Furnace Spec — Translucent Alumina 1850C

Atmosphere     Achievable Density    Translucency        Verdict
---------------------------------------------------------------
Air @ 1800C    99.0-99.5%            Marginal            Trapped N₂/O₂ in closed pores
Vacuum         99.5-99.9%            Good to excellent   Gas escapes as pores close
H₂             99.7-99.95%           Excellent           H₂ cleans + diffuses out
Vacuum + HIP   99.95-99.99%          Best possible       Gold standard

Tube Furnace: Your Secret Weapon

Why Your Tube Furnace May Be the Best Path

You already own a hot tube furnace. Tube furnaces have structural advantages for this exact process:

Tube Furnace Advantages

Uniformity	Cylindrical geometry → radially symmetric heating. ±2-3C typical vs ±5-10C box
Atmosphere	Trivial to seal: two end caps with O-rings + gas ports. Vacuum or flowing H₂
Vacuum	Round tube is naturally pressure-vessel shaped. Resists atmospheric load better than flat walls
Cost to upgrade	End caps + vacuum pump + fittings = $2-5k vs $15-60k for a new vacuum box furnace
Hot zone length	The uniform zone is typically the middle 60% of element length

Tube Furnace Limitations

Diameter	Workpiece limited by tube ID. Common: 50-100mm ID for high-temp alumina tubes
Loading	Horizontal loading, samples on a boat/crucible pushed in
Tube material	Al₂O₃ tube above 1600C. Mullite OK to ~1500C
Tube cost	99.8% alumina tubes, 80mm OD x 1m: $200-600. Fragile.
Batch size	Small per run. Production needs multiple runs or larger tube

Tube Furnace Atmosphere Retrofit: $2,000-5,000

COMPONENT                    EST. COST       SOURCE
----------------------------------------------------------
Vacuum end caps (KF flanges)   $300-800      LDS Vacuum, Kurt Lesker
Rotary vane pump (2-stage)     $800-1,500    Edwards, Welch, Agilent
Vacuum gauge (Pirani+TC)       $200-400      Kurt Lesker, Lesker
KF fittings + bellows          $200-400      Kurt Lesker
H₂ gas regulator + flowmeter   $300-500      Matheson, Airgas
H₂ safety: flashback arrestor   $100-200      Western Enterprises
Thermocouple feedthrough       $100-200      Omega, Kurt Lesker
----------------------------------------------------------
TOTAL VACUUM PATH:             $1,600-3,300
TOTAL H₂ PATH:                  $1,000-2,100
TOTAL BOTH:                    $2,200-4,500

Buyable Furnaces: Air Atmosphere (with links)

1800C Box Furnaces

1700C Box Furnaces

1700-1800C Tube Furnaces

Vacuum / Atmosphere Options

Thermal Physics: 13.2 kW Budget

Vendor	Model	Hot Zone	Max T	Price	Link
Nabertherm	LHT 02/18	~2L	1800C	$29,631	MSE Supplies
Nabertherm	LHT 04/18	~4L	1800C	$35,021	MSE Supplies
MTI Corp	KSL-1800X-KA3	8x8x12" (12.5L)	1800C	~$6-10k	MTI Store
MTI Corp	KSL-1800X-A1-W	6x6x6" (3.6L)	1800C	~$5-8k	MTI Store
Henan Sante	STM-20-18	~20L	1800C	$3,400-3,600	Made-in-China
Sentrotech	ST-1800 box	various	1800C	~$8-20k	Sentrotech
SH Scientific	1800C muffle	4.5-11L	1800C	~$15,400	Lab&Furnace
Brother	BR-18HM	various	1800C	~$3-8k	Brother Furnace

Vendor	Model	Hot Zone	Max T	Price	Link
Nabertherm	LHT 03/17 D	~3L	1700C	$22,682	MSE Supplies
MTI Corp	KSL-1700X-KA2	8x8x8" (8.4L)	1750C	~$4-7k	MTI Store
MTI Corp	KSL-1700X-A7	60x60x60cm (216L)	1700C	~$15-25k	MTI Store
Henan Sante	various	1-20L	1700C	$600-6,500	Made-in-China
Sentrotech	ST-1700 box	various	1700C	~$6-15k	Sentrotech

Vendor	Model	Tube ID	Max T	Price	Link
MTI Corp	GSL-1700X series	25-100mm	1700C	~$3-8k	MTI Store
MTI Corp	GSL-1800X series	25-80mm	1800C	~$5-12k	MTI Store
Sentrotech	ST-1700/1800 tube	various	1800C	~$5-15k	Sentrotech
Brother	BTF-1800C	various	1800C	~$2-6k	Brother Furnace
Thermansys	RCT-AM2-T-1800	various	1800C	quote	Thermansys

Vendor	Type	Max T	Atm	Price	Link
SH Scientific	Vacuum muffle	1800C	Vacuum	$39,400-58,100	Lab&Furnace
MTI Corp	H₂ atmosphere box	1700C	H₂	~$8-15k	MTI KSL-1700X-H2
YOUR TUBE + RETROFIT	Tube + end caps	existing	Vacuum / H₂	$2,200-4,500	BEST VALUE
Custom build	Mo/Vacuum box	1850C+	Vacuum / H₂	$8-15k DIY	See physics section

POWER SUPPLY: 60A x 220V = 13,200W = 13.2 kW

Current hot zone: 120 x 200 x 300mm = 7.2L
Inner surface area: 0.24 m²

INSULATION STACK (150mm composite):
Layer              Thick    k (W/m·K)    R (m²·K/W)
Dense Al₂O₃        25mm     2.0            0.013
Al₂O₃ fiberboard   50mm     0.35           0.143
Microporous        50mm     0.05           1.000
Ceramic blanket    25mm     0.12           0.208
                                    TOTAL: 1.364

Q_walls = 0.24 x 1830 / 1.364 = ~322 W (idealized)

REALISTIC STEADY-STATE LOSSES:
                    Vacuum       H₂
Wall conduction     300-500 W    300-500 W
Door/seal           200-400 W    200-400 W
Structural          100-300 W    100-300 W
Radiation gaps      100-400 W    100-400 W
Gas convection        ~0 W      800-1500 W    ← H₂ thermal conductivity 10x air
Feedthroughs        100-200 W    100-200 W
                   ----------   ----------
TOTAL:             0.8-1.8 kW   1.6-3.3 kW

RAMP DEMAND (80 kg thermal mass, Cp ~1000 J/kg·K, ΔT ~900K):
4-hour ramp: 5.0 kW avg + losses = 7-9 kW peak     margin: ~4-6 kW  ✔
8-hour ramp: 2.5 kW avg + losses = 4-6 kW peak     margin: ~7-9 kW  ✔✔

VERDICT: 13.2 kW is sufficient. Vacuum = very comfortable. H₂ = tighter, use 8hr ramp.

Element x Atmosphere Compatibility

               Air     N₂      Ar     Vacuum   H₂
MoSi₂          ✔1800  ✔      ✔     ✘ ¹     ✘ ¹
SiC            ✔1600  ✔      ✔     ✘ ¹     ~
Mo             ✘ ²    ✘ ³    ✔     ✔ 2200  ✔ 2200
W (tungsten)   ✘ ²    ✘      ✔     ✔ 2800  ✔ 2500
Graphite       ✘ ⁴    ✔      ✔     ✔ 3000  ✔

¹ SiO₂ glass layer can't form → element crumbles
² Catastrophic oxidation >400-500C
³ Nitriding embrittlement
⁴ Burns in air >400C

CRITICAL: Vacuum/H₂ eliminates MoSi₂ and SiC.
          For your atmosphere: Mo or W elements only.
          (Graphite reduces alumina: Al₂O₃ + 3C → 2Al + 3CO)

Breaker Panel: 60A / 220V

MAIN DISCONNECT: 60A 2-pole 220V
│
├─ HEATING ZONE 1 (Top)       20A → SCR → Mo elements   (~4 kW)
├─ HEATING ZONE 2 (Center)    20A → SCR → Mo elements   (~4 kW)
├─ HEATING ZONE 3 (Bottom)    20A → SCR → Mo elements   (~4 kW)
│                                                         12 kW heating
├─ VACUUM SYSTEM              15A
│   ├─ Roughing pump (rotary vane)    5-8A run, 12A inrush
│   └─ Turbo/diffusion pump           3-5A
│
├─ CONTROLS                   10A
│   ├─ PID controllers (x3)
│   ├─ Thermocouples, vacuum gauges
│   └─ Safety interlocks
│
└─ COOLING                    15A
    ├─ Water recirculator             5-10A
    └─ Shell cooling fans             2-3A

DEMAND ANALYSIS:
Phase            Heat   Pump  Ctrl  Cool  TOTAL   MARGIN
Pumpdown          0A     12A   2A    2A    16A     44A ✔✔
Ramp (8hr)       35A      6A   2A    6A    49A     11A ✔
Ramp (4hr)       48A      6A   2A    8A    64A     -4A ✘
Soak @1850C    8-15A      6A   2A    8A   24-31A   29A ✔✔
Cooldown          0A      6A   2A   10A    18A     42A ✔✔

NOTE: 4hr ramp clips 60A near peak. Use 6-8hr ramp (alumina prefers it anyway).

Hyperdimensional Options Map

CONFIG          A: Mo/Vac     B: Mo/H₂      C: W/Vac      D: W/H₂       E: Tube+Retro
Element         Mo wire       Mo wire        W rod/mesh     W rod/mesh     Existing MoSi₂
Atmosphere      Vacuum        H₂ (forming)   Vacuum         H₂             Vac or H₂

DIMENSION       A             B              C              D              E
──────────────────────────────────────────────────────────────────────────────────
Temp ceiling    ████████░░    ████████░░     ██████████     █████████░     ███████░░░
                2200C         2200C          2800C          2500C          1800C (existing)

Power margin    █████████░    ██████░░░░     █████████░     ██████░░░░     ██████████
                >4 kW         ~2 kW          >4 kW          ~2 kW          already built

Safety          ████████░░    ████░░░░░░     ████████░░     ████░░░░░░     █████████░
                vac=safe      H₂ explosive   vac=safe       H₂ explosive   minimal new risk

Cost            ████░░░░░░    █████░░░░░     ███░░░░░░░     ███░░░░░░░     █████████░
                $8-15k        $6-12k         $12-25k        $10-20k        $2-5k

TPA quality     ████████░░    █████████░     ████████░░     █████████░     ███████░░░
                excellent     superb         excellent      superb         good (if T enough)

Uniformity      ███████░░░    ████████░░     ██████░░░░     ███████░░░     █████████░
                ±5-10C box   H₂ convection  W hard shape   better         ±2-3C tube!

Time to first   ████░░░░░░    ███░░░░░░░     ████░░░░░░     ███░░░░░░░     ██████████
  sinter        weeks-months  weeks-months   weeks-months   weeks-months   DAYS

SCORE           ★★★★          ★★★★           ★★★            ★★★            ★★★★★

Recommendation

Path E: Retrofit Your Tube Furnace → First Sinter in Days

If your tube furnace already hits 1700C+, the fastest and cheapest path to translucent alumina is:

Seal the tube ends with KF-flanged vacuum caps ($300-800)
Add a rotary vane pump for ~10⁻³ torr vacuum ($800-1500)
First test sinter at 1750-1800C under vacuum
If translucency is marginal, add H₂ flowing gas path ($500-1000 more)
If you need more volume, THEN buy a box furnace for production

Total: $2,200-4,500 vs $30k+ for a new vacuum box furnace.

Tube uniformity (±2-3C) is actually better than box (±5-10C). The only downside is batch size limited by tube ID.

What You Need to Check About Your Tube Furnace

What is the tube ID? (determines max workpiece diameter)
What is the max rated temperature? (need 1700C+ for TPA)
What elements does it use? (MoSi₂ = air only, can't do vacuum. Mo/W = vacuum OK)
If MoSi₂ elements: you can still do flowing inert gas (Ar) or mild vacuum at the tube level while elements stay in air
Hot zone length? (need ~150-300mm uniform zone for useful batch)

Vendor Directory

US-Based / US-Warehouse

MTI Corporation	Richmond, CA. China-made, US stock.
Sentrotech	Berea, OH. US-made. Good customization.
CM Furnaces	Bloomfield, NJ. Premium custom.
Across International	Livingston, NJ. Budget elements + furnaces.
MSE Supplies	Tucson, AZ. Nabertherm distributor.
Stanford Adv. Materials	Lake Forest, CA. MTI reseller.

International / Direct-from-China

Nabertherm	Germany. Premium lab furnaces.
Brother Furnace	Zhengzhou, China. Budget MoSi₂.
Henan Sante	Luoyang, China. Cheapest 1800C.
SH Scientific	Korea. Vacuum options listed.
Thermansys	Turkey. Fast-ramp tube furnaces.
TMAX	Xiamen, China. Box + tube.

Kurt J. Lesker	KF flanges, feedthroughs, pumps, gauges
LDS Vacuum	Budget vacuum fittings + valves
Edwards	Rotary vane + turbo pumps
Omega Engineering	Thermocouples, feedthroughs
Matheson / Airgas	H₂ supply, regulators, safety

Furnace Spec Map

Mission: Translucent Polycrystalline Alumina