- Email: [email protected]
The use of a transparent furnace for vapor transport experiments
Journal of Crystal Growth 66 (1984) 679—681 North-Holland. Amsterdam
679
LETTER TO THE EDITORS TUE USE OF A TRANSPARENT FURNACE FOR VAPOR TRANSPORT EXPERIMENTS G. FOURCAUDOT, J. MERCIER and H. GUYOT LEPES/CNRS
~,
JO6X. F-38042 Grenoble Cedex, France
Received 28 January 1984
A simple low-cost transparent furnace, well suited for vapor transport experiments is described. Its application to the growth of Mo
401 crystals is briefly reported.
Transparent furnaces for crystal growth from the vapor have been built, according to two types of design: (a) The so-called gold furnace [1.2]. making use of a thin transparent, thermally insulating gold layer. (b) A resistance heating furnace, making use of a
*
Associated with the University of Grenoble.
transparent, electrically conducting oxide layer as a resistor [3]. Both set-ups utilize rather fragile layers, somewhat costly, suffering from a more or less limited range of operating temperatures. We describe a home-made, simple and low-cost system. Its lifetime has been tested, exceeding up to now six months of almost continuous operation~ with not the least sign of loss of visibility.
Fig. 1. General view of the furnace.
0022-0248/84/$03.OO © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
680
G. Fourcaudot ci al.
/
Transparent furnace for i’apor transport
Basically, it is a two-zone furnace (fig. 1), the temperature of each zone being monitored by two electronic controllers. It consists of two concentric, horizontal pure silica tubes, maintained by two bored end plates, made of brass. The inner tube (external diameter = 3 cm, length L = 50 cm) hears the resistance windings over 33 cm. The low ternperature zone, in the middle, is heated via 14 turns (4.8 Q/m) by 70 V of the secondary of a transformer. From each side beyond this zone, 22 and 31 turns are wound in series to heat the high temperature zone, supplied by 220 V. The spacing of the turns is 0_S cm. A camel-back temperature profile along the length is thus obtained, The attachment of the wiring elements to the inner tube is the keypoint of the system. In a first version, the tube was threaded externally, the wires maintained in the grooves. But due to machining, a loss of transparency made this solution of little use. Then we tried to maintain the wirings by depositing alumina-based cement along two opposite generatrices of the tube. Unfortunately, on heating a rapid devitrification took place from these deposits, presumably due to the binder. Finally, two silica rods (diameter = 3 mm) were
Fig. 2. Detail of the windings assembly.
torch-welded in place of the cement. Notches were machined along these rods, where the wires were inserted (fig. 2). This furnace can withstand temperatures up to 850°C. During the heating cycle, the windings “breathe” according to the power sequence of the controllers, but with no detectable temperature change in a steady state. This furnace is being used to grow Mo
401 single crystals of both crystallographic modifications in sealed silica ampoules (diameter = 1 .5 cm. L = 10 cm). The starting material is the y-form prepared as a powder by solid state reaction at 800°C between Mo and MoO3, in a sealed tube. Usually the transport agent is TeCI4. The process lasts I week and only 2 to 3 large single crystals. limited in size by the wall, are obtained. This nucleation control is a unique, as yet unexplained advantage of this furnace compared to conventional opaque ones, operating under quite similar conditions which give rise to the transport of numerous small crystals. Typically (fig. 3): T~= 570°C. T2 = 550°C, ~ Mo4011, T1 = 690°C, T2 = 650°C, y — Mo401
G. Fourcaudot et al.
,
/
Transparent furnace for vapor transport
,
Fig. 4. ~~—gros~ fl UeMi~0 ~~ Cr’s sial’. ( n~nigrid).
.
,
681
.
tiphase occurrence, indicated by the MoC1
5 expert-
ments, but also to attempting to dope crystals with Fe: in this instance, TeMo5O16 [51crystals as well
F ~ 3. As-groun Mo4O1~crystals (mm grid). Top. ~i-modification. Bottom: y-modification.
The as-grown n-crystals are twinned. Since Te contamination was also suspected, we used MoCI5 as a transporting agent. But this alone failed to give the desired phase: at T1 = 680°Cand T, = 606°C, MoO3 forms; at T~= 700°Cand T2 = 680°C, Mo4011 decomposes into MoO2 at T~and MoOs sublimes at T2. s~-Mo4o1igrew using 2 mg TeC14 + 9 mg MoC15 at T= 524°C (T = 542°C). Whatever the transport agent. ~-Mo4O11 exhibits a plate-like morphology, the large face being the (100) plane but
the elongated direction is different: [0101 in the first case and [011] in the second case. More significant, this second procedure gives twin-free crystals, according to X-ray diffuse scattering experiments. This chemical system is highly sensitive to mul-
as sj-Mo4011 grew in the same run. Intentionally grown TeMo5 016 crystals can be obtained, starting from a mixture of 5 Mo4011, 4 Te07 (TeMo5O1576 nominal), T1 = 600°C, T2 = 573°C, and 20 mg TeC14 4). is currently being investigated. In Iron(fig. doping addition, in order to improve the accuracy of resistivity measurements on these highly conducting materials (p 1 -~ 70 ~sQ cm at 300 K [61), it is desirable to increase the resistance, changing the geometry of the sample. In this way, the flexibility of our furnace is of great help.
References [11 TB.
Reed. Solid State Research Reports Lincoln Lab. MIT (1973 1).
[21 E.
SchOnherr, The Growth of Large Single Crystals from
the Vapor Phase, in: Crystals: Growth.
Properties and
Applications. Vol. 2 (Springer, Berlin, 1980) p. 53. ~•
[51Y. [61 H.
Kib
A
~
~l;~o~Ferrand
(1983).
Arnaud and J. Guidot, Acta Cryst. B33 (1977) 2151. Guyot. C. Escribe-Filippini, G. Fourcaudot. K. Konate
and C. Schlenker. J. Phys. C (Letters), to he published.
679
LETTER TO THE EDITORS TUE USE OF A TRANSPARENT FURNACE FOR VAPOR TRANSPORT EXPERIMENTS G. FOURCAUDOT, J. MERCIER and H. GUYOT LEPES/CNRS
~,
JO6X. F-38042 Grenoble Cedex, France
Received 28 January 1984
A simple low-cost transparent furnace, well suited for vapor transport experiments is described. Its application to the growth of Mo
401 crystals is briefly reported.
Transparent furnaces for crystal growth from the vapor have been built, according to two types of design: (a) The so-called gold furnace [1.2]. making use of a thin transparent, thermally insulating gold layer. (b) A resistance heating furnace, making use of a
*
Associated with the University of Grenoble.
transparent, electrically conducting oxide layer as a resistor [3]. Both set-ups utilize rather fragile layers, somewhat costly, suffering from a more or less limited range of operating temperatures. We describe a home-made, simple and low-cost system. Its lifetime has been tested, exceeding up to now six months of almost continuous operation~ with not the least sign of loss of visibility.
Fig. 1. General view of the furnace.
0022-0248/84/$03.OO © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
680
G. Fourcaudot ci al.
/
Transparent furnace for i’apor transport
Basically, it is a two-zone furnace (fig. 1), the temperature of each zone being monitored by two electronic controllers. It consists of two concentric, horizontal pure silica tubes, maintained by two bored end plates, made of brass. The inner tube (external diameter = 3 cm, length L = 50 cm) hears the resistance windings over 33 cm. The low ternperature zone, in the middle, is heated via 14 turns (4.8 Q/m) by 70 V of the secondary of a transformer. From each side beyond this zone, 22 and 31 turns are wound in series to heat the high temperature zone, supplied by 220 V. The spacing of the turns is 0_S cm. A camel-back temperature profile along the length is thus obtained, The attachment of the wiring elements to the inner tube is the keypoint of the system. In a first version, the tube was threaded externally, the wires maintained in the grooves. But due to machining, a loss of transparency made this solution of little use. Then we tried to maintain the wirings by depositing alumina-based cement along two opposite generatrices of the tube. Unfortunately, on heating a rapid devitrification took place from these deposits, presumably due to the binder. Finally, two silica rods (diameter = 3 mm) were
Fig. 2. Detail of the windings assembly.
torch-welded in place of the cement. Notches were machined along these rods, where the wires were inserted (fig. 2). This furnace can withstand temperatures up to 850°C. During the heating cycle, the windings “breathe” according to the power sequence of the controllers, but with no detectable temperature change in a steady state. This furnace is being used to grow Mo
401 single crystals of both crystallographic modifications in sealed silica ampoules (diameter = 1 .5 cm. L = 10 cm). The starting material is the y-form prepared as a powder by solid state reaction at 800°C between Mo and MoO3, in a sealed tube. Usually the transport agent is TeCI4. The process lasts I week and only 2 to 3 large single crystals. limited in size by the wall, are obtained. This nucleation control is a unique, as yet unexplained advantage of this furnace compared to conventional opaque ones, operating under quite similar conditions which give rise to the transport of numerous small crystals. Typically (fig. 3): T~= 570°C. T2 = 550°C, ~ Mo4011, T1 = 690°C, T2 = 650°C, y — Mo401
G. Fourcaudot et al.
,
/
Transparent furnace for vapor transport
,
Fig. 4. ~~—gros~ fl UeMi~0 ~~ Cr’s sial’. ( n~nigrid).
.
,
681
.
tiphase occurrence, indicated by the MoC1
5 expert-
ments, but also to attempting to dope crystals with Fe: in this instance, TeMo5O16 [51crystals as well
F ~ 3. As-groun Mo4O1~crystals (mm grid). Top. ~i-modification. Bottom: y-modification.
The as-grown n-crystals are twinned. Since Te contamination was also suspected, we used MoCI5 as a transporting agent. But this alone failed to give the desired phase: at T1 = 680°Cand T, = 606°C, MoO3 forms; at T~= 700°Cand T2 = 680°C, Mo4011 decomposes into MoO2 at T~and MoOs sublimes at T2. s~-Mo4o1igrew using 2 mg TeC14 + 9 mg MoC15 at T= 524°C (T = 542°C). Whatever the transport agent. ~-Mo4O11 exhibits a plate-like morphology, the large face being the (100) plane but
the elongated direction is different: [0101 in the first case and [011] in the second case. More significant, this second procedure gives twin-free crystals, according to X-ray diffuse scattering experiments. This chemical system is highly sensitive to mul-
as sj-Mo4011 grew in the same run. Intentionally grown TeMo5 016 crystals can be obtained, starting from a mixture of 5 Mo4011, 4 Te07 (TeMo5O1576 nominal), T1 = 600°C, T2 = 573°C, and 20 mg TeC14 4). is currently being investigated. In Iron(fig. doping addition, in order to improve the accuracy of resistivity measurements on these highly conducting materials (p 1 -~ 70 ~sQ cm at 300 K [61), it is desirable to increase the resistance, changing the geometry of the sample. In this way, the flexibility of our furnace is of great help.
References [11 TB.
Reed. Solid State Research Reports Lincoln Lab. MIT (1973 1).
[21 E.
SchOnherr, The Growth of Large Single Crystals from
the Vapor Phase, in: Crystals: Growth.
Properties and
Applications. Vol. 2 (Springer, Berlin, 1980) p. 53. ~•
[51Y. [61 H.
Kib
A
~
~l;~o~Ferrand
(1983).
Arnaud and J. Guidot, Acta Cryst. B33 (1977) 2151. Guyot. C. Escribe-Filippini, G. Fourcaudot. K. Konate
and C. Schlenker. J. Phys. C (Letters), to he published.