 920 °C being considered optimal [12,13,14,15,16,17]. However, the impact of varying sintering temperatures on the superconducting and transport characteristics of Gd123 produced by COP remains insufficiently explored.

This study aims to produce a Gd123 ceramics superconductor via the coprecipitation method to synthesize fine precursor powder before the calcination and sintering stages. Comprehensive characterizations are carried out using several techniques, including a scanning electron microscope (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Direct-Current (DC) system with a cooling facility for electrical measurement within the range of superconductivity.

2. Results and Discussion

2.1. TGA Curve of the Produced Blue Nano-Powder

The TGA curve showed five significant weight drops as a function of temperature, as illustrated in Figure 1. The first drop (4.3%) was the moisture loss from the sample, which ended at approximately 125 °C. The second drop, occurring at 125–226 °C, (15%) represented the water loss due to crystallization from the metal oxalate mixtures. The third drop in weight of (20%) at 226–293 °C could be due to the decomposition of CuC₂O₄, BaC₂O₄, and Gd₂(C₂O₄)₃·6H₂O into CuO [27], BaCO₃, and Gd₂(C₂O₄)₃. The fourth drop (13.1%) at 310–860 °C is associated with the decomposition of Gd₂(C₂O₄)₃ to Gd₂O₃ [27], and the formation of BaCuO₂, which agrees with previous reports [26,28]. The final drop shows a complete decomposition and the formation of GdBa₂Cu₃O_7−δ that begins at 860 °C, as indicated in

Table 1. Different stages of the reaction mechanism during the TGA test.

Weight Losses (Drops)		Reaction Mechanism
Drop 1 (4.3%)	30–125 °C	Moisture loss from the surface of metal oxalate complex
Drop 2 (15.0%)	125–226 °C	BaC2O4· 0.5H2O ⟶ BaC2O4 + 0.5H2O CuC2O4· H2O ⟶ CuC2O4 + H2O
Drop 3 (20.0%)	226–310 °C	Gd2(C2O4)3· 6H2O ⟶ Gd2(C2O4)3 + 6H2O BaC2O4 ⟶ BaCO3 + CO CuC2O4 ⟶ CuO + CO2 + CO
Drop 4 (12.1%)	310–860 °C	Gd2(C2O4)3 ⟶ Gd2O3 + 3CO2 + 3CO CuO + BaCO3 ⟶ BaCuO + CO2
Drop 5 (4.0%)	Above 860 °C	0.5 Gd2O3 + 2 BaCuO2 + CuO + 0.5 O2 ⟶ GdBa2Cu3O7

. Therefore, from the TGA thermogram, it can be concluded that the suitable calcination and sintering temperatures for Gd123 are in the range of 860–950 °C [27,29,30].

2.2. Microstructure of the Produced Powder and Sintered Pellets

The SEM image of the produced blue powder confirms the precipitation of nanoparticles, as shown in Figure 2. The image reveals the agglomeration state of the nanoscale particles, which is a probable result because of the mutual diffusion between particles during the precipitation process, in the absence of any capping substance. The particle size is within the range of 50 nm or less. All sintered pellets exhibited large, densely packed grains with reduced porosity structures, as demonstrated in Figure 3.

In addition, the grains become closer and more coherent as the sintering temperature increases, thus improving the sintering density as a common result, which is in good agreement with the porosity and relative density measurements recorded in Table 3.

2.3. XRD Results

Figure 4 presents XRD patterns for sintered Gd123 at temperatures ranging from 920 °C to 950 °C. The dominant phase of the orthorhombic structure (Gd123) is presented for all samples. This phase was identified and labeled as (h k l) planes and the perovskite structure with the space group of Pmmm, No. 47, Z = 1, α = β = γ = 90°. The phase identification, size calculation, and lattice parameters were calculated Using Highscore Plus software (Version 5.2), the measured lattice parameters for all samples match the standard JCPDS, # 01-089-5733 (International Centre for Diffraction Data, USA, 2015) as shown in

Table 2. Lattice parameters and unit cell volume for all sintered samples.

Sintering Temp. (°C)	a (Å)	b (Å)	c (Å)	Volume (Å)3
920	3.84921 ± 0.00033	3.90079 ± 0.00079	11.7179 ± 0.0028	175.923 ± 0.021
930	3.84691 ± 0.00025	3.90007 ± 0.00039	11.7158 ± 0.0011	175.799 ± 0.029
940	3.84163 ± 0.00016	3.90276 ± 0.00021	11.7157 ± 0.0004	175.807 ± 0.021
950	3.84311 ± 0.00032	3.90030 ± 0.00056	11.7234 ± 0.0018	175.853 ± 0.022

. Few peaks belonging to impurities at 28.42°, 29.27°, 30.08°, 35.68°, 39.99°, 41.87°, 42.54°, and 49.27° were also detected and belonged to the non-superconducting phases, such as BaCuO₂ (Gd011) and (Gd211).

Phase contents “Gd123%” for samples sintered at 920 °C, 930 °C, 940 °C, and 950 °C were 96.2%, 99.1%, 98.2%, and 97.8%, respectively. Impurities at 920 °C result from incomplete sintering, while 930–950 °C yields high Gd123 purity.

The Gd123% was calculated using the following Equation (1) [13]:

$$\mathrm{G}\mathrm{d}123\mathrm{\%}={\displaystyle \frac{\mathsf{\Sigma }\mathrm{I}123}{\mathsf{\Sigma }\mathrm{I}123+\mathsf{\Sigma }\mathrm{I}\mathrm{I}\mathrm{m}\mathrm{p}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{e}\mathrm{s}}}\times 100\mathrm{\%}$$

(1)

where I₁₂₃ and I_mpurities are the peak intensities of the observed phases for Gd 123 and the impurities.

2.4. Electrical Measurements

The normalized electrical resistance as a function of temperature, in the absence of a magnetic field, was measured for all sintered samples and showed normal metallic behavior with a single-drop transition feature, as shown in Figure 5. All sintered samples showed a zero-resistance temperature (T_C,R=0) of 94–95 K, with no notable changes as the sintering temperature increased.

The transport critical current density, J_C, was calculated using Equation (2) [15]:

$$J_C={\displaystyle \frac{I_C}{A}}$$

(2)

where I_C represents the maximum current (A) through the sample’s cross-sectional area (cm²) before superconductivity is lost at 77 K and zero magnetic field. Figure 6 presents J_C values of 4.35 ± 0.11, 6.4 ± 0.4, 7.9 ± 0.7, and 12.9 ± 0.9 A/cm² for samples treated at 920, 930, 940, and 950 °C, respectively.

The J_C values presented in Figure 6 and

Table 3. Summarized data of T_C(R=0) (K), T_C-onset (K), Gd123%, relative density, porosity, and the maximum current density (Jc).

Sintering (°C)	TC(R=0)	TC (onset)	Impurities %	Relative Density %	Jc (A/cm2)
920	95	97	3.8	80.3	4.35 ± 0.11
930	94	97	0.9	83.2	6.4 ± 0.4
940	94	96	1.8	89.9	7.9 ± 0.7
950	95	97	2.2	95.3	12.9 ± 0.9

increased with higher sintering temperatures, attributed to a reduction in the weak link formation at grain boundaries, thereby facilitating improved current flow. Additionally, the Gd 211 phase acts as a strong pinning center in Gd123 superconductors due to interfacial boundaries and strain fields. These non-superconducting inclusions locally suppress the order parameter, providing favorable sites for vortices, while the lattice mismatch enhances vortex anchoring. A small amount of 211 phase improves transport properties and significantly increases critical current densities and pinning force, as confirmed by XRD data and discussed in references [2,17,18,28,30].

2.5. Relative Density and Porosity

The relative density (D_relative %) and the porosity for all samples were calculated from the actual and theoretical densities using the following Equations (3) and (4):

$${\mathrm{D}}_{\mathrm{R}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}}\mathrm{\%}={\displaystyle \frac{{\mathsf{\rho }}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}{{\mathsf{\rho }}_{\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}}}}\times 100\mathrm{\%}$$

(3)

$$\mathrm{P}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{\%}={\displaystyle \frac{{\mathsf{\rho }}_{\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}}-{\mathsf{\rho }}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}{{\mathsf{\rho }}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}}\times 100\mathrm{\%}$$

(4)

where the relative density of sample D_Relative is given by the percentage ratio of the density ρ_sample, obtained for the bulk samples, to theoretical density ρ_theoretical obtained from the XRD data (Table 3). Increasing the relative density of ceramics plays a pivotal role in improving their critical current density. This enhancement is primarily attributed to the reduction in insulating cavities within the material, which facilitates better current flow between the grains.

The critical current density ($J_c$) in bulk polycrystalline high-temperature superconductors is lower than in thin films or single crystals, mainly due to grain boundaries acting as weak links, poor connectivity from porosity and cracks, and oxygen inhomogeneity. These factors disrupt current flow and reduce superconducting performance. In contrast, thin films and single crystals have excellent connectivity, controlled oxygen content, and aligned current paths; thin films can also incorporate engineered nanostructured pinning centers, while single crystals are free of grain boundaries, resulting in much higher $J_c$ values [31].

When comparing this work with other methods, the crystal structure of Gd123 remains similar (a ≈ 3.82 Å, b ≈ 3.88–3.90 Å, c ≈ 11.68 Å); however, the T_C(R=0) values vary, as shown in the table below (

Table 4. Transition temperature (T_C(R=0)) by method.

Method	TC(R=0)	Reference
Solid-State Reaction (SSR)	~92 K	[4,5]
Pulsed Laser Deposition (PLD)	78–93 K	[10,11]
Chemically Modified Photosensitive (CMP)	~90 K	[18]
Co-precipitation (COP)	94–95 K	This work

).

3. Materials and Methods

3.1. Materials

Gadolinium(III) acetate tetrahydrate {Gd(CH₃COO)₃· 4H₂O}, barium acetate {Ba(CH₃COO)₂}, and Copper(II) acetate monohydrate {Cu(CH₃COO)₂· H₂O}, were purchased from Sigma Aldrich (USA), supplied with a purity greater than 98%, and used as received. Oxalic acid (C₂H₂O₄·2H₂O) was supplied by Thermos Fisher Scientific (USA), isopropanol (C₃H₈O) were purchased from Loba Chemie (India), and distilled water were used as required.

3.2. Superconductor Powder Synthetization

A clear aqueous solution of a mixture of 6.92 mmol Gd(CH₃COO)₃· 4H₂O, 13.85 mmol Ba(CH₃COO)₂, and 20.78 mmol Cu(CH₃COO)₂· H₂O was prepared, and then 300 mL of 0.5 M oxalic acid and isopropanol/water mixture (3:2 volume ratio) were added. The addition of isopropanol reduces the overall polarity of the water medium, slowing down nucleation and growth, leading to finer and more uniform particles synthesis. The result was a stable blue suspension product that was separated and dried overnight, gaining a blue powder, as reported in the ref [13].

Figure 7 shows the workflow of the research project, which includes the synthesis of the required powders, sintering, electrical measurement, and characterization.

3.3. Compaction and Sintering

The blue powder obtained was heated at 900 °C for 12 h in a muffle furnace (Nabertherm, Lilienthal, Germany), then cooled to room temperature at a rate of 2 °C per minute. After calcination, the powder was ground for 10 min using an agate mortar and pestle, then pressed into pellets about 1.25 cm in diameter with a Carver pressing machine (Carver, MN, USA) at a compaction pressure of 300 MPa. These pellets were sintered at 920, 930, 940, and 950 °C under an oxygen flow of 5 mL/min for 15 h in a tube furnace (Nabertherm, Germany), which is recommended to enhance the formation of the Gd123 superconductor and reduce the oxygen deficiency until room temperature is achieved [14], followed by slow cooling to room temperature at a rate of 1 °C per minute.

3.4. Electrical Resistance Measurement

Electrical resistance measurements for the sintered pellets were conducted over a temperature range of 50–200 K using a standard four-probe method. A direct current (DC) of 30 mA was applied, and the experiments were carried out in a closed-cycle refrigerator system model ARS-EA202A (Pittsburgh, PA, USA).

3.5. Characterization

Some of the sintered pellets were milled, and their powders were examined using X-ray powder diffraction (Malvern Panalytical, Aeris, monochromatic Cu k_α1, 1.5406 Å, 0.02 step angle, with 2θ ranging from 4° to 60°, Almelo, The Netherlands) for phase, and lattice parameters’ determination. TGA of the blue powder was conducted using a Netzsch STA 409 PG/PC thermal analyzer (Selb, Bavaria, Germany) from 30 to 1000 °C under nitrogen (50 mL/min) at a rate of 5 °C/min to measure the weight loss as temperature increased. SEM (FEI QUANTA 200, Eindhoven, The Netherlands) and (Hitachi S 3400 N, Tokyo, Japan) were employed for microstructural investigations of the produced blue superconductor powder and sintered pellets.

4. Conclusions

GdBa₂Cu₃O_7−δ superconducting ceramics (Gd123) were synthesized from nanoscale metal–oxalate precursors via co-precipitation, followed by sintering at temperatures ranging from 920 °C to 950 °C. Elevated sintering temperatures resulted in an increase in the orthorhombic Gd123 phase fraction from 96.2% to 99.8%. Electrical measurements demonstrated metallic characteristics, with (Tc, R = 0) consistently observed between 94 and 95 K.

Both XRD and SEM analyses show that raising the sintering temperature improves the critical current density. According to XRD patterns, small amounts of secondary phases are present, which can serve as efficient flux-pinning centers. Meanwhile, SEM images demonstrate that higher temperatures progressively decrease the gaps and voids between superconducting grains. This reduction in insulating cavities strengthens intergranular coupling and increases the material’s relative density, resulting in better current transport efficiency.

Overall, a sintering temperature of 950 °C not only produces the highest Jc but also indicates that large-scale production of high-performance Gd123 tapes or bulk magnets could be feasible.